Rust mites on citrus

Q: Our 8-year-old son was 5 when he stuck a seed from his Clementine orange into a flower pot with old potting soil in it. We all kidded him, telling him that it would never grow. And just like in the book about the little boy with the carrot seed, he took care to water it from time to time, and endured our kidding. A couple of months later, he announced that he had a clementine tree. Sure enough, he did. It has grown very well, sitting out on our patio during warm months and coming inside to the living room window for winter. We have repotted twice, as it has outgrown pots, as well as our expectations. It has always had lustrous deep green, slightly waxy leaves, and the typical sharp thorns.

Late this summer, the leaves on one small branch become mottled turning yellow and dry. We thought that plant had gotten too dry, and we watered it well over the next couple of weeks. This fall, we moved it into the living room. Over the past couple of months, all the leaves have progressed to the same mottled appearance, some are pale, others more yellow, and all are drying. In addition, in the past couple of weeks, there are tiny webs at some of the junctions of branches and leaves, with very small bugs in the webs.

Any ideas about the cause?

A: Your son will surely become a great gardener as a result of his success! However, teach him that insects and mites love citrus trees just as much as we do.

This is a classic case of spider mite infestation. These fast-breeding creatures suck sap from the leaves and make tiny webs for protection.

A big spider mite population can cause complete leaf drop. Try donning a plastic glove and an old cotton sock on one hand and spraying it with . Wipe the backside of each leaf gently to coat with oil and suffocate the mites. This method is least likely to hurt beneficial insects that might be present.

You could also use a trigger-sprayer filled with as long as you spray underneath all leaves.

spider mite on clementine

spider mite on clementine

spider mite on clementine

Tags For This Article: citrus, clementine, lemon, lime, Meyer, orange, spider mite

Many species of the spider mite (family: Tetranychidae), so common in North America, attack both indoor and outdoor plants. They can be especially destructive in greenhouses.

Spider mites are not true insects, but are classed as a type of arachnid, relatives of spiders, ticks and scorpions. Adults are reddish brown or pale in color, oval-shaped and very small (1/50 inch long) – about the size of the period at the end of this sentence. Immature stages resemble the adults except only smaller.

Mites live in colonies, mostly on the underside of leaves, and feed by piercing leaf tissue and sucking up the plant fluids. Feeding marks show up as light dots on the leaves. As feeding continues, the leaves turn yellow and may dry up and drop off.

Spider mites are most common in hot, dry conditions, especially where their natural enemies have been killed off by insecticide use. Some of the many species common in North America are predators of the plant-feeding mites, which make up the vast majority. They are also very prolific, which is why heavy infestations often build up unnoticed before plants begin to show damage.

Large populations are often accompanied by fine webbing. Host plants are many and include strawberries, melons, beans, tomatoes, eggplant, ornamental flowers, trees and most houseplants.

Life Cycle

Most mite species overwinter as eggs on the leaves and bark of host plants. In early spring, as temperatures warm, tiny six-legged larvae begin hatching and feed for a few days before seeking shelter where they molt into the first nymphal stage. Nymphs have eight-legs and pass through two more molts before becoming mature adults.

After mating, females continuously produce as many as 300 eggs over a couple of weeks. Hot, dry weather favors rapid development of these pests. During such conditions the time it takes to pass from egg to adult may occur in as little as 5 days. There are several overlapping generations per year.

Note: Spider mites are wind surfers. They disperse over wide areas riding their webbing on the breezes. Careful containment and disposal of infested plants is crucial.

Damage

Spider mites, almost too small to be seen, pass into our gardens without notice. No matter how few, each survives by sucking material from plant cells. Large infestations cause visible damage. Leaves first show patterns of tiny spots or stipplings. They may change color, curl and fall off. The mites activity is visible in the tight webs that are formed under leaves and along stems.

The University of California Agricultural and Natural Resources division’s Integrated Pest Management website says the following about the damage mites cause:

  • On annual vegetable crops — such as squash, melons and watermelons — loss of leaves can have a significant impact on yield and lead to sunburning.
  • On crops such as sugar peas and beans, where pods are attacked, spider mites can cause direct damage.
  • On ornamentals, mites are primarily an aesthetic concern, but they can kill plants if populations become very high on annual plants. Spider mites are also important pests of field-grown roses.

How to Control

Chemical pesticide use actually encourages the spread of spider mites by killing the beneficial insects that prey on them. Mites are also known to develop quick resistance to various pesticides. For these reasons, it’s important to control mites with effective natural and organic methods.

  1. Prune leaves, stems and other infested parts of plants well past any webbing and discard in trash (and not in compost piles). Don’t be hesitant to pull entire plants to prevent the mites spreading to its neighbors.
  2. Use the Bug Blaster to wash plants with a strong stream of water and reduce pest numbers.
  3. Commercially available beneficial insects, such as ladybugs, lacewing and predatory mites are important natural enemies. For best results, make releases when pest levels are low to medium.
  4. Nuke Em, a relatively new organic insecticide containing food-grade ingredients, works fast and kills most indoor gardening pests at the egg, larvae or adult stage. Best of all, it does this without leaving a residue on the leaves that can impact flavor.
  5. BotaniGard ES is a highly effective biological insecticide containing Beauveria bassiana, an entomopathogenic fungus that attacks a long-list of troublesome crop pests – even resistant strains! Weekly applications can prevent insect population explosions and provide protection equal to or better than conventional chemical pesticides.
  6. Mix Pure Neem Oil with Coco-Wet and apply every 3-5 days to kill pest eggs indoors and interrupt the reproductive cycle. Make sure to spray all plant parts, including the undersides of leaves. Do NOT apply when temperatures exceed 90˚F and wait at last six hours before turning lights on.
  7. If populations are high, use a least-toxic, short-lived pesticide (Take Down Spray, Doktor Doom Foggers) to reduce infestations, then release predatory mites to maintain control.
  8. Insecticidal soap or botanical insecticides can be used to spot treat heavily infested areas.
  9. On fruit trees, horticultural oil should be applied early in the season or late in the fall to destroy overwintering eggs.
  10. Dust on leaves, branches and fruit encourages mites. A mid-season hosing (or two!) to remove dust from trees is a worthwhile preventative.
  11. Water stress makes both trees and garden plants more susceptible to mite infestations. Make sure your plants are properly watered.

Tip: Management strategies must take into account the fast development time of this pest, especially during warm weather when eggs are laid continuously. Just targeting the adults will do little good if eggs and larvae survive. Repeat treatments are almost always necessary. The use of leaf shines and washes helps control and prevent further infestations.

Citrus Rust Mites

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We have seen quite a few citrus fruit samples at the Extension Office in which the fruit is brown on the outer rind, primarily on one side but sometimes all over. This damage is caused by citrus rust mites. These small arthropods chew on the surface of the rind causing the scarring/browning. Although you may notice the browning now, when you are harvesting the fruit, the damage probably dates back to summer and early fall when the mites were more active. The mites avoid the sunny side of the fruit, thus the tendency for the browning to be mostly on one side, unless the fruit was shaded on all sides by the foliage.

Since the rind damage is superficial, doing nothing is an option because the fruit’s eating quality is not significantly affected. Mites are most active in the hottest months of the year and therefore treatments at this time are not recommended. If you wish to prevent some of this damage next year, apply horticultural oil sprays in July, August and September. Make sure and get good coverage of all foliage and fruit surfaces for best results. Oil should be mixed at a 2% concentration in water and the spray tank should be shaken often to prevent separation of the oil and water mixture. Avoid applying oil to trees that are drought stressed or when temperatures are above 95 degrees F.

By Lukasz L. Stelinski, Jawwad A. Qureshi and Philip A. Stansly

Florida citrus is host to four main groups of mites categorized by differences in lineage and morphology. However, only rust mites, spider mites and broad mites historically cause economic damage to commercially grown citrus in the state. The fourth group, referred to as false spider mites, are vectors elsewhere of the causal virus of the disease leprosis, but are currently not classified as causing serious economic concern in Florida.

Figure 1. Rust mites can cause “sharkskin” damage in citrus. (Photos by Jamie Burrow, University of Florida)

RUST MITES
Rust mites are the most important of the economically relevant groups and include the citrus rust mite and the pink citrus rust mite. Both species feed on fruit and leaves, which causes damage to epidermal cells. Rust mite injury has a known history of reducing fruit grade in Florida citrus. Severe infestations can cause significant fruit water loss and thereby contribute to fruit drop.

Figure 2. This sweet orange fruit shows severe “bronzing” damage from rust mites.

Early-season damage is characterized by destroyed epidermal cells, which fracture as the fruit enlarges, causing a rough form of russeting known as “sharkskin” (Figure 1). Mature fruit maintains intact epidermal cells and a wax layer to take on a polished look when damaged by rust mites, referred to as “bronzing” (Figure 2). Leaf damage caused by mites appears in two distinct forms visually: 1) a dull, bronze-like and glossy appearance and 2) patches of yellowish discoloration where wounded epidermal cells have released ethylene, which causes the change in color from green to yellow. Sunburst tangerine is especially prone to rust mite damage.

Figure 3. Small, chlorotic leaf spots caused by spider mite feeding are referred to as “stippling” damage.

SPIDER MITES
The other two taxonomic groups of concern are the spider mites and broad mites. In Florida, Texas citrus mite and citrus red mites are the species of economic concern. Spider mites are found primarily on mature leaves. Unlike rust mites, spider mites feed underneath the epidermis in the palisade cell layer. Spider mite feeding causes mesophyll tissue to collapse, resulting in small chlorotic spots called “stippling” (Figure 3). Severe damage can cause leaf desiccation, termed “firing,” which can be exacerbated by cold, dry winds.

Spider mites are typically kept in check by a broad range of biological control agents, including predatory mite species and pathogens. These beneficial organisms are hindered by dry conditions, which favor spider mite populations. Petroleum oil kills eggs. Application of miticides may only be necessary if populations rise to the point where 10 mobile mites are observed per leaf between September and May.

BROAD MITES
Broad mites are particularly important in greenhouse settings and on lemons and limes grown in the field. Broad mites primarily feed on the youngest and most tender leaf flush tissues. New leaf growth is distorted by a toxicant in the saliva that is injected by these mites. Favorable environmental conditions, which include warm temperatures, high humidity and shade, can contribute to increases in broad mite populations.

SEASONALITY
Monitoring for rust mites should begin in early April and continue every two to three weeks throughout the season. In general, rust mite populations begin to increase in May and then decline in late August. While mite populations are historically highest from May to July with a second smaller peak in the fall, significant increases in late October and early November have been seen in recent years. Thus, monitoring for mites should not be neglected during the fall peak.

UNINTENTIONAL FLARING OF MITE POPULATIONS
Natural enemies of rust mites include predaceous mites, small inconspicuous ladybeetles and the fungus Hirsutella thompsonii. Current control measures for Asian citrus psyllid and citrus canker could impact rust mite control programs. Applications of broad-spectrum insecticides, especially pyrethroids like Danitol, Baythroid or Mustang, can cause rust mite populations to flare.

Multiple copper sprays to control canker or fungal diseases of citrus may also increase rust mite populations. Therefore, more intense monitoring and/or additional sprays may be necessary for rust mite management after pyrethroid or copper applications. Use of a summer petroleum oil spray in place of copper for greasy spot control could reduce the need for a later miticide treatment; however, fungal pathogen management will, in part, dictate these decisions.

Florida citrus was affected by destructive Hurricane Irma in the fall of 2017. Mite populations in Central Florida immediately following Hurricane Irma and into November 2017 did not appear drastically different from the previous year. After the destructive hurricanes of 2005, mite populations were also not immediately affected, but did increase where they were not monitored and controlled soon after the storms of 2004, causing bronzing of fruit (Albrigo et al., 2005). Higher than normal rainfall and humidity in the spring of 2005 were thought to have contributed to an early rise in rust mite populations, rather than any possible lingering effects of the storms (Albrigo et al., 2005).

Perhaps a more important factor affecting rust mite populations locally in 2017 was pyrethroid sprays for psyllids. Psyllid populations increase rapidly following mass defoliation events as trees produce new flush. It is imperative to kill off adult psyllids prior to a new mass flushing cycle before the populations can take off by depositing eggs on highly abundant new flush. Broad-spectrum insecticides, such as pyrethroids, can be some of the more effective tools to knock back adult populations prior to a new flush. In areas where these pesticides were used heavily following Irma, one would have expected to see flare-ups of rust mite populations. Mite populations may require additional monitoring following intense broad-spectrum sprays for psyllids to determine if a miticide is warranted.

MANAGEMENT
Selection of an appropriate miticide depends on the intended target of control. With the current emphasis on psyllid and citrus leafminer control, it would be wise to choose a miticide that may also have some activity against one of these two pests, such as diflubenzuron (Micromite 80 WGS) or spirotetramat (Movento). With the exception of petroleum oil, no miticide should be applied more than once per season to avoid development of resistance. For more detailed information on the latest miticides and recommended rates, see Chapter 10 of the Florida Citrus Pest Management Guide (http://edis.ifas.ufl.edu/cg002) on mite control.

Fruit appearance is a much greater priority in fresh-market fruit and thus will require more intensive management to prevent mite damage. Fruit growth and abscission will not be affected until 50 to 75 percent of the fruit surface is injured by rust mites. So there is less need for chemical control where fruit is destined for processing. Groves producing fresh-market fruit may be treated with miticides three to four times annually, typically during April, June, August and October, depending upon level of pest population. However, groves producing fruit for the processing market may receive zero to two applications for rust mites annually.

Lukasz L. Stelinski is an associate professor with the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Citrus Research and Education Center in Lake Alfred. Jawwad A. Qureshi is an assistant professor at the UF/IFAS Indian River Research and Education Center in Fort Pierce. Philip A. Stansly is a professor at the UF/IFAS Southwest Florida Research and Education Center in Immokalee.

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