Corn earworm control mineral oil

Corn Earworm Management in Sweet Corn

ENTFACT-318: Corn Earworm Management in Sweet Corn | Download PDF

Ric Bessin, Extension Entomologist
University of Kentucky College of Agriculture

Corn earworm is potentially the greatest threat to sweet corn production in Kentucky. Because it feeds directly on the market product, is difficult to control, and is common in high numbers at the end of the season, most insecticides used on sweet corn target this pest. Once earworm has become established within the ear, control is impossible. Earworms spend a relatively short period of their life feeding in a site that can receive an adequate insecticide application. A effective program, especially on late season corn, is necessary to ensure that damaged ears are kept to a minimum.

Currently, the primary insecticides used for corn earworm control in sweet corn belong to the pyrethroid class. There is growing concern that corn earworm in some regions of the Midwest has developed resistance to this class of insecticides. Some field failures have been reported.

Corn Earworm Larva

Earworms are variable in color, but they have a brown head without markings and numerous microscopic spines covering their body. Corn earworms are moderately hairy larvae that vary from yellow, to green, to red to brownish black. They may be found feeding in the ear tips following silking. The larvae are cannibalistic, rarely is their more than one per ear or whorl.

The moth has a wing span of 1 to 1-1/2 inches. The front wings of the male are usually a light yellowish olive; those of the female are yellowish brown to pinkish brown. Each forewing has a dark spot in the center. The dome-shaped-egg is usually white when first laid but develops a reddish-brown band before hatching.

Biology

Corn earworms overwinter as pupae in underground cells. Some adults from these pupae begin to emerge as early as late March, others may not appear until August. There are generally four generations each year, however, overlap is great and adult moths that can lay eggs may be present in significant numbers throughout most of the growing season.

Female moths search out fresh silks on which to lay single eggs. Following hatch, the small larvae often eat the egg shell before beginning to feed on the silk. Within a day or so they move down the silks to the tip of the ear. Corn earworms generally complete their development in 14 to 16 days. Full grown worms leave the ear and pupate in the soil. The new adult will be active in another 10 to 14 days. Damage to the kernels in the tip make the ear more attractive to sap beetles.

Cultural Controls

There are a number of approaches that growers can implement to control corn earworm besides just spraying insecticides. This includes selecting the best varieties and planting dates. Varietal selection is very important. Corn hybrids having a long, tight fitting shuck appear to suffer less damage than those with loose shucks. The key factor determining the relative risk of corn earworm attack is planting date. Early and midseason we typically have fewer corn earworm moths to lay eggs on the silks. But late-planted corn will be late-silking corn, and many more moths are searching for egg-laying sites at this time. In addition, late in the season, the field corn crop is producing high numbers of corn earworm moths and is not attractive to the moths for egg laying.

Spray Coverage

Spray solution should be driven deep into the silks to be of maximum benefit. The center third of the plant is the only zone that needs to be protected. Ground application has always been shown to be superior to aerial application, particularly when using drop nozzles on each side of the row directed towards the ears. A spray pressure of 30 psi or higher is recommended.

Preventive Management

A preventive program against corn earworms may begin when 10% of the ears are silked. Repeated sprays at three to five day intervals until 90% of the silks have wilted should give a high percentage of worm free ears during early and midseason. Control is more difficult late in the season. Even shortening spray intervals may produce only 90% clean ears.

An IPM Approach

Since moth intensity varies considerably during the season, it makes good sense to monitor adult activity and adjust the need for sprays accordingly. Pheromone traps need to be examined twice a week for corn earworms beginning in early June to determine moth activity and the need to spray. Special attention should be given to late planted fields and fields with green silks. Moths should be removed from traps, counted, destroyed, and removed from the field during each visit.

Economic Threshold: When tassels emerge and silks are still green, numbers of corn earworm moths captured in pheromone traps will determine the frequency of insecticide applications.

Bt Sweet Corn

Similar to Bt field corn, Bt sweet corn has been genetically engineered with a protein that kills certain caterpillars. It is very selective, safe to beneficial insects, and has been approved for commercial production. However, it is not “bullet proof” when planted late there can be considerable earworm damage to Bt sweet corn. Bt sweet corn still needs to be monitored and in some situations may need to be sprayed to prevent infestation with corn earworm.

Issued: 11/06
Revised: 11/06

CAUTION! Pesticide recommendations in this publication are registered for use in Kentucky, USA ONLY! The use of some products may not be legal in your state or country. Please check with your local county agent or regulatory official before using any pesticide mentioned in this publication.

Of course, ALWAYS READ AND FOLLOW LABEL DIRECTIONS FOR SAFE USE OF ANY PESTICIDE!

Photos courtesy Ric Bessin, University of Kentucky Entomology

Jan 15, 2013Strategies to control sweet corn pests

Sweet corn producers must rely on timely pest monitoring and effective insecticide sprays to minimize ear damage by corn earworm, European corn borer and fall armyworm. The fresh-market can tolerate only minimal damage to the ears. Pyrethroids are the popular choice for worm control, but efficacy has declined in recent years due to resistance in corn earworm populations.

Based on insecticide trials in the mid-Atlantic area, pyrethroids have lost about one-third of their efficacy since 2002, although efficacy varies from year to year depending on the relative susceptibility of migrant moths from the south. Spray mixtures of Lannate or Larvin plus a pyrethroid have become a common practice to circumvent a potential resistance problem. Also, rotations and mixtures with the newer insecticides such as Coragen, Belt and Radiant, as well as some premix products (VoliamXpress, Hero EC) are recommended options and provide excellent control.

However, despite the insecticide used, timing the first spray at the first signs of silking followed by a prescribed schedule based on moth pressure and adequate spray coverage of the ear zone are critical steps to achieve effective control. For example, most corn earworm eggs are oviposited directly on sweet corn silks. Once larvae hatch, they quickly move down the silk channel and begin feeding on the ear tip, where they are protected from insecticidal sprays. It is thus important to maintain a residual level of insecticide on freshly emerged silk tissue at all times.

As an alternative, the most potent bioinsecticide for sweet corn insect control is provided by transgenic hybrids expressing one or more insect-active toxins from the bacterium, Bacillus thuringiensis (Bt sweet corn). Attribute Insect Protected hybrids from Syngenta Seeds have been commercially available since 1996. Acreage of Bt sweet corn has increased significantly in recent years with the introduction of improved Rogers Brand fresh-market hybrids. The availability of seed in 25,000 units has also made it easier for the small producer to use the Bt technology.

Attribute Bt hybrids express a single Cry1Ab toxin that is highly effective against European corn borer, but this toxin alone does not provide 100 percent control of corn earworm or fall armyworm. Based on multiple-year field trials in Maryland, Attribute hybrids eliminated all whorl treatments and reduced silk sprays by at least four applications. Although these hybrids provide excellent protection against the caterpillar complex, two and sometimes three supplemental insecticide sprays may be needed to ensure fresh-market-quality ears, especially during high moth activity.

The development and commercialization of new Bt field corn events by Syngenta Seeds has isolated a novel vegetative insecticidal protein from B. thuringiensis – Vip3A. This toxin is highly effective against a range of agriculturally important lepidopteran larvae including black cutworm, fall armyworm, corn earworm and western bean cutworm. In field studies conducted in Maryland and Minnesota, hybrids expressing the Vip3A trait and pyramided with the Cry1Ab Bt protein were compared to near isogenic non-Bt hybrids.

Over all years and locations, the non-Bt hybrids, without insecticide protection, averaged between 43 and 100 percent ears infested with a range of 0.24 to 1.74 H. zealarvae per ear. By comparison, no larvae were found in the pyramided Vip3A x Cry1Ab hybrids, indicating virtually 100 percent of all lepidopteran larvae. Syngenta Seeds plans to commercialize its next generation Attribute II technology in sweet corn in 2013.

Monsanto’s Seminis Seeds also has developed and is marketing pyramided Bt sweet corn seed under the Performance Series trade name. Bt hybrids available are Temptation II, Obsession II and Passion II, and these hybrids express three insecticidal proteins: Cry1A.105 and Cry2Ab to control lepidopteran larvae, and Cry3Bb1 to control rootworms; and herbicide-tolerant traits. Maryland studies have shown that the Performance Series hybrids provide virtually 100 percent control of corn borers and fall armyworms, and more than 95 percent control of corn earworms, of which all surviving larvae were small and caused very minor injury on the ear tip.

The Attribute II and Performance Series sweet corn hybrids ideally fit the IPM philosophy by combining host plant resistance traits, different modes of action to prevent resistance and a reduced-risk bioinsecticide, and by providing an environmental safe option to conserve beneficial insects. Clearly, the Bt technology can significantly reduce pesticide use and control costs, but control efficacy may vary under adverse growing conditions or very high insect pressure. And finally, the Bt hybrids will not be insect pest free, so regular monitoring of insect pests not affected by the expressed proteins will be essential for successful IPM.

By Galen P. Dively, University of Maryland

Tags: Crop Management, Labor, Marketing, Sweet corn

Organic Corn Earworm Control

Corn Earworm Reproduction

Corn earworms overwinter as pupae from Zone 6 southward. When adults emerge in spring, some are transported northward on south winds. A few days after emergence, adult moths begin laying eggs on host plants. Eggs hatch in three to four days, and the larvae begin feeding. The larval stage lasts about two weeks in warm summer weather, or more than a month in spring and fall. This allows time for one or two generations annually in Northern regions, compared to up to seven generations in the far South. After the larvae attain full growth, they burrow into the soil to pupate — a state that can be as brief as two weeks in the summer or as lengthy as six months during winter hibernation.

Natural Enemies of Corn Earworms

Many natural controls are at work to offset the phenomenal reproductive capacity of corn earworms. Birds, bats, toads, spiders, and numerous predatory insects inflict a heavy toll on the earworm population. The minute trichogramma wasp is an important parasite, and the earworm helps control itself through cannibalism, especially in severely infested ears.

Organic Controls for Corn Earworms

Many gardeners get good organic control of corn earworms by using eyedroppers or small squirt bottles to place a few drops of canola or olive oil in ear tips, as soon as the silks show signs of drying. You also can use a standard solution of Bt (Bacillus thuringiensis) or spinosad in the same way.

Frequently early corn is more seriously damaged by this sweet corn pest than midseason corn, mostly because robust midseason varieties have tight ear tips that prevent earworms from getting inside. Experiment with varieties and planting dates to find the best combination for escaping earworm damage in your area.

Corn earworms are too high for chickens to reach, but they can be employed to remove larvae that may be overwintering in the soil. You can also feed your chickens earworm larvae found in the tips of your corn.

More Advice on Organic Corn Earworm Control

Corn earworm damage disappears when you pop off the end of an infested ear, so you still get beautiful and delicious sweet corn.

Midseason sweet corn varieties with tight husks naturally resist earworm infestation.

In small plantings, you can open the end of an immature ear, remove the corn earworms with tweezers, and secure the husks back in place at the tip with a clothespin.

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for Corn Earworms

Common in vegetable gardens throughout North America, the corn earworm (Helicoverpa zea) is one of the most destructive insect pests attacking corn. It’s also known to bore into other fruiting vegetables and feed on lettuce. One worm can do extensive damage, and often a single larvae is all that’s found on a fouled ear. The corn earworm is also a serious pest of cotton where it is known as the cotton bollworm. On tomatoes it is known as the tomato fruitworm.

Identification

Full grown larvae (1-1/2 inch long) are lightly striped and vary in color from a light green or pink to brown. They darken as they grow older. Adults are night-flying, dull greenish gray or brown moths (1-1/2 inch wingspan) with irregular darker lines and spots near the outer margins of the fore and hind wings. During the day they hide in nearby vegetation, but may occasionally be seen feeding on nectar.

Note: Adult moths are good flyers, and able to move long distances. Each year they migrate from warm southern areas back to northern states where they are subject to winterkill.

Life Cycle

In areas where this insect survives the winter, pupae hibernate in the soil. Adult moths emerge anywhere from February through June, depending upon temperatures, and deposit their eggs singly on corn silks and other plant parts. Each female can lay up to 3,000 eggs, which hatch in two to ten days. When larvae emerge, they burrow directly down through the silks into the ear tip, becoming fully grown in 3-4 weeks. Corn earworm are extremely cannibalistic, which tends to limit the number of larvae to one per ear. When full grown, they drop to the ground to enter the soil for pupation. Usually two generations develop in the north, with as many as six in the extreme south.

Damage

Damage usually begins in the corn’s silk, where the moth deposits its eggs. The caterpillars follow the silk down to the ear, eating as they go. Extensive damage is often found at the ear’s tips, where the worms devour kernels and foul them with excrement. The larvae often destroy the silks before pollination is complete. The resulting ears are deformed and susceptible to mold and disease.

Worm damage is often confined to the tip of corn ears and can easily be cut away. Worms frequently follow leaves down the ear, leaving excrement and settling into kernels well in from the tip. It doesn’t take much of this kind of damage to make the entire ear undesirable.

Each year, copious amounts of pesticides are sprayed on commercial corn fields in attempts to kill larvae. Runoff from these sprayings contribute to watershed and water table contamination while the spray itself decimates beneficial insects. Genetically engineered corn, each kernel producing its own pesticide, was developed with corn earworms in mind.

Corn Earworm Control

An Integrated Pest Management plan that deals with the earworm at all three stages is the best way for corn growers to combat them. As moths, corn earworms are great travelers. Continued vigilance is needed. Just because you had them well-managed the previous season doesn’t mean they won’t glide back in on the first warm breeze come springtime.

Seasonal maintenance:

  • Don’t mulch ahead of winter without first getting as many pupae as you might have out of the soil.
  • Till your soil fall and spring to expose the pupae to wind, weather, birds and other predators.
  • Got chickens? Turn them loose after harvest to pick the grubs from your soil. Watching them work can be very entertaining!
  • If you suspect your former corn patch might harbor corn earworm pupae, try broadcasting beneficial nematodes into moist soil well ahead of first frost. Word of mouth suggests this is a helpful component of any earworm IPM program.

Spring moth arrival:

  • Use pheromone traps to determine the main flight period for moths. Moths mostly fly under cover of night and go unspotted.
  • At first sign of moths, release trichogramma wasps to destroy eggs.

Growing season:

  • Inspect silk for larvae, eggs regularly.
  • Employ beneficial insects, such as green lacewings, minute pirate bugs and damsel bugs. All will feed on corn earworm eggs and small larvae.
  • Spray or inject silks weekly with Beneficial Nematodes to control larvae.
  • If corn earworms persist, apply Safer Garden Dust (Bacillus thuringiensis) or Monterey Garden Insect Spray (Spinosad) to silks at 5-10% formation and continue weekly until tassels turn brown. Both products are listed in the Organic Materials Review Institute’s products certified for use in the USDA’s National Organic Program.

Tip: When using mineral or vegetable oils to suffocate feeding larvae at the ear’s tips, include a botanical insecticide in the oil as an added punch.

Control Of Corn Earworm – Tips To Prevent Corn Earworms

Earworm control in corn is a concern of both small and large scale gardeners. The Heliothus zea has the distinction of being the most destructive corn pest in the United States. Thousands of acres are lost each year to the larvae of this moth and many home gardeners have been discouraged by its damage. There are, however, ways to prevent corn earworms from wreaking havoc in your corn patch.

Earworm Lifecycle

Before we discuss how to get rid of corn earworms, we need to talk about the life cycle of the moth since many of the treatments, particularly organic control of corn earworms, are dependent on the stage of development to be most effective.

Corn earworm moths are most active during the evening and at night. They’re small moths with wingspans of only 1 to 1 1/2 inches. They appear in early June and search out corn silk on which to lay their eggs. One female moth can lay anywhere from 500 to 3,000 eggs and each egg is but half the size of a pinhead.

Larvae appear in two to 10 days and start feeding immediately. The larvae eat their way along the silk to the ears where they continue to feed until they are ready to fall to the ground.

They then burrow into the soil where they remain until their pupal stage has passed. New adults emerge in 10-25 days except for the last batch of fall. Those will remain underground until the following spring.

How to Prevent Corn Earworm

Organic control of corn earworms in sweet corn begins with early planting. The moth’s population is at its lowest in spring. Corn that matures early will have fewer problems. Choosing resistant varieties will also aid earworm control in corn. Staygold, Silvergent, and Golden Security are a few of the reliable resistant strains available.

As odd as it may sound, to prevent corn earworms from gaining entry to the ears, try placing clothespins where the silk joins the ear. This will block the worm’s access and can be quite successful on a small scale. In the fall, get rid of earworm’s overwintering pupae by turning the soil and exposing them to freezing temperatures.

How to Kill Corn Earworms

There are several biological answers to how to kill corn earworms. Trichogamma is an egg parasite wasp that lays its eggs inside the eggs of the earworm. Control in corn is 50-100% successful.

Green lacewings and soldier beetles are also effective answers to how to kill corn earworms. Bacillus thuringiensis is another. It’s a natural pathogen sold under the name Dipel and it kills only moth larvae and not beneficial insects.

Applying mineral oil to the silk where it inserts into the ear is an effective treatment to get rid of earworms. The oil suffocates the larvae.

There are insecticidal sprays that are used for earworm control in corn, but great care should be taken in the use of these products. While they may prevent corn earworm’s infestations, they can also harm beneficial insects and pose a toxic hazard to honeybees. Apply these products before 6 am or after 3 pm to reduce their environmental damage. Time your spraying to egg laying and hatching to reap the greatest benefit.

Whether you choose chemical, biological or organic control of corn earworm infestations, there are answers and treatments out there. Don’t let those devilish insects ruin the pleasure of raising your own sweet corn.

It’s getting to be the time of year in which more worms are found munching on the tops of our sweet corn. Growing up in the city, I was always told that you wanted the corn with the worm on it because that meant that ear was the sweetest ear. True or not, I don’t know…my mom usually ate that ear. Some people refuse to take an ear of corn that has a worm in it; we just cut or flick it out (the chickens love them). I know it’s good corn if the worms want it. We take pride in not spraying our corn with insecticide. It’s not something I would want my children exposed to, so I certainly won’t have yours exposed to it either. The truth of the matter is that ear worms are sprayed for when the corn is in the silk stage. That is where and when the moths lay their eggs in the silk. So, this insecticide is sprayed on top of the silk while the ear is inside it being formed. You may as well pick up the jug and take a sip! (At least that is my best analogy). Anyway, I would rather deal with a hundred worms than to leave spray residues on the food we are about to consume. As is, we only spray our corn for grasses and weeds before the corn is ever out of the ground. Without that practice we coiuld never grow the quantity of corn that we do.
Then over the summer some information was re-released about worms in canned and frozen corn. I decided to look it up (now that we are seeing more worms again) and pass it on. For those that are concerned when they see a worm on one of our ears, just note that thankfully you can see our worms, you can’t see the worms in the canned and frozen corn. As a result, we freeze our corn (as this is the best tasting and best for us) and I make sure we have enough to last us until the next sweet corn harvest. Generally I make enough that the pigs end up getting a little treat last spring because we don’t eat it all.
As per the FDA, Canned corn is allowed — 2 insect larvae per 100 grams. Or as more specifically written, any insect larvae (corn ear worms, corn borers) longer than 2 or more 3mm or longer larvae, cast skins, larval or cast skin fragments of corn ear worms or corn borer and the aggregate length of such larvae, cast skins, or if the larval or cast skin fragments exceeds 12 mm in 24 pounds (24 No. 303 cans or equivalent) is not allowed. Anything under that is a-okay. (Source: http://www.fda.gov/Food/GuidanceComplianceRegulatoryInformation/GuidanceDocuments/Sanitation/ucm056174.htm)
If you want the instructions/recipe for freezing corn, I’d be happy to pass it on. It came from Dave’s grandmother and anyone around here knows how wonderful of a cook she is. I’ve heard stories of people eating in shifts because there wasn’t enough room for everyone…Dave’s out of luck because my cooking will never compare!

Bryan Jensen, UW Extension and IPM Program

CEW Larvae and damage

I’ve had a few calls as well as personnel observations regarding “worms” and their damage in corn ears. European corn borer seems to be making a bit of a comeback. Although these reports have been locally heavy, they have not been widespread. Western bean cutworm complaints have certainly increased from previous year, especially on traited corn. Fall armyworms, which migrate to Wisconsin, had a surprisingly early migration and their damage can mimic injury caused by other ear feeding insects. Finally, corn earworm is another one of those late-season migrants which arrived in very heavy numbers at some locations during the 2016 growing season. Normally it is not a field corn pest but when migrating populations are this high, field corn can be a host. One clue that you have an infestations of corn “worms” can be significant bird damage to the ears. Diagnosis of the injury can be important when planning for the future. However, diagnosis of injury symptoms can be a little tricky and not always definitive based on symptoms. Below is a quick synopsis that can be used for identification of both the larvae as well as type of damage.

ECB

European corn borer (ECB), as mentioned earlier, is making a bit of a comeback presumably because more conventional corn is being planted. They are normally controlled by hybrids expressing above ground trait(s). ECB have a very dark head (usually black) and a lighter cream to tan colored body and may grow up to an inch in length. Depending on the area of state, there is either 1 or 2 generations/year. However, I have had a few reports of a third generation this summer. Larvae from the third generation do not mature in time to overwinter successfully. Diagnosing injury can always be a little difficult, however, ECB larvae may still be present in the ear and the injury is typically confined to a smaller area on the ear tip if compared to other insects. ECB are also more likely to burrow into individual kernels and/or the cob. You also would expect to find ECB injury in ear shanks as well as tunneling within the stalks.

Western Bean Cutworm. Photo credit: Frank Peairs, Colorado State University, Bugwood.org

Western bean cutworms (WBC) are dark to light brown, without distinguishable stripes or dots and their skin is smooth. They will grow up to 1 ½ inch and the later instars will have two short/broad strips behind the head. WBC complete 1 generation/year and leave the ear to pupate in the soil. Injury to the ear can vary from light surface feeding on kernels to complete consumption of large areas of kernels. Molds may be found on the ear but this is not diagnostic of only WBC. They are not cannibalistic; therefore, more than one larvae may feed on each ear. WBC injury is often associated with sandy soils. Several states in the Midwest, have had repots of significant WBC injury to traited corn.

Fall Armyworm (FA) is a species which occasionally migrates to our state. Larvae may be up to 1 ½ inch long, have variable coloration (green, light brown to almost black), smooth skin and light striping on their backs. A diagnostic feature of FA is an inverted white “y” on their head located between their compound eyes. Damage depends on crop stage and can range from leaf feeding to significant kernel injury on both the tip and sides of the ear.

Fall Armyworm, note inverted “y” on head. Photo credit: Steve L. Brown, University of Georgia, Bugwood.org

Corn earworms (CEW) feed on a variety of crops, including sweet corn. Although an infrequent field corn pest, feeding can be significant during years of heavy pressure. CEW vary in color and can be green, yellow, brown, tan to almost black. All CEW larvae will have a tan head. CEW can easily grow to 1 ½ inches long and later instars have easily recognizable striping. Early instars will have small black hairs identifiable if you have magnification. Earworms usually enter the ear through the silk and damage is concentrated at the ear tip, however injury can be severe on any part of the ear. What can separate CEW injury from WBC and FA is that the latter two species may chew holes into the husks.

There are no effective control treatments for these insects once these insects have entered the ear. However, proper identification can help develop management plans for the future, if needed.

Corn Diseases, Insects, and Pests

Most of the trouble gardeners have with corn is easily controlled. Diseases aren’t much of a problem, and insects can easily be kept in check. Birds and four-footed visitors who want to share your corn harvest can be kept out with any number of scare-off devices and fences. Prevention can be 100 percent of the cure. If you sow your corn in well drained soil that has balanced nutrient levels, you’re on the road to having healthy plants. Healthy plants can withstand nibbling or insect damage better than weakened ones. In many cases, a crop that’s healthy will often be spared disease and insect attack altogether.

A very important step you can take for disease-free corn is to clean up all the cornstalks as soon as the harvest is over. Till healthy cornstalks into the soil as additional organic matter, or, if you prefer, shred, compost or simply discard them. Dealing with old cornstalks will prevent many diseases and insects from overwintering, which is crucial to the health of crops grown in future seasons.

Diseases

Stewart’s Bacterial Wilt can affect sweet corn at any stage, but is most harmful to young plants. It causes dwarfing and wilting of the plants, and the tassels often develop early and die without completing pollination. Leaves develop yellow-brown streaks and wavy edges. The leaves of young plants may dry out, and the stem eventually dies. This wilt is often characterized by a yellow slime on the inner husks and in the stem. Bacteria overwinter in the gut of the corn flea beetle. This disease is prevalent after a mild winter, when more disease-carrying flea beetles have survived. To prevent an outbreak, clean up all crop residues, rotate your corn crop each year, plant resistant varieties and control corn flea beetles.

Root Rot is caused by fungi in the soil and shows up as stunted plants or irregular plants with rotten roots. You risk root rot when planting seed in cold, damp soil. Use treated seed, plant on raised beds if soil drainage is a problem and wait until the soil temperature is 55° F before planting.

Corn Smut is caused by a soil fungus, and can strike corn anywhere it’s grown. Smut looks awful, but it’s not a disastrous condition. Smut is edible and actually is sought after by gourmet chefs. In the early stages of the infection, grayish white, spongy growths called “galls” usually appear on the corn ear or tassel. As these galls ripen, they turn black and eventually burst open, releasing powdery spores that spread the smut. The disease thrives in hot, dry weather and often infects weak or injured plants first. To prevent, rotate crops, and if you notice any galls, pick them and burn them before they blacken and burst. This will halt the smut’s spread and is often all it takes to keep the disease in check from one season to the next.

Southern Corn Leaf Blight is another fungal disease. It is characterized by tan streaks or lesions on the leaves, and may cause early seedling death, mold-covered kernels or rotten cobs. A similar disease, northern corn leaf blight, results in grayish green or tan lesions on the leaves and reduced yields. These fungi overwinter in infected seed and plant debris. Plant resistant varieties, using healthy, certified disease-free seed. Rotate crops and remove or till under crop debris. If the disease has been severe in your area, check with your local Extension agent for a preventive fungicide program. In 1970 this blight reached epidemic proportions, wiping out 15 percent of the total United States corn crop, for an estimated loss of one billion dollars.

Insects

These are the insects most likely to affect corn, along with the best control measures for the home gardener.

Corn earworm is also known as the tomato fruitworm or cotton bollworm. This 1- to 2-inch-long caterpillar ranges from light green to purplish brown. Moths lay eggs on corn plants in early summer and larvae feed first on the silks, then on the kernels at the tip of each ear. The insect can prevent pollination, and it opens kernels to fungus invasion. To discourage this pest, select varieties with tightly closed husks. Earworms can be controlled somewhat by squirting mineral oil into each ear after silks have started to dry, using half a medicine dropper per ear. You can also spray the plant and silks with Bt. If earworm damage occurs, clip off the tip of the ear and any affected kernels. The rest of the ear should be fine to eat.

European corn borer is a 1-inch-long tan or brown caterpillar sporting rows of dark brown spots and a dark brown head. The moths fly mostly at night and lay eggs on the undersides of corn leaves in early summer. The hatched larvae bore into cornstalks and ears to feed. Broken tassels, bent stalks and “sawdust” around corn are all signs of borer damage. If you catch it in time, you can often cut out the borer from the stalk with no permanent damage. To prevent infestation, treat ears and leaves with Bt as soon as silk has partially emerged. Spray with an organic pesticide at five-day intervals from the time you first spot borer activity or when the tassels begin appearing. Make at least two applications for best results. Be sure to till or spade under crop residues at the end of the season so the borer has no place to overwinter.

Corn Sap Beetles are small, black beetles that spawn maggot-like larvae that eat into the kernels of roasting corn. Larvae are whitish and up to 1/4 inch long. The beetles are attracted to the scent of damaged corn, so preventing feeding by other pests helps keep them at bay. To prevent, plant resistant varieties and clean up all crop residues. Southern corn rootworms are small, yellowish grubs of the spotted cucumber beetle. They weaken corn plants by feeding on roots, causing the stalks to blow over easily in wind or heavy rain. Adults lay eggs around roots of cornstalks in the fall. The eggs hatch in spring. You can avoid damage from corn rootworms by tilling under cornstalks and rotating crops each year.

Corn Root Aphids are tiny, light green insects that feed on corn roots, causing the plants to be stunted and yellowed. The aphids overwinter in the nests of cornfield ants. The best way to control this pest is to plow the garden in the fall, destroying ant nests. Corn Flea Beetles are small but dangerous. Only 1/16th of an inch long, these jumping black beetles chew corn foliage and transmit Stewart’s bacterial wilt. The pests abound during cool, wet periods and after mild winters. They hibernate in weeds and plant debris over the winter, so keep the garden and surrounding areas clean. Many of the later-maturing white corn varieties are resistant to wilt. To repel flea beetles, sprinkle a light dusting of wood ash over plants and soil.

Wireworms are slender, yellowish or brown larvae of click beetles. These 1/2- to 1 1/2-inch-long worms resemble a jointed wire. They damage corn plants by feeding on the roots. These pests are most often present in newly worked sod. Rotate crops and till or spade your garden thoroughly in the fall. Heavy infestations may require soil treatment with beneficial nematodes before planting.

Seed corn maggots are cream-colored and legless. The 1/2-inch-long larvae bore into sprouting seeds and prevent further growth. When early corn is planted in cool, wet soil, the slower germination makes the seeds more susceptible to maggot attack. If maggots are a problem in your area, delay planting until weather warms.

Uninvited Guests

One of the biggest challenges in growing corn is keeping it for yourself. From the day you plant to the day you’re ready to harvest, it seems there’s always some critter who’d just love to share in the bounty. Fortunately, most of the animals and birds that invade corn can be outwitted. Raccoons are smarter than we’d like them to be, but they, too, can be kept at bay. Here are some ideas to keep corn free of uninvited guests. Some of these tricks can solve pest problems in other parts of your garden, as well.

Raccoons are well known for their expertly timed raids on the sweet corn patch. Many people start their tales of raccoon damage with the words, “The night before we were going to pick the first, ripe, sweet corn -.” When you lose ripening sweet corn to raccoon raiders, you swear they were on hand at planting time reading your seed packets, jotting down the days to harvest and keeping track of the time back in the woods. Actually, raccoons are attracted by the smell of the sweet corn tassels.

There are many old-time tricks to keep raccoons out of the corn patch, but only one rule: Put your defense in action before the raccoons can set a single foot in your garden. Once an animal has tasted your sweet corn, it will be almost impossible to keep it out of the garden.

To protect sweet corn, try these ideas:
* Erect a three-foot-high chicken-wire fence topped by an electrical wire.

* Play a portable radio in the cornfield all night.

* Plant a crop such as pole beans, pumpkins or winter squash between rows of corn. Supposedly, raccoons don’t like to tread on vines or foliage covering the ground around cornstalks. Also, the lush foliage of pole beans cuts down the raccoons’ ability to see, and this, too, is said to discourage their corn raids.

Other Critters

Some of the methods that keep raccoons out will also work for skunks, woodchucks, deer and squirrels. An electric fence is the best all-around pest barrier, except when it comes to squirrels. These agile creatures aren’t put off by fences, electric or not. One way to keep squirrels away is to sprinkle red pepper or Tabasco sauce on some ears on the outer rows of the corn. It won’t affect the corn’s flavor when it’s cooked, but any squirrel, skunk or raccoon who takes a nibble of the “hot” ears isn’t likely to come back for seconds.

It seems there’s an endless list of home remedies to try to keep the corn patch free of four-footed pests: running barefoot around the corn rows to leave a strong human scent; tying your dog near the corn to guard it; placing a paper bag over each ear; the bag of tricks seems bottomless. Some of these methods work some of the time, so try anything you think might work for you. Using several methods in succession or simultaneously increases your chances of success.

Birds

Unlike four-footed critters, birds do as much good in the garden by eating insect pests as they do damage, but some species have a weakness for corn. There are a few ways to keep birds away from the corn at each stage of the game.

After planting, cover each row with a long strip of chicken wire, bending it in an inverted U-shape about 10 inches high in the middle. The close mesh keeps out prying beaks, and by the time the seedlings touch the top of the wire, the birds are no longer interested. You can remove the wire and store it for the next season.

Don’t bother with scarecrows. Although they might be fun for the kids to put together, they’ll only serve as a handy perch for most birds. However, there are other effective ways to scare birds away. Try putting a realistic life-size plastic owl on a tall post near the corn. The owl will ward off birds, and may help with neighborhood mice and rabbits as well. Moving the owl frequently will keep critters from getting used to it too quickly.

If you can keep birds away from your newly planted corn, they shouldn’t bother the crop again until the ears start to fill out. Then you need an effective bird-scarer or chaser. Rig up noisemakers or aluminum pie plates around the corn to frighten them. A cat or dog near the garden often does the trick. Flashy mylar tape and scare-eye balloons can help, too.

One time-consuming, but surefire bird barrier is to tie a paper bag around each ear of corn, but only after the corn has been pollinated. This also can ward off invading insects, but it’s too much work for a large cornfield. When it rains, the bags break and need replacing; and when it’s windy, they often blow off.

Ministry of Agriculture, Food and Rural Affairs

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Corn Earworm

Table of Contents

  1. Introduction
  2. Migration
  3. Life History
  4. Damage
  5. Monitoring
  6. Control

Introduction

The corn earworm (Helicoverpa zea, formerly known as Heliothis zea) is a major pest of late-season sweet corn in Ontario and is responsible for a large percentage of grade-out corn. Earworms feed almost exclusively on the tips of the ears, leaving no visible damage on the husks or leaves. Fall armyworms – another late-season pest – feed extensively on the leaves, and often enter the ears through the side. European corn borers, which are present throughout the season, feed on all parts of the corn plant. Most farmers consider an intensive spray program necessary to control these pests.

The earworm has a wide host range, feeding on many cultivated crops and weeds. Elsewhere in its range, it is also known as the cotton bollworm, tomato fruitworm or tobacco budworm. In Ontario, however, it is a major problem only on sweet corn. Most seed corn fields are harvested before the worst of the earworm damage, and the damage to field corn is not considered economic.

Migration

The corn earworm is one of several pests that does not overwinter in Ontario. The entire population is killed by low winter temperatures. Each spring, the moths must reestablish themselves in Ontario, from the overwintering populations in the southern United States and Mexico. They begin their northward migration around May and usually reach Ontario sometime in August. Since moths are not strong enough to fly that distance on their own, they are carried by high-level winds. The warm southerly winds that blow into Ontario in the summer carry the moths long distances. These winds often bring rain, which commonly brings the moths back to earth.

Since their migration is so weather-dependent, the moths arrive in Ontario at different times each summer. Although they may arrive in late July some years, at other times they may not arrive until September.

Earworm distribution in Ontario also differs each year. While generally found in higher numbers in the counties that border Lake Erie and Lake Ontario, they may not infest parts of the province some years. For example, in 1994 there were no reports of earworms from any sites east of Toronto. Even when earworms are present in large numbers in one area, other corn fields that are only a few kilometres away may not be infested.

Life History

There are four stages in the corn earworm’s life cycle. Earworms arrive in Ontario as moths. The moths lay eggs, which then hatch into larvae (caterpillars). After feeding for two to four weeks, the larvae become pupae. In Ontario, larvae are typically killed by frost before pupation. In warmer climates, adult moths emerge from the pupae to repeat the cycle.

Moth

The corn earworm adult is a buff- or tan-colored moth with a wingspread of 3.5 to 4 cm (1.25 to 1.5 inches) (Figure 1). The forewing may have several darker markings and always has a central brown dot, clearly visible on the underside of the wing and faintly visible from the top. The hindwings are very pale in color, with a darker brown border. Eyes of living moths are bright green, which fade to dull olive green or brown a few days after death. There is no easy way to differentiate male from female moths.

Figure 1. Corn earworm moth.

Egg

Although earworms may lay their eggs anywhere on the corn plant, almost all will be laid on fresh silks if these are available. Although eggs are laid individually, one female can lay over 100 each night, and over 1000 during her lifetime. Each egg is nearly spherical, and about the same color and diameter as a corn silk (Figure 2 and Figure 3). Most eggs are laid on the outer third of the silks and hatch in 2 to 10 days, depending on the temperature.

Figure 2. Corn earworm egg on fresh silk (actual size).

Figure 3. Egg magnified 10 times.

Larva

Upon hatching, young earworms (Figure 4) crawl down the silks toward the ear. After feeding on the silks inside the husk for a few days, they begin feeding on the kernels at the ear tip. The worms will grow up to 3.7 cm long (1.5 inches), with prominent stripes running the length of their bodies (Figure 5). The size and the presence of stripes differentiate earworms from European corn borers, while their head color (tan) differentiates them from fall armyworms. See Table 1 for more complete descriptions of these three pests.

Figure 4. Corn earworm larva, recently hatched. Note the stripes running longitudinally.

Figure 5. Fully grown larva, up to 3.7 cm (1.5 inches) long. Note the stripes and the tan-colored head.

Damage

When corn earworms are present, all sweet corn with exposed fresh silk is susceptible to damage. Corn in the tassel stage does not need to be protected from earworms but may be attacked by other pests. Where sequential plantings are located close together, the field with the most fresh silk will likely receive the bulk of the egg-laying; other fields, however, are not immune.

Earworms normally feed only on the kernels of sweet corn, beginning by feeding at the tip of the ear and moving down the ear as they grow. Feeding is almost always confined to the top third of the ear. Fecal matter is found as large moist pellets in the silk channel and at the ear tip. Earworms do not bore into the cob, as European corn borers and fall armyworms sometimes do.

Although earworms damage only a small percentage of the kernels, their presence and droppings are very distasteful to most consumers. Where control has been less than perfect in the field, growers are forced to check ears at harvest and cull the damaged ones. Earworm-infested ears can sometimes still be marketed if the tips are cut off, although this practice significantly reduces the shelf life of the corn. Sweet corn destined for the processing plant may be able to sustain some earworm damage at the ear tips, as the tips are not used in the finished product.

Table 1. Comparison of Worms Feeding in Sweet Corn.

European Corn Borer Fall Armyworm Corn Earworm
Body Colour Spotted
Dirty pinkish-white with dark spots: pale stripes may also be visible
Striped
Greyish or brownish with distinct stripes
Striped
Yellowish, greenish or brownish. with distinct stripes
Head Color Medium to dark brown Dark brown to black. with an obvious white inverted “Y” Tan-colored: an inconspicuous “Y” is sometimes visible
Length of full-grown larva 2.5 cm (1 inch) 3.7 cm (1.5 inches) 3.7 cm (1.5 inches)
Mid-dorsal line
(stripe down centre of back)
None Single wide line Double fine lines
Proleg hooks *
(“Claws” on legs in middle of body – use a l0X lens to examine)
Hooks form a circle or nearly complete circle Hooks form a straight line, arc or half circle Hooks form a straight line, arc or half circle
Feeding location Anywhere on plant Extensive feeding on leaves: usually enter ear through the side Almost exclusively in the ear tip zone, entering through the silk channel

* A hand lens is the most reliable means of separating corn borers from the other two species.

In addition to direct damage, earworms can also predispose the crop to attack by other pests. Sap beetles will be attracted to the smell of fermenting sugars after earworms begin feeding on sweet corn kernels. In field corn, ear molds developing in the damaged kernels can cause toxicity problems for livestock.

Monitoring

The biology of the corn earworm makes it a very good candidate for an on-farm insect-monitoring program. The pest is present only at certain times of the year, its distribution is sporadic, and a cheap, effective monitoring method gives growers adequate time to implement a control program once the pest has been detected.

OMAFRA monitors corn earworms and other sweet corn pests at several sites across the province. Information about pest activity in your local area can be obtained by calling a regional vegetable agriphone. While regional information is helpful, it is not as reliable as insect counts from traps located on your own farm.

Corn earworms are monitored using a Heliothis trap, manufactured by Scentry Inc. (Figure 6) A pheromone lure (corn earworm Luretape manufactured by Hercon), which imitates the female earworm’s sex attractant, is placed in the trap. Male earworm moths are attracted to the lure and are caught inside the trap. Since pheromone traps only catch males, they cannot be used to control an infestation. Their value lies in detecting the presence of the pest and in estimating the size of the infestation. (For information about ordering traps, call your local Pest Management Advisor.)

Figure 6. A Heliothis trap, baited with a pheromone lure, is a simple way to monitor for corn earworms

Sweet corn growers should use at least two Heliothis traps to monitor for earworms. Place one trap per field in the two corn fields that are farthest apart. If two traps are used in a single field, place them at opposite ends. Move traps as often as necessary to ensure that each trap is always next to a field with fresh silk. Establish traps in early July, then check them twice a week. Replace the pheromone lure every two weeks. (Store extra lures in the refrigerator or freezer.)

Most years, the first earworms will not be trapped until sometime in August. When moths are caught in the traps, compare them with Figure 1 to see if they are earworms and not a similar species. After the first earworms arrive, begin checking the traps three times a week. Continue monitoring until the last planting no longer has any fresh silk showing.

Control

Non-Chemical Controls

Before modern insecticides became available, corn growers applied mineral oil to the silk of each ear to prevent invasion by corn earworms. This method is still used occasionally by some organic farmers and home gardeners. It effectively controls the pest, but is time-consuming and the oil left at the ear tip may be distasteful to consumers.

Several predators and parasites, including ladybird beetles, lacewings, predatory bugs, and parasitic flies and wasps, attack earworm eggs and larvae. These beneficial insects, present naturally in the field, exert an ongoing influence in keeping the pest population in check. They are not, however, adequate for economic control. To date, no commercially available biological control agents will effectively control the earworm.

Those who want to avoid earworm damage without the use of insecticides must plant early. Sweet corn harvested before the middle of August is usually free of earworms. After that time, insecticides are necessary to protect the crop.

Chemical Controls

Corn earworms can be controlled with insecticide sprays applied every three to seven days while fresh silks are present. For best results, sprays should be based on trap counts and temperature. (See Table 2 for spray intervals.) Since there is more risk of damage, spray intervals are shortened as the earworm population increases. Intervals are also shortened as the temperature increases, because unexposed silks are growing faster and because the insecticides break down more rapidly.

The choice of insecticide is very important in controlling earworms, which have developed resistance to some insecticides in the carbamate family (such as Sevin and Furadan) and are no longer controlled by these chemicals. They can be controlled by synthetic pyrethroids; currently available products are listed in OMAFRA Publication 363, Vegetable Production Recommendations.

Since egg-laying occurs at dusk and since pyrethroids are more effective in cool temperatures, it is best to spray in the evening. Aim the nozzles at the ear zone and ensure that the silks are well covered.

Table 2. Spray Intervals for Corn Earworm (from Univ. of Massachussetts Cooperative Extension)

Moths/Trap/Week Daily Maximum Temperature
Less than 27 °C More than 27 °C
1-6 5-7 days 5-7 days
7-90 5 days 4 days
more than 90 4 days 3 days

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