- Treating and Preventing Scale
- Crapemyrtle Bark Scale
- Insect Life Cycle
- Landscape & Ornamentals
- SCALE INSECTS ON SHADE TREES AND SHRUBS
- Scale insects
- Identifying scale insects
- What is the usual season for scale insects to appear?
- What conditions help them to reproduce?
- What are the risks for the plant?
- How to fight scale insects?
- Different types of scale insects
- Plants often attacked by scale insects and mealybugs
- Smart tip about scale insects
- Controlling Citrus Scale – How To Treat Types Of Scale On Citrus Plants
- What are Citrus Scale Pests?
- What are the Types of Scale on Citrus Plants?
- Controlling Citrus Scale
- How to Manage Pests
- UC Pest Management Guidelines
- Introduction – Synonymy – Distribution – Life Cycle and Biology – Description – Hosts – Economic Significance – Management – Selected References
- Synonymy (Back to Top)
- Distribution (Back to Top)
- Life Cycle and Biology (Back to Top)
- Description (Back to Top)
- Hosts (Back to Top)
- Economic Significance (Back to Top)
- Management (Back to Top)
- Selected References (Back to Top)
- Related posts:
Treating and Preventing Scale
Scale insects can be devastating to citrus trees. The pests are tiny insects which suck sap from the citrus tree and then excrete honeydew which accumulates on leaves, branches and fruit. Honeydew then turns to sooty mold and that interferes with photosynthesis in leaves, and can cause leaf drop and branch die back.
There are two types of scale, armored scales that are hard bodied consisting primarily of wax, the females of this family insert their long mouthparts as crawlers and never move again on their own, but can be moved by ants that are farming them. The soft bodied scale is not fastened permanently to the tree. The soft scale gives off large amounts of honeydew upon which sooty mold fungus forms, they can move on their own but often they are moved to other areas when ants relocate them.
It is important to have an ant barrier of a 3-4 inch band with tangle-foot around the trunk of the citrus to help protect ants from invading citrus. Since ants have a symbiotic relationship with scale insects (and other pests as well) a tape barrier is crucial. Ants domesticate many pests that damage plants and citrus trees, and ants will move those bugs from one food source to the next.
If you have an orchard, it is always a good idea to have a tape/tangle-foot barrier and to purchase Ladybugs and release those yearly. Ladybugs will attack all stages of scale.
If you notice beetles on your citrus trees, do not use pesticides. Beetles are keeping your trees healthy and will protect your investment.
Spraying soapy water onto the tree does little to remove scale insects. So it is always a good idea to completely wash citrus trees with Dawn dish-soap and warm water, with a wash-cloth. After washing treat tree with Horticultural Oil or Neem Oil.
Check tree again in a week and repeat process if required. It is always a good idea to wash the tree every time you fertilize.
If you need further assistance with a scale infestation, please message us here.
Term applies to small insect of the order Hemiptera, suborder Sternorrhyncha. There are currently around 8000 known species of scale insects.
Scale are small sap-sucking insects. Many look like the scale of a fish or lizard, while others can even appear fluffy. The scale insects that most gardeners are familiar with are the adult females. The adults are generally immobile and are permanently attached to the plant that they are sucking the sap from. Scale feeding on a plant can affect the vigour and also produce honeydew.
Honeydew is a sweet waste product that can attract wasps and cause your plants to become covered in sooty mould.
Healthy plants are less likely to suffer from insects like scale. Feed plants regularly, do what you can to improve the soil and make sure that plants are well watered.
Spray plants thoroughly (until they start dripping) with Bugtrol.
It’s an organic spray but, if possible, spray in the evening to avoid harming any beneficial insects in your garden.
Don’t use oil-based sprays on ferns or fine-leafed palms. For these plants either use pyrethrum or see the other treatment option.
Use Conqueror oil over your infected plants. Make sure to spray on non-flowering plants, or take the flowers off any bee-friendly plants before spraying.
If possible spray in the evening to reduce harm to the beneficial insects in your garden.
Our lemon tree is bursting into bloom with the warmer weather
We’ve got a lemon tree growing in a large pot at the back doorstep. It’s just bursting in to bloom at the moment. Unfortunately we’re also starting to get a bit of scale appearing on the undersides of some of the leaves.
Scale are usually located on the underside of leaves and sometimes the stems of plants
If you’ve got scale on your citrus trees, you’ll probably also notice that there are ants crawling over your tree too. Scales shoot a sweet substance called honeydew. Ants like to ‘farm’ the scale to feed on the honeydew. They’ll even pick the scale up and move them all over the tree. Honeydew also leads to sooty mould, a black dusty fungus that grows over the leaves and stems. Controlling the scale will also get rid of the sooty mould.
Scale insects secret honeydew which in turn attracts ants
Scale in large numbers can cause leaf yellowing, leaf drop and die-back. Don’t worry though, it’s easily treated with a spray of white oil, which suffocates them. The great news is that it’s easy to make your own white oil to treat scale and aphids.
We’ve got a Meyer Lemon tree in a container at the back doorstep
Crapemyrtle Bark Scale
September 2019 Distribution map for CMBS from EDDMaps.org
The crapemyrtle bark scale (Acanthococcus lagerstroemiae) is a recently introduced pest from Asia that initially infested crapemyrtles (Lagerstroemia spp.) in Texas during 2004. Since then, it has spread rapidly through Oklahoma, Arkansas, Louisiana, Mississippi, and Georgia. Now it has been discovered in North Carolina and Virginia, and the distant spread of this pest has likely been through the movement of plant material. Crapemyrtle bark scale (CMBS) has been confirmed in Richland County (Columbia) in South Carolina this year. With the recent appearance of CMBS in Mecklenburg County (Charlotte) in North Carolina, this insect pest may also appear soon in Upstate South Carolina.
The CMBS is a bark or felt scale, which is slightly different from soft scales. However, they have a waxy coating and exude honeydew, as do soft scales. Bark scales are in a different scale insect family (Eriococcidae) than soft scales (Coccidae), and they look very similar to mealybugs.
Crapemyrtle bark scale is a new insect pest to the Southeastern US. Above on a branch are the white adults and ovisacs containing their pink eggs.
Helene Dougherty, Virginia Polytechnic Institute & State University, Bugwood.org
Crapemyrtle branches can become completely covered with crapemyrtle bark scale.
Jim Robbins, University of Arkansas CES, Bugwood.org
The CMBS infestations appear as white or gray, waxy crustations on stems, large twigs, and trunks, but rarely on foliage. They especially congregate in branch crotches and at pruning sites. This scale will settle to feed under loose, exfoliating bark of the crapemyrtle, which makes control by both predators and pesticides more difficult.
These bark scales produce copious amounts of honeydew, the sugary waste the scale produces as it feeds on the plant’s phloem. As a result, the leaves, branches, and trunk become covered with black sooty mold, which grows on the honeydew.
The spent crapemyrtle flower clusters may also be covered with crapemyrtle bark scale and sooty mold.
Mengmeng Gu, Texas A&M AgriLife Extension Service, Bugwood.org
Insect Life Cycle
The small CMBS males are winged and will fly to find females and to mate. Once the mated females produce their ovisacs (egg-containing capsules) and lay eggs, they die. The eggs remain protected within the white colored ovisacs until the crawlers (immatures) hatch and disperse onto the branches. Each female lays about 60 to 250 eggs, which may over-winter within their ovisacs, and then hatch during mid- to late April to May. The crawlers are pink, very small, and may not be noticed without a hand lens. A second peak in crawler activity occurs in late summer. Double-sided sticky tape wrapped around small branches can be used to trap the crawlers to see when they hatch and to base the timing of additional contact insecticide applications. These mobile crawlers move out to new twigs and branches to settle down and begin feeding on the sugary phloem layer beneath the bark.
Crapemyrtles suffer aesthetic damage because of the CMBS infestations. These bark scales may not kill the plants, but there may likely be a reduction in plant vigor, number of flowers, and flower cluster size. Infested plants typically leaf out later than healthy plants. Branches and trunks can be covered in the white scale infestation. Another striking symptom is the extensive amount of black sooty mold that may completely cover the foliage, branches, and trunks. However, do not confuse the honeydew and resulting black sooty mold caused by an aphid infestation with that caused by the crapemyrtle bark scale. Aphids are small insect pests that feed on new tender growth on the ends of branches. With a scale heavy infestation, there may be premature bark peeling. Often there will be more female adults congregated on the lower (and shadier) sides of branches.
The crapemyrtle on the left was treated to control crapemyrtle bark scale. The tree on the right is infested and shows reduced flowering. A closer inspection of infested crapemyrtles will show copious sooty mold on leaves, branches, and trunks.
Jim Robbins, University of Arkansas CES, Bugwood.org
Cultural Control: Keep crapemyrtles healthy by properly mulching, irrigating, fertilizing (based on soil test recommendations), and proper pruning. Please see HGIC 1009, Crapemyrtle Pruning for best pruning practices. Crapemyrtles in sunnier sites often have smaller infestations than plants growing in more shade, and plants grown in shade typically have more crawlers (immatures) than in full sun. So, always plant crapemyrtles in the full sun areas of the landscape.
Several other common landscape plants are susceptible to CMBS infestation. These include pomegranate, persimmon, edible fig, boxwood, American beautyberry, cleyera, privet, and raspberries. These plants should be closely inspected for the CMBS, especially if crapemyrtles are planted nearby.
Natural predators may take a while to build up in numbers, but both lady beetles and mealybug destroyers are very effective in controlling CMBS.
Scraping the soft crapemyrtle bark scales can reveal if the scales are alive or dead. If alive, scraping will result in “bleeding” of their reddish body fluids. Mengmeng Gu, Texas A&M AgriLife Extension Service, Bugwood.org
Chemical Control: The most effective chemical control is a soil drench in the spring with dinotefuran. This systemic insecticides is available in a number of brands as concentrates for use as a soil drench, and in a few brands as granular products to scatter around the plants and water into the soil. These systemic insecticides will move up into the plants and give control for at least a year. They are most effectively applied in spring as new plant growth begins. See Table 1 for examples of products containing this systemic insecticides.
Alernatively, sprays for crawlers are best applied in the late April and May, then again in late summer when immatures appear. Use a bifenthrin spray mixed with 2% horticultural oil (i.e., 5 tablespoons of horticultural oil per gallon of water in a sprayer) added for best crawler control. Follow the label directions on bifenthrin products for rate per gallon. See Table 1 for examples of products containing bifenthrin and horticultural oil.
To determine if the soil drench treatments have been effective, scrape the soft bodies of the CMBS adults on a branch. If the result is the presence of a reddish body fluid of the scales, they are still alive. No “bleeding” will occur if they have been killed.
Table 1. Insecticides to Control Crapemyrtle Bark Scale on Crapemyrtles.
|Insecticide Active Ingredients|| Examples of Common Insecticide Products Labeled for Use on
|Dinotefuran||Gordon’s Zylam Liquid Systemic Insecticide
Gordon’s Zylam 20SG Systemic Turf Insecticide
Valent Brand Safari 20SG Insecticide
Valent Brand Safari 2G Insecticide (2% granules)
Ortho Tree & Shrub Insect Control Ready To Use Granules (2%)
|Bifenthrin||Ferti-lome Broad Spectrum Insecticide Concentrate
Hi-Yield Bug Blaster Bifenthrin 2.4 Concentrate
Monterey Turf & Ornamental Insect Spray
Up-Star Gold Insecticide Concentrate
Bifen I/T Concentrate
Talstar P Concentrate
|Horticultural Oil||Ferti-lome Horticultural Oil Spray Concentrate
Bonide All Seasons Spray Oil Concentrate
Southern Ag ParaFine Horticultural Oil Concentrate
Espoma Earth-tone Horticultural Oil Concentrate
Monterey Horticultural Oil Concentrate
Summit Year Round Spray Oil Concentrate
|These products are typically found for sale in small containers at feed & seed, farm supply, or landscaper supply stores. Follow all directions for mixing and safe use.
Horticultural oil sprays should be applied when temperatures are above 45 °F and below 90 °F. Always spray in the late evening to slow drying time and increase effectiveness.
Safety and Insecticide Treatment Notes: Many cultivars of crape myrtles can grow very tall, and some may reach heights of 20 or 30 feet. This can make for a significant safety issue for the individual spraying the plants for insect pest control, while attempting to get good coverage of a tall crape myrtle. Do not spray if it is windy, and wear the recommended protective gear stated on the label, especially if the plants are tall. Minor pruning of the crape myrtle will reduce some height and remove flowers present to lessen the impact of the bifenthrin insecticide spray on pollinators. This will also allow for better spray coverage. Prune, then spray with the bifenthrin and horticultural oil mix. Repeat the spray in 2 weeks and repeat 2 weeks later, which will be before the crape myrtle begins flowering again. Keep in mind that there can be a spray drift from spraying upward, and the insecticide can have an impact on pollinators of nearby flowering shrubs and herbaceous plants. Therefore, the best time to spray is in the very late evening to reduce the impact on pollinators.
For the use of soil systemic treatments (dinotefuran), the amount to apply is based upon the height of the crape myrtle (if shrub-like) or the cumulative diameters of the trunks (if tall and tree form). Therefore, a light pruning, which reduces the height, may reduce the amount of product required to treat the plant. The soil systemic insecticides may have a minor harmful effect on pollinating insects that feed on the pollen and nectar, but this should be much less of an impact than spraying a plant in bloom, which would likely kill pollinating insects. The pruning will delay bloom and lessen this harmful effect. A soil drench is far safer for the applicator and for protecting wildlife and beneficial insects (as these do not feed on the plant), much more so than spraying an insecticide all throughout the plant.
However, if one does not want to spray or apply a soil drench with an insecticide on the crape myrtles, there is another option. If the plant is highly infested, cut the crape myrtle off flush with the ground. Promptly burn the removed trunk and limbs, or cut up and place in garbage bags for trash pickup. Do not place on curb for regular brush pickup. Once cut, the stumps will send up sprouts very quickly. Select three sprouts that are equally spaced around the cut stump. Prune out the rest. Mulch around the plant area to cover the stump. Fertilize the sprout growth twice during the spring (April 1st and again mid-May) with a slow-release tree & shrub fertilizer, and within 3 years, the crape myrtle will again be a beautiful flowering plant.
Landscape & Ornamentals
SCALE INSECTS ON SHADE TREES AND SHRUBS
Clifford S. Sadof, Extension Entomologist
If you want to view as pdf, click here
Scale insects are common pests of shade trees and shrubs. More than 60 different kinds occur in Indiana, yet they are often overlooked or ignored until tree or shrub branches “mysteriously” start to die. Upon closer examination, these branches are likely to be covered with small bumps that are actually scale insects. They damage plants by sucking out plant juices.
From a damage standpoint, there are two types of scales, those that excrete a sugary liquid (honeydew), and those which do not. Honeydew is both a nuisance and a threat to plant health. Parked cars, walks, and benches beneath infested trees often become a sticky mess. The sugary liquid attracts ants, flies and wasps. Plants become unsightly when this liquid becomes a food for a black fungus called sooty mold. This mold can shade leaves and reduce plant growth.
Soft (Lecanium), kermes, and bark scales produce honeydew. These scales feed directly on plant parts that transport fluid and nutrients. Armored scales and pit scales do not produce honeydew. The armored scale’s straw-like mouth moves like a plumber’s snake to burst plant cells and feed on their contents. Pit scales are likely to do the same to the raised plant tissue that surrounds them.
GENERAL SCALE LIFE CYCLE
Scales spend most of their lives feeding on the same spot of a plant, and unable to walk. After the eggs hatch beneath females the young scales are called crawlers because they can walk at this time. Crawlers are small (<1/32”) and flattened, looking like dust on the plant surface. Scale infestations spread when crawlers walk or are blown by the wind to nearby plants or plant parts.
After an armored scale crawler begins to feed, it becomes very flat and covered with a clear wax shell. As it continues to grow, it remains beneath its waxy armor. This armor is difficult to penetrate with insecticides. Winged males crawl out from beneath their cover and mate with covered females who produce eggs. Females can produce about 100 eggs each.
Soft scales, are not covered by a waxy shell. Crawlers that hatch from eggs in mid-summer will usually crawl directly to leaves. They spend most of the summer feeding on leaves and excreting honeydew. They return to the twigs and bark where they spend the winter as settled second stage scales. They continue to grow on twigs in the spring until winged males mate with wingless females, who swell with up to 1,000 eggs.
Check plants for live scale infestations. Flip over suspicious looking bumps on twigs and branches with a thumbnail. Bark is usually intact beneath a scale. When a soft body is beneath a cover, the plant is likely to have live armored scales. When the bump itself can be squashed it is likely to be some other type of scale. When honeydew falls from a tree, leaves should be inspected for live soft scales or mealybugs.
Scales will thrive on trees that are under stress. Plant trees that are correctly suited to your landscape site. Slower growing plants with variegated leaves can require more care. Keep them watered. Carefully inspect newly purchased plants for scales. If a twig is unusually bumpy and leaves are somewhat yellowed it may have scales.
If a plant is normally a rapid grower, such as red-osier dogwood, or wintercreeper euonymus, consider cutting out heavily infested branches with a pruning shears to foster growth of uninfested shoots.
The stationary life of scales makes them easy targets for many natural enemies including lady beetles and microscopic wasps. These beneficial insects can keep the numbers of scales quite low in a natural woodland setting.
Conventional pesticides cannot penetrate a scale’s tough skin or waxy cover. Scale crawlers are killed by these pesticides when they are covered during foliar application, or as they walk along treated surfaces. To achieve maximum kill, pesticides in this group should be sprayed at the begining of the crawler period. Several obstacles make conventional materials undesirable for managing scales. Thorough coverage on tall trees is difficult and these materials do not kill scales after they settle. More importantly, these materials kill the scale’s natural enemies responsible for lasting control in the landscape.
Conserve natural enemies and kill armored scales on infested trees by using a biorational material like horticultural oil. This material works by smothering scales. Unlike other conventional pesticides, this material can kill armored scales after they have settled while the scale body is still somewhat clear. After it dries, it is not toxic to natural enemies that can fly back to an infested plant and feed on the remaining scales. When applied in winter at the dormant rate, horticultural oil kills scales that do not winter as eggs beneath the scale cover (See Table 1 for winter stage).
For persistent armored scale problems or those scales that winter in the egg stage (pine needle or oystershell scle) insect growth regulators can be a promising biorational alternative. Apply when scales are crawling or when the scales are still clear. These materials (e.g., Pyriproxifen) kill insects as they molt. Soil applied systemic insecticides are effective rescue treatments.
Control of soft scales an be achieved by targetting egg laying females with bifenthrin in the spring. A second spray of pyriproxifen targetting crawlers on leaves. The next year apply pyriproxifen sprays to leaves, if needed. To get the most from both natural enemies and pesticides, do the following:
- Identify the scale. Use picture sheet and Table 1.
- Inspect plants for live scales in early spring and for active crawlers in summer
- Use Table 2 to make decisions about pesticide use, and Table 3 to select a pesticide.
|Live armored scales on plant in spring when plant is dormant||Dormant season spray||Low impact on natural enemies, only for armored scales that do not winter as eggs|
|Live armored or pit scales on plant. Crawlers are present or have recently settled. Some new leaf discolor or branch dieback.
Live honeydew producing scales on plant. Crawlers have not settled and are actively walking. Some new leaf discolar or branch dieback.
|Summer biorational spray||Low to moderate impact on natural enemies. Some parasitic wasps active at this time.
Wash honeydew from cars, benches, and patios to manage honeydew nuisance.
|Live armored or pit scales on plant. Crawlers are present or have recently settled. New leaf discolor or branch dieback is severe.
Live honeydew producing scales on plant. Crawlers are present or have recently settled. Female scales are filling with eggs. New leaf discolor or branch dieback is severe.
|Summer conventional spray||High impact on natural enemies. Bifenthrin has been shown to kill soft scales as they fill with eggs.|
|1 Always conserve natural enemies when plant health and customer satisfaction can be maintained|
|Response||Insecticide||Use in 1 Gal.||Use in 100 Gal.||Comment|
|Dormant season sprays||Superior Oil (Sunspray, Volk Oil, Clean Crop, Scalecide, and others)||4 oz. or 7.5 Tbsp.||3 gal.||Not effective for soft scales and armored scales that winter as eggs. Use before spring growth when temperature is above 40°F. Do not follow with Captan, Morestan, Sevin, or Cygon for 1 month. Use ultrafine oil at low rate for soft maples. Can temporarily remove “bloom” from blue-needled conifers.|
|Ultra-fine Oil (Ultra-Fine, Sunspray 6E Plus, Rockland, and others)||4-5 oz. or 8-10 Tbsp||3-4 gal.|
|Summer biorational spray on actively crawling or recently settled scales||azadirachtin (Azatrol EC)||1.6 – 2.1 fl. oz.||160 – 218 fl. oz.||OMRI approved.|
|Insecticidal soap||2.5 oz. or 5 Tbsp.||2 gal.||On actively crawling scales, especially soft scales.|
|Buprofezin (Talus 70 DF)||0.14 oz.||14 oz||None.|
|pyriproxifen (Distance, Fulcrum)||1 1/3 tsp.
1/2 – 3/4 tsp
|21.5 fl. oz.
|Do not use more than twice a season. Does not kill adults.|
|spirotetramat (Kontos)||See label||See label||Nursery and greenhouse only.|
|Ultra-fine oil (Ultra-Fine, Sunspray 6E Plus, Rockland and others)||2.5 oz. or 5 Tbsp||2 gal.||Can temporarily remove “bloom” from blueneedled conifers. Drought stressed plants, dwarf Alberta spruce and soft maples can be sensitive. Do not follow with compounds as listed above. Avoid spraying on wet foliage.|
|Summer conventional spray (when crawlers are active)||bifenthrin (Talstar 10WP)||2 tsp.||2 1/4 cup||For licensed applicators only|
|cyfluthrin (Bayer Lawn & Garden)||2 Tbsp.||–||Homeowner use.|
|cyfluthrin (Tempo, Decathalon)||See label||See label||For licensed applicators only.|
|deltamethrin (Suspend SC)||3/4 – 1.5 tsp.||4-8 oz||For licensed applicators only.|
|lambda-cyhalothrin (Battle WP, Scimitar WP)||–||1.5-5 fl. oz.||For licensed applicators only.|
|malathion (Malathion 57EC)||4 tsp.||2 pts.||Injury may ocur on hickory, virburnum, lantana and elm.|
READ AND FOLLOW ALL LABEL INSTRUCTIONS. THIS INCLUDES DIRECTIONS FOR USE, PRECAUTIONARY STATEMENTS (HAZARDS TO HUMANS, DOMESTIC ANIMALS, AND ENDANGERED SPECIES), ENVIRONMENTAL HAZARDS, RATES OF APPLICATION, NUMBER OF APPLICATIONS, REENTRY INTERVALS, HARVEST RESTRICTIONS, STORAGE AND DISPOSAL, AND ANY SPECIFIC WARNINGS AND/OR PRECAUTIONS FOR SAFE HANDLING OF THE PESTICIDE.
It is the policy of the Purdue University Cooperative Extension Service that all persons have equal opportunity and access to its educational programs, services, activities, and facilities without regard to race, religion, color, sex, age, national origin or ancestry, marital status, parental status, sexual orientation, disability or status as a veteran. Purdue University is an Affirmative Action institution. This material may be available in alternative formats.
This work is supported in part by Extension Implementation Grant 2017-70006-27140/ IND011460G4-1013877 from the USDA National Institute of Food and Agriculture.
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Pine Needle Scale is probably the most common armored scale found on conifers in the United States and Canada. The white, oystershell-shaped scale can completely cover needles causing plant discoloration, needle yellowing, and even branch death. This scale settles on the needles of its host and forms a 1/8-inch white, oystershell-shaped cover. Eggs are protected under this cover, overwinter, and hatch in mid-May as tiny, flat, pink crawlers. These crawlers search for suitable needles on which to feed and once settled, begin to form their protective armor. Males molt into a prepupa for a week and then emerge as winged adults. Females, however, molt into wingless nymph-like adults. After mating, the females lay eggs under their protective shell. There are two generations each year.
Soft Scale Species
European elm scale
European elm scale males and females differ considerably in appearance and life cycle. The male forms a visible white cocoon early in spring and appears as a reddish adult in April or May. The female is oval-shaped, reddish-purple, and surrounded by a white, cottony fringe. The female deposits her eggs beneath herself on a twig of the host plant. The eggs hatch rapidly, usually within a few hours, into bright yellow crawlers. The crawlers then migrate to feeding sites along the midrib and other prominent veins on the underside of leaves. Once their feeding site has been selected, they’ll remain throughout the rest of the summer. In the fall, the crawlers return to a limb or trunk crevice where they hibernate as nymphs (immature females). Hibernating females often resemble small mealy bugs: oval-shaped and covered in short, white, waxy filaments. All native elms are susceptible to this scale. There is one generation per year.
Fletcher scale is common in the northern parts of the Midwest and Canada and is most frequently found on arborvitae (Thuja sp.) and yew (Taxus sp.). Pachysandra and Eastern red cedar are also susceptible. Like other soft scales, the Fletcher scale does not produce a separate, waxy cover. Instead it secretes a thin, transparent film, which does not totally cover the insect. The amber to reddish-brown nymph overwinters on a branch. The following spring, it feeds heavily as it grows into an adult. At maturity, a single female can produce 500-600 eggs in May, which hatch in late June or early July. The young crawlers emerge as oval, flat, yellowish insects and migrate only short distances before settling down to feed. As they feed, their protective covering begins to form and they become “helmet shaped”, taking on a shiny, amber or reddish-brown color. One generation per year is produced.
Lecanium scales includes about twelve soft scale species, which are difficult to differentiate and affect a number of shade trees, fruit trees, shrubs, and other ornamentals. The scales can vary in length from 1/8-to-1/2-inch, depending on species. Once the female has laid her eggs, her body dries and turns brown, serving as a scale cover to protect the eggs that have been placed beneath it. Eggs hatch beneath the females in late spring or early summer, and crawlers then migrate to leaves of the host plant to feed. Excessive amounts of honeydew can attract black sooty mold fungus. In late summer, immature females return to twigs to overwinter. There is one generation per year.
Magnolia scale is our largest soft scale insect, reaching ½ inch in length. This scale spends the winter on one-to-two-year-old twigs as tiny, dark-colored nymphs. In the spring, the scales begin to feed, mature, and change color. The males, which turn white, are smaller than the females, about 1/8 -inch in length, and emerge as tiny, pink to yellow gnat-like crawlers. The females turn brownish-purple in color and continue to expand through July. Magnolia scale eggs hatch internally, and the crawlers are born alive. Crawler emergence occurs in the fall. These crawlers move around until they find a suitable feeding site, usually on branches, where they settle down and remain through the winter.
Management of scale insects varies with the species. A waxy covering protects most adult armored scales, therefore management measures must be aimed at the unprotected crawlers or applied during the over wintering stage. Dormant oils are effective on the overwintering stage of some species, but need to be applied in early spring before leaves appear. They are less effective on armored scale species. Insecticidal soaps can be effective against the crawler stage but usually have no effect on the adult scale. Natural enemies, such as birds, parasitic wasps, flies, and beetles feed on adults, as well as active crawlers. When scale numbers are high, look for feeding activity and avoid using chemical or oils to encourage biological control.
Scale insects are very vulnerable in the crawler stage when the young are looking for a place to feed. Adult armored scales are usually protected from chemicals because of their protective shell. Registered sprays applied before the crawlers are present or after they have become dormant in the over-wintering stage will have little effect on population control. Timing of application is critical and will vary with species. Contact the Plant Clinic (630-719-2424 or [email protected]) for current recommendations. Use pesticides safely and wisely; read and follow label directions.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
SCALE INSECT (CRAWLER)/ TREATMENT TIME
Cottony maple scale / Mid- July,
Euonymus scale / Early June,
European elm scale / Early April
Fletcher scale / Early April and early July
Juniper scale / Early July
Lecanium scale / Mid-June
Magnolia scale / Late September or early spring when buds are opening
Oystershell scale / Early June and late August
Pine needle scale / Late May
Spruce bud scale / Mid-to-late July
Scale insects like mealybugs are among the most common parasites on indoor plants.
They can also be found in the garden at the end of spring, during summer, and in the fall.
Quick scale insect facts
Name – Coccidae, Pseudococcidae, Diaspididae
Lifespan – up to a year (dormant in winter)
Size – from 1/16th to 1/4th inch (1-5mm)
Danger to plant – fatal if infested
Contagious – often
Main treatment – fermented nettle or oil/soap/alcohol mix
The damage they inflict is variable, as are also the shapes they can take to hide.
Have a special question on scale insects?
- Ask it on the Nature & Garden forum and let an expert gardener quickly give you an answer.
Identifying scale insects
What do scale insects look like?
They usually have an oval shape, and a hard or soft shell made from special wax. The smallest are 1/16th of an inch (1mm) but some grow as large as 1/4th inch (5mm).
- In particular, mealybugs cover themselves in a white or pinkish velvet that gives the infestation a cottony appearance.
- It is also possible to find brown-shelled scale insects that look like small black hulls attached to stems and/or leaves.
There are thousands of species. Each looks slightly different. Some only attack a single plant species while others feed on many different plants.
Symptoms of scale infection
- Small spots on leaves, leaf joints, stems (especially in hidden areas)
- Plant growth slows
- Leaf buds and tips of branches die
- Heavily infested leaves die and fall off
- Leaves are gnarled, twisted, curled and stunted.
What is the usual season for scale insects to appear?
Scale insects and mealybugs don’t necessarily appear in any given season, but rather emerge when a set of conditions is met. Scale insects like heat and high humidity.
- They can appear all year round on indoor plants.
- They are common in spring, summer and until the beginning of fall in our gardens.
- They cannot develop in winter outdoors, but it is nonetheless recommended to clear infested areas.
What conditions help them to reproduce?
As we’ve just mentioned, scale insects love heat and humidity.
- They particularly like closed and humid environments, and most of all high temperatures.
- A high relative humidity indoors or regularly spraying water on leaves will accelerate their development.
- Also, the end of spring or the beginning or fall are seasons when temperatures are fairly high and outside air is moist.
Having scale insects on a plant will often lead to other plants being contaminated as well.
What are the risks for the plant?
Scale insects and mealybugs must be dealt with immediately because an onslaught can very well kill your plant.
- The insects feed on the plant’s sap and weaken it.
- This hinders proper plant development and growth, and can even lead to contamination by a black fungus called sooty mold.
- On fruit trees, especially citrus trees, scale insects and mealybugs can compromise the entire harvest. It can even kill a whole tree if they aren’t dealt with early enough.
Often, scale insects also carry diseases from one plant to another.
- This is particularly true for viral diseases and for fungal plant diseases.
Since some scale insects are specific to a single plant or plant family, they can be used as a form of biological control.
- This is currently being experimented to fight certain invasive plants.
How to fight scale insects?
Preventive treatment against scale insects and mealybugs
- For inside plants, treat against scale insects from fall until the beginning of spring, 3 to 4 times in all.
Curative treatment against scale insects and mealybugs
- As soon as mealybugs appear, destroy their protective shell with a rag dipped in beer, 90-proof alcohol or soapy water, and then treat with scale insect spray.
- If a major invasion is ongoing, burn or remove all infected branches.
A natural and organic scale insect spray
The best recipe is to mix following ingredients:
- 1 quart (1 liter) of water
- 1 teaspoon dishwashing soap
- 1 teaspoon vegetable oil
- and 1 teaspoon 90-proof alcohol.
Spray this solution once a day for 3 to 4 days.
- There are also white oil-based scale insect sprays that have proven their worth.
You can find them in specialized stores.
- A spray based on fermented stinging nettle tea is also very efficient in case of mealybug attacks.
It is also possible to slow their spread with a powerful blast of water or by cleaning leaves with a rag dipped in 90-proof alcohol and rinsing afterwards with water.
Take note: it is important to treat quickly, otherwise the plant will be entirely invaded and will die.
Different types of scale insects
Since treatments are very similar, usually only the general term “scale” is used. But there are actually three major families of scale insects.
- One of these is the famous mealybug. This is the white one that weaves tiny cottony cocoons near where leaves are attached to stems.
- The other two are called scale insects, with the following distinction: armored scale and soft scale, referring to whether the shell is hard or soft.
- Apart from these three families, there are 17 more within the scale insect “Coccoidea superfamily” which totals over 7,000 species.
- There are many more species which haven’t yet been described, though!
|Main families of Scale insects|
|Soft scale||Mealybugs||Armored scale|
|Over 1,000 described species||Nearly 2,000 described species||Nearly 2,400 described species|
Plants often attacked by scale insects and mealybugs
Most indoor plants such as ficus tree or hibiscus, orchid, rose, and even succulent-like plants such as Zamioculcas and Aloe vera can contract this pest.
Outdoor plants are also attacked, including citrus, lime tree (or basswood), hortensia, oleander, lilac, olive tree, mulberry tree, red currant, peach tree, and also, albeit less often, the apple tree.
- Note that citrus are particularly vulnerable to scale insects.
An efficient and timely treatment against scale insects and mealybugs will help contain spreading to other plants in your house or garden.
Smart tip about scale insects
A regular inspection of your houseplants and garden will help you catch infestations at the start. It’s very easy to contain them early on.
Credits for images shared to Nature & Garden (all edits by Gaspard Lorthiois):
Scale infesting branch by Scot Nelson under Public Domain
Brown soft wax scale by Kim & Forest Starr under © CC BY 4.0
Long-tailed mealybug on leaf by Robert Briggs under © CC BY-SA 4.0
Yellow hard wax scales by Katja Schulz under © CC BY 2.0
Controlling Citrus Scale – How To Treat Types Of Scale On Citrus Plants
So your citrus tree is dropping leaves, twigs and branches are dying back, and/or the fruit is stunted or distorted. These symptoms may indicate an infestation of citrus scale pests. Let’s find out more about citrus scale control.
What are Citrus Scale Pests?
Citrus scale pests are tiny insects that suck sap from the citrus tree and then produce honeydew. The honeydew is then feasted upon by ant colonies, further adding insult to injury.
The female adult scale is wingless and often has no legs while the adult male has one pair of wings and notable leg development. Male scale bugs on citrus look similar to a gnat and are generally not visible and they do not have mouth parts to feed. Male citrus scale pests also have a very short lifespan; sometimes only a few hours.
What are the Types of Scale on Citrus Plants?
There are two major types of scale on citrus plants: armored scales and soft scales.
- Armored scale – Female armored scales, from the family Diaspididae, insert their mouthparts and never move again – eating and reproducing in the same spot. Male armored scales are also immobile until maturity. This type of scale bugs on citrus exudes a protective coating made up of wax and cast skins of prior instars, which creates its armor. These citrus scale pests not only wreak the havoc mentioned above, but the armor will also remain on the plant or fruit long after the insect is dead, creating disfigured fruit. Types of scale on citrus plants in the armored scale family may include Black Parlatoria, Citrus Snow Scale, Florida Red Scale and Purple Scale.
- Soft scale – Soft scale bugs on citrus also form a protective coating via wax secretion, but it is not the hardened shell that the armored scale produces. Soft scales cannot be lifted from their shell and females roam the tree bark freely until eggs begin to form. The honeydew secreted by the soft scale attracts the sooty mold fungus, which in turn covers the citrus leaves preventing photosynthesis. Once dead, the soft scale will fall from the tree instead of remaining stuck as the armored scale. Types of scale on citrus plants in the soft scale group are Caribbean Black Scale and Cottony Cushion Scale.
Controlling Citrus Scale
Citrus scale control can be accomplished with the use of pesticides, biological control via the introduction of indigenous parasitic wasps (Metaphycus luteolus, M. stanleyi, M. nietneri, M. helvolus, and Coccophagus) and an organically approved petroleum spray, such as 440 oil Pure Spray Green. Neem oil is also effective. When utilizing any pesticide for controlling citrus scale, follow manufacturer’s instructions and spray the entire tree until it is dripping wet.
When controlling citrus scale, one may also need to eliminate the ant colonies, which thrive upon the honeydew extruded from the scale. Ant bait stations or a 3-4 inch band of “tanglefoot” around the trunk of the citrus will eliminate the ant marauders.
Citrus scale pests can spread rapidly as they are highly mobile and may also be transported on clothing or by birds. The best and first line of defense in controlling citrus scale is to buy certified nursery stock to prevent infestation from the get go.
How to Manage Pests
UC Pest Management Guidelines
| All citrus pests | All crops | About guidelines |
Brown Soft Scale
Scientific Name: Coccus hesperidum
(Reviewed 2/17, updated 2/17, corrected 1/19)
In this Guideline:
Description of the Pest
Female brown soft scales lay a few eggs at a time during summer. Eggs hatch almost immediately and crawlers start to feed. Young scales move around until they are about half grown. They have mottled, yellowish, rounded shells. The young molt twice and reach maturity on leaves or twigs; they rarely move onto fruit. There are three to five overlapping generations a year. Brown soft scale numbers are usually highest from mid-summer to early fall.
Citricola scale, another soft scale that is similar to brown soft scale, may be found infesting the same trees, but because brown soft scales have multiple overlapping generations, colonies of this pest contain multiple life stages.
Heavy feeding by the soft brown scale reduces tree vigor, kills twigs, and reduces yields. Sooty mold grows on excreted honeydew and may affect fruit grade. The honeydew also attracts ants, which interfere with the biological control of a number ofpests.
Management of brown soft scale focuses on preserving its natural enemies and controlling ants. Avoid the repeated use of broad-spectrum insecticides for other pests that disrupt the biological control of soft scales. Individual treatment of this scale is rarely necessary. If natural enemies do not control the scales, a spot treatment with an oil spray is usually sufficient. In areas with citricola scale or black scale, populations of brown soft scale may be beneficial if they are not too large because their generations overlap and provide parasites with susceptible life stages to attack throughout the year, thus allowing parasite numbers to increase to higher levels.
A complex of Metaphycus spp. parasites attack brown soft scale. The most common of these is M. angustifrons in Southern California. In addition, the lady beetles Rhyzobius (Lindorus) lophanthae, Chilocorus orbus, and C. cacti prey on brown soft scales. Ants will protect brown soft scale from parasitism and predation because they feed on the honeydew that soft scales produce. Maximizing parasitism by controlling and reducing ants is critical for brown soft scale control because pesticides are not very effective against this scale species.
Organically Acceptable Methods
Use biological control and organically approved oils, such PureSpray Green (NR 440), on an organically certified crop.
Monitoring and Treatment Decisions
Monitor brown soft scale from June through October when biological control may be disrupted Check the level of parasitism by looking for parasite exit holes and for developing parasites within the scale body. Organophosphate and carbamate insecticides are not very effective in controlling this pest. Usually, reduction of these insecticides in combinations with ant control will resolve the brown soft scale problem.
|Common name||Amount to use||REI‡||PHI‡|
|(Example trade name)||(type of coverage)**||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide’s properties and application timing. Always read the label of the product being used.|
|A.||NARROW RANGE OIL (UR92%)|
|(415, 440)||1.2–1.4% (TC)||See label||When dry|
|. . . or . . .|
|NARROW RANGE OIL (UR99%)|
|(415, 435, 440, 455)||1.2–1.4% (TC)||See label||When dry|
|RANGE OF ACTIVITY: Pests: broad (unprotected stages of insects and mites); Natural enemies: most|
|PERSISTENCE: Pests: short; Natural enemies: short|
|MODE OF ACTION: Contact including smothering effects.|
|COMMENTS: To avoid phytotoxicity problems, see timings for California red scale. Apply higher rate of narrow range oil in July or August only. Narrow range 440 spray oil (or higher) is preferable in the Central Valley during warmer months because of greater persistence, but risk of phytotoxicity increases unless using products with 99% unsulfonated residues (UR). Caution: Serious hazards are associated with oil sprays to green lemons because of phytotoxicity after sweating; check label for preharvest interval.|
|(Sevin XLR Plus)||3–5 qt/acre (TC)||See label||5|
|RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most|
|PERSISTENCE: Pests: long; Natural enemies: long|
|MODE-OF-ACTION GROUP NUMBER1: 1A|
|COMMENTS: For use on all varieties. May cause outbreaks of citrus red mite and citrus thrips.|
|(Sevin XLR Plus)||3–5 qt/acre (TC)||See label||5|
|RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most|
|PERSISTENCE: Pests: long; Natural enemies: long|
|MODE-OF-ACTION GROUP NUMBER1: 1A|
|COMMENTS: For use on all varieties. May cause outbreaks of citrus red mite and citrus thrips.|
|. . . PLUS . . .|
|NARROW RANGE OIL|
|(415)||0.5–1.4%||See label||When dry|
|RANGE OF ACTIVITY: Pests: broad (unprotected stages of insects and mites); Natural enemies: most|
|PERSISTENCE: Pests: short; Natural enemies: short|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|COMMENTS: For use on all varieties. Reducing the rate of carbaryl increases survival of natural enemies. Reducing the rate of the oil reduces the risk of phytotoxicity, especially in warmer growing areas of the state. Do not apply during bloom. May increase citrus red mite numbers. Caution: Serious hazards are associated with oil sprays to green lemons because of phytotoxicity after sweating; check label for preharvest interval for tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i.|
|(Malathion 8 Flowable)||7.5 pt/acre (TC)||72||7|
|RANGE OF ACTIVITY: Pests: broad (many insects); Natural enemies: most|
|PERSISTENCE: Pests: intermediate; Natural enemies: intermediate|
|MODE-OF-ACTION GROUPNUMBER1: 1B|
|COMMENTS: For use on all varieties.|
|**||LV – Low-volume uses 20 to 100 gal water/acre.|
|TC – Thorough coverage uses 750 to 2,000 gal water or more/acre, depending on tree size.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers (un = unknown or uncertain mode of action)are assigned by IRAC (Insecticide Resistance Action Committee).|
- Photos of citricola and brown soft scale
UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Insects, Mites, and Snails
Acknowledgments for contributions to Insect, Mites, and Snails:
J. Barcinas, E.S.I., Corona, CA
R. Dunn, Badger Farming Co., Exeter, CA
J. Gorden, Pest Management Associates, Exeter, CA
H. Griffiths, E.S.I., Corona, CA
D. Machlitt, Consulting Entomology Services, Moorpark, CA
C. Musgrove, retired entomologist, Riverside, CA
K. Olsen, S & J Ranch, Pinedale, CA
T. Roberts, E.S.I., Corona, CA
T. Shea, UC Cooperative Extension, Riverside County
J. Stewart, Pest Management Associates, Exeter, CA
P. Washburn, Washburn & Sons Citrus Pest Control, Riverside, CA
Top of page
Introduction – Synonymy – Distribution – Life Cycle and Biology – Description – Hosts – Economic Significance – Management – Selected References
The citrus snow scale, Unaspis citri Comstock, is an armored scale belonging to the family Diaspididae, which is comprised of 2400 species of armored scales. The family Diaspididae constitutes one of the most successful groups of plant-parasitic arthropods and includes some of the most damaging and unmanageable pests of perennial crops and ornamentals (Beardsley and Gonzalez 1975). Citrus snow scale is considered to be a pest on citrus species and has been reported on all varieties, with mandarins being the least favorable (Dickens 1968).
Figure 1. Immature male and adult female citrus snow scales, Unaspis citri Comstock. Photograph by J. L. Castner, UF/IFAS.
Citrus has been commercially farmed since the mid-1800s in Florida, but the citrus snow scale did not become a problem until grove expansion significantly increased in the 1960s. The freeze of 1962 was the main factor which increased the spread of citrus snow scale across the state because so many trees were damaged. In response to this devastating damage, many growers replaced the damaged trees with new saplings from nurseries. These new citrus trees were already infested with the citrus snow scale, and once transplanted within the new groves it spread to already established trees.
Although citrus snow scale may be commonly found in Florida citrus groves, populations today rarely result in economic damage. Several historical citrus surveys conducted by the Florida Department of Agricultural Services-Division of Plant Industry (FDACS-DPI) indicated that citrus snow scale is quite common within Florida (Hodges 2013). Current citrus sample submissions for FDACS-DPI focus on more recent invasive pests of concern such as the Asian citrus psyllid, Diaphorina citri, and the accompanying citrus greening disease transmitted by the psyllid (Stocks 2013).
Figure 2. Immature male citrus snow scales, Unaspis citri Comstock, covering a citrus branch. Photograph by Jeffrey W. Lotz, FDACS-Division of Plant Industry.
Synonymy (Back to Top)
Chionaspis citri Comstock
Howardia citri Comstock
Prontaspis citri Comstock
Dinaspis veitchi Green & Laing
Trichomytilus veitchi Green & Laing
Dinaspis annae Malenotti
Distribution (Back to Top)
Citrus snow scale is thought to have originated from Asia, but it is now found in many citrus-growing regions around the world, including:
Australasian: American Samoa, Australia, Queensland, Cook Islands, Federated States of Micronesia, Truk Islands, Fiji, Hawaiian Islands, Kiribati, New Caledonia, Niue, Papua New Guinea, Solomon Islands, Tonga, Vanuatu, Wallis and Futuna Islands, and Western Samoa.
Neotropical: Antigua and Barbuda, Argentina, Barbados, Bermuda, Bolivia, Brazil, Mato Grosso, Rio Grande do Sul, Rio de Janeiro, Sao Paulo, Chile, Colombia, Cuba, Dominican Republic, Ecuador, El Salvador, Grenada, Jamaica, Netherlands Antilles, Panama Canal Zone, Peru, Puerto Rico and Vieques Island, Saint Croix, Saint Lucia, Trinidad and Tobago, and Uruguay.
Palaearctic: Algeria, Armenia, Azores, and Egypt.
Figure 3. World distribution map of citrus snow scale, Unaspis citri Comstock. Map by Courtney R. Buckley, University of Florida, Entomology and Nematology Department.
Life Cycle and Biology (Back to Top)
Armored scales are distinguishable from soft scales by the removable cover or ‘armor’. As crawlers, the females of this family insert their long mouthparts into suitable hosts and never move again (Fasulo and Brooks 2010, Miller and Davidson 2005). The immature males become immobile once they begin feeding and remain immobile until they emerge as winged adults after the pupal stage.
Video 1. Armor from a false oleander scale, Pseudaulacaspis cockerelli Cooley, being removed to show adult female with eggs residing beneath the armor. Video by Courtney R Buckley, University of Florida.
Female scale insects have three distinct life stages: the egg, nymph, and adult stage. Male citrus snow scales have these stages in addition to a prepupal and pupal stage. The second instar male produces mature armor that protects its bright orange body underneath. The third instar or prepupal stage has distinct eye spots, a more pointed abdomen and faintly visible appendages (Dickens 1968). The fourth instar is referred to as the ‘pupal’ stage and is quiescent or inactive. During the pupal stage the male develops distinct antennae, legs and a posterior stylet. After the pupal stage, the winged adult male emerges and no longer possesses the waxy covering of the immature stages. The adults do not possess mouthparts and their main goal is to find females and mate.
In comparison, the female adults remain in a larval body form underneath their armor. Even though the females acquire mature sexual organs and can reproduce, their adult larval body form does not have the same characteristics as the adult male, with no antennae, legs or wings (Watson and Berger 1932). Several overlapping generations occur each year in Florida (ScaleNet 2013). Arias Reverón (1995) found development to be temperature dependent with an optimal range between 25 to 38°C.
Description (Back to Top)
Adults: The males and females look considerably different from each other.
Female: The females are 1.5 to 2.25 mm long and have oyster-shell shaped armor with a central longitudinal ridge (Futch, McCoy and Childers 2012). The shell is brownish-purple to black with a grey border (Fasulo and Brooks 2010). Because of this coloring, the females are difficult to see against tree bark. Beneath the armor, the female body ranges from creamy to bright orange in color (Dickens 1968).
Figure 4. Adult female citrus snow scale, Unaspis citri Comstock. Photograph by Lyle Buss, University of Florida.
Figure 5. Adult female citrus snow scale, Unaspis citri Comstock, showing larval body form underneath the armor. Photograph by Ian Stocks, FDACS-Division of Plant Industry.
Figure 6. An enlarged section of Figure 5 the yellow arrow points to the the median lobes of the adult female citrus snow scale Unaspis citri Comstock, which is one characteristic used for identification of this species. Photograph by Ian Stocks, FDACS-Division of Plant Industry.
Male: The winged adult male is bright orange in color, has long 10-segmented filiform antennae, four dark purple eye spots, and no mouthparts (Dickens 1968).
Figure 7. Immature male and adult female citrus snow scales, Unaspis citri Comstock. Photograph by S.H. Futch, C.W. McCoy and C.C. Childers, University of Florida.
Egg: The egg is ovoid, bright orange in color and approximately 0.30 mm in length. Eggs are laid singly underneath the protective covering of the adult female’s armor and usually hatch 30 minutes to three hours after being laid. Another egg is not laid until the previous one hatches. Over a two to three month period a female may lay up to 150 eggs.
Nymphs: This life stage begins after the scale insect emerges from the egg. The nymphs are called ‘crawlers’ because this is the dispersal stage. Nymphs are ovoid, bright yellow in color, with six legs, two five-segmented antennae and two laterally opposed eye spots (Dickens 1968). Crawlers are most abundant in autumn, but may occur throughout the year.
After the first molt, sex differentiation occurs and nymphs begin to form a waxy armor. Males form a white armor and females form a grey armor (Figure 7). The snow white armor color of the immature male is the reason for the species’ descriptive name. The armor has three longitudinal ridges; one prominent center ridge, and two marginal ridges (Dickens 1968). The immature male is about 1 mm in length. Females begin producing a white semi-transparent wax covering at the posterior end of the molt’s shed exoskeleton (also known as the exuvium).
Hosts (Back to Top)
In Florida, the citrus snow scale infests a wide range of citrus species, including:
- Lime (Citrus aurantifolia)
- Sour orange (Citrus aurantium)
- Lemon (Citrus limon)
- Pummelo (Citrus maxima)
- Navel orange (Citrus sinensis)
- Grapefruit (Citrus x paradisi)
In other parts of the world the insect also infests other hosts, including pineapple, soursop, jackfruit, peppers, mandarin, coconut, kumquats, rosemallows, banana, guava, and Spanish moss.
Economic Significance (Back to Top)
Commercial citrus production is currently a $9 billion industry, and employs thousands of Floridians (Florida Department of Citrus 2013). Florida’s 2011-2012 growing season produced ~338 boxes of fruit per acre, with just over 433,000 acres being farmed. Citrus is usually grown for either the fresh market or for juice production, with the fresh fruit returning a larger profit for the grower. The profit for 2011/2012 was ~$4400 per acre for fresh fruit, and ~$3500 per acre for juice production. When the fruit skin is damaged by citrus snow scale, the fruit is not sellable in a fresh market scenario.
The citrus snow scale feeds primarily on the trunk and tree limbs of older citrus trees, feeding on plant juices (Russo and Longo 2004). The scale may also be seen on leaves and fruit, if the population is large enough. Early symptoms include reduced tree vigor and fruit production. With prolonged high population densities, the normal growth of the bark is prevented and the toughened bark is not able to expand and may become split or ‘hidebound’ (Russo and Longo 2004). This split in the bark may allow access for other insect borers and pathogens to invade the damaged tree.
Figure 8. Citrus branch with citrus snow scale, Unaspis citri Comstock, crawlers and adult males. Photograph by Michael Rogers, University of Florida.
Figure 9. Bark splitting caused by severe infestation of citrus snow scale, Unaspis citri Comstock. Photograph by Michael Rogers, University of Florida.
Management (Back to Top)
Biological control: The parasitic wasp Aphytis lingnanensis Compere is effective in helping to control citrus snow scale if it is already established within the grove (Fasulo and Brooks 2010). The introduced ladybird beetle Chilocorus circumdatus Gyllenhal has also shown to be a successful biological control agent (Rogers 2012).
Cultural control: As the citrus snow scale crawler is the mobile life stage, preventing the distribution of this stage is crucial. Crawlers can be moved by wind, farming equipment and workers in the field. New research has also shown that they may phoretically disperse by attaching themselves to other species, such as pollinating insects (Magsig-Castillo et al. 2010). Farming equipment, such as pruners and fruit shakers, should be cleaned before moving to new groves, and workers should brush their clothing before entering a new area of a grove.
Chemical control: The effectiveness of biological control agents may be reduced with the application of chemical control agents used in the citrus groves to combat other insect pests. For example, sulfur applications used to control fungal pathogens can adversely affect the parasitic wasp Aphytis lingnanensis (Fasulo and Brooks 2010).
Scouting is an important tool when deciding if an insecticide should be applied. To determine if citrus snow scale crawlers are active, a tree trunk section can be brushed clean with a handheld brush and the same area checked a week later. The need for an application can become evident when high populations of crawlers are seen on patches of bark that had been brushed clean the previous week (Browning et al. 2012).
Figure 10. Citrus tree covered in citrus snow scales, Unaspis citri Comstock. The arrow shows an area where crawlers have been brushed away to monitor crawler activity. Photograph by Michael Rogers, University of Florida.
An insecticide can be applied to manage this pest, but the concentration of the spray and adequate coverage of the tree limbs is important in achieving control. Citrus snow scales will layer themselves on top of each other, seven to eight individuals deep, so a chemical control agent may need to be applied multiple times in order for the layers to slough off (Fasulo and Brooks 2010).
Florida Citrus Pest Management Guide for soft-bodied insects
Selected References (Back to Top)
- Anonymous. 2004. Scales. Florida Grower. August: 12-13.
- Arias Reverón, JM. 1995. Population studies on the citrus snow scale, Unaspis citri (Comstock). University of Florida Ph.D. Dissertation.
- Beardsley Jr JW, Gonzalez RH. 1975. The biology and ecology of armored scales. Annual Review of Entomology 20: 47-73.
- Brooks RF, Vitelli MA. 1976. An easily erected tree cage for introducing insect parasites. The Florida Entomologist 59: 1: 67-70.
- BugGuide (2003). Citrus snow scale. (29 June 2017)
- Culik MP, Martins DS, Ventura JA, Wolff VS. 2008. Diaspididae (Hemiptera: Coccoidea) of Espírito Santo, Brazil. Journal of Insect Science. 8: 17: 1-6.
- Dekle GW. 1976. Florida armored scale insects. Arthropods of Florida and neighboring land areas, Volume 3. Florida Department of Agriculture and Consumer Services, Division of Plant Industry. 152.
- Dickens TH. 1968. Life history of citrus snow scale Unaspis citri (Comstock). Master of Science Thesis, University of Florida.
- Fasulo TR, Brooks RF. (February 2010). Scale pest of Florida citrus. EDIS. (19 May 2013.)
- Florida Department of Citrus. 2013. Florida Citrus. (29 June 2017)
- Futch SH, McCoy CW, Childers CC. 2012. A guide to scale insect identification. Horticultural Sciences Department, Institute of Food and Agricultural Sciences, University of Florida HS-817.
- Hatton Jr TT, Reeder WP. 1963. Effects of the December 1962 freeze on lula and taylor avocado fruits. Proceedings of the Florida State Horticulture Society. 76: 370-374.
- Hodges G. 2013. Bureau Chief of Entomology, Nematology and Plant Pathology at Florida Department of Agricultural and Consumer Services, Division of Plant Industry. Personal Interview.
- Magsig-Castillo J, Morse JG, Walker GP, Bi JL, Rugman-Jones PF, Stouthamer R. 2010. Phoretic dispersal of armored scale crawlers (Hemiptera:Diaspididae). Journal of Economic Entomology 103: 4: 1172-1179.
- Miller DR, Davidson JA. 2005. Armored scale insect pests of trees and shrubs (Hemiptera: Disaspididae). Comstock Publishing Associates. 392-395.
- Parsons L. 2007. Freeze forecast. Florida Grower. p. 32.
- Plantwise, CABI (2013). Knowledge Bank: citrus snow scale. (29 June 2017)
- Robinson FA. 1964. The effects of the December 1962 freeze on citrus honey production in Florida. The Florida Entomologist. 47: 1: 55-56.
- Rogers J. 2000. Book Review: A history of Florida citrus freezes. Bulletin of the American Meteorological Society. 81: 3: 590-591.
- Rogers M. 2012. Citrus pest spotlight: Citrus snow scale. Citrus Industry May p. 12.
- Russo A, Longo S. 2004. Diagnostic protocols for regulated pests: Unaspis citri. European and Mediterranean Plant Protection Organization. Bulletin 34: 299-301.
- ScaleNet: A database of the scale insects of the world. (June 2017). Unaspis citri (Comstock). (29 June 2017)
- Soares AO, Elias RB, Schanderl H. 1997. Encarsia citrina (Crawford) (Hymenoptera, Aphelinidae), a parasitoid of Unaspis citri (Comstock) and Lepidosaphes beckii (Newman) (Homoptera, Diaspididae) in citrus orchards of S達o Miguel island (Azores). Bol. San. Veg. Plagas. 23: 449-456.
- Stansley PA, Rogers ME. December 1995. Revised September 2013 and April 2016. 2016 Florida Citrus Pest Management Guide: Ch. 11 soft-bodied insects attacking foliage and fruit. EDIS. (29 June 2017).
- Stocks I. 2013. Biologist Scientist IV, Florida Department of Agricultural and Consumer Services, Division of Plant Industry. Personal Interview.
- Stout, RG. 1964. Florida citrus fruit and tree losses from the December 1962 freeze. Department of Agricultural Economics, University of Florida. Agricultural Economics Mimeo Report EC 64-7.
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