Corn Root Borer: Tips For Controlling Corn Borers In The Garden

Corn Root Borer: Tips For Controlling Corn Borers In The Garden

By: Susan Patterson, Master Gardener

The European corn borer was first reported in the United States in 1917 in Massachusetts. It was thought to have come from Europe in broomcorn. This insect is one of the most damaging corn pests known in the United States and Canada, causing over $1 billion dollars of damage to corn crops annually. Even worse, corn borers don’t limit their damage to corn and can damage over 300 different garden plants such including beans, potatoes, tomatoes, apples and peppers.

Corn Borer Life Cycle

Also known as the corn root borer, these destructive pests do their damage as larva. Young larvae eat leaves and munch on corn tassels. Once they are done eating leaves and tassels, they tunnel their way into all parts of the stalk and ear.

The 1-inch long, fully mature larvae are flesh colored caterpillars with a red or dark brown head and distinct spots on each body segment. These fully grown larvae spend the winter in plant parts that they have been eating.

Pupation happens in late spring, and adult moths appear in May or June. Mature female moths lay eggs on host plants. Eggs hatch as soon as three to seven days and the young caterpillars begin to eat the host plant. They are fully developed in three to four weeks. Pupation takes place inside corn stalks and second-generation moths start laying eggs early in the summer to begin yet another corn borer life cycle.

Depending on the climate, there could be one to three generations with the second generation being most destructive to corn.

Controlling Corn Borers in Corn

It’s imperative to shred and plow under cornstalks in fall or early spring before the adults have a chance to emerge.

Several beneficial insects find corn borer eggs a delicacy, including ladybugs and lacewings. Stink bugs, spiders and hover fly larvae will eat young caterpillars.

Other known corn borer control methods include using garden insect sprays to kill young caterpillars. It is important to spray plants every five days until tassels start to brown.

Another beneficial corn borer treatment method involves keeping garden and surrounding areas free of weeds. Moths like to rest and mate on tall weeds, which will increase the number of eggs laid in your garden area.

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How to Control Corn Borers & Earworms

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Nothing is more frustrating than pouring your heart and soul into growing a garden only to discover that pests have destroyed much of your edible crop. Corn borers and corn earworms are major pests of your corn crop, but they aren't too picky and will also destroy other fruits and vegetables such as strawberries and tomatoes, too. If you've discovered these pesky creatures in your garden, take action immediately to prevent extensive damage and protect your precious produce.

Plant your crops early and harvest them before late August. This prevents the late summer population of pests from causing devastating damage.

Apply mineral oil to the silk of the corn. This controls the growth of eggs and newly hatched larvae. Drop the mineral oil onto the silk as soon as the pests first appear, and repeat applications once a week until the silk starts to turn brown.

Handpick corn earworms from the plants. Gently, but firmly pluck each visible cornworm from the stalks and leaves of your plants.

Introduce parasitic and predatory insects into your garden. Lacewings and lady beetles will eat pests so they don't damage your crop, and can be purchased from many home and garden stores. Frogs, birds and spiders also will feast on the pests.

Treat crops with Bacillus thuringiensis, which is a naturally occurring bacteria that acts as an insecticide. This can be particularly effective if you've had trouble with corn borers in the past.

Remove old stalks, plow them under or rototill after you've harvested. This reduces the risk of a repeat infestation in the spring.


Biology

Scouting and management is complicated by two biotypes of corn borer. They respond the same to current Bt hybrids (99.5% control) but because of the timing of egg laying with respect to corn development, they do not respond equally well to insecticide applications.

The univoltine biotype has a fixed diapause and only one generation occurs. The multivoltine biotype has multiple generations, typically two in Minnesota, and diapause is determined in the second instar larvae. In warm years, a third generation may be triggered, but these larvae may not reach the 5th larval stage (instar) and be able to overwinter.

The univoltine biotype can overlap a 2nd generation of the multivoltine biotype where they co-exist creating the potential for egg laying over an extended period and making the timing of insecticide applications difficult. Historically, the univoltine biotype is predominant in the north and the multivoltine in the south.

Overwintering

The larvae of the both biotypes overwinter in corn stalk residues, but they break diapause at different times. In the spring, multivoltine larvae pupate earlier ( 350 + DDs). This leads to differences in time of year and corn growth stage when moths are active.

Moth emergence and egg laying

After the moths emerge (Figure 1), they congregate and mate in areas of dense vegetation called “aggregation sites” or “action sites”. You may encounter 1st generation univoltine moths in dense grass action sites near corn. The second generation and univoltine will use both dense grasses and soybeans for action sites.

ECB prefer particular corn stages for egg laying. Small corn contains a high concentration of a chemical compound (DIMBOA) that is toxic to ECB larvae and their survival is very low on corn less than 17-inches tall (extended leaf).

First generation females seek out taller, early planted whorl stage corn and lay eggs (Figure 2) near the midrib of a whorl leaf. Corn nearest tassel is most attractive to univoltine moths and 2nd generation moths are attracted to late planted or less mature fields.

Larvae hatch

Larvae hatch in 3-7 days or approximately two weeks after moths emerge. Newly hatched larvae (Figure 3) feed on corn leaf surfaces or pollen if available. The feeding of second instar larvae feeding creates small, round “shotholes” (Figure 4) in whorl stage corn. Ten-day to two-week old 3rd instar larvae can tunnel into leaf mid ribs and stalks. Young univoltine and 2nd generation larvae often feed on pollen and begin tunnels in leaf axils.

ECB damage in corn

Feeding by ECB larvae causes direct, physiological yield losses when stalk tunnels (Fig. 5) disrupt the movement of water, nutrients and photosynthates within the plant. This is the primary way ECB larvae cause corn yield loss. The impact of larval feeding varies with the stage of corn development when the feeding occurs. Losses can range from over 6% loss/borer/plant in pre-tassel corn to 3% loss/borer/plant after pollination.

Corn borer feeding can produce indirect losses by providing entry points for ear and stalk pathogens. The tunnels in stalks and ear shanks create harvest losses due to stalk lodging and ear drop.


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Corn Insects

European Corn Borer, Ostrinia nubilalis.

Egg mass of European corn borer.

First Generation

First-generation control is advisable in non-BT cornfields where approximately 50 percent of the plants are infested with an average of at least one live larva per plant. If control is attempted, insecticides must be applied early in the infestation while the small larvae are confined to the whorl area. Granules should be applied directly over the planted rows into the whorls of the plants. Granular products have sometimes been inconsistent in K-State experiments. Liquids probably should be applied so that sprays are delivered into the whorl if ground travel equipment is used. Reports indicate that some products work better when applied with high volumes of water (check labels for chemigation labeling). Retreatment within seven to 14 days may be necessary if egg-laying continues for an extended interval.

Second Generation

Typically, there is a greater need to control second-generation European corn borer than first generation in Kansas. Egg-laying for the second generation is generally heaviest in silking-stage corn during the July to August moth flight. The prolonged emergence of adults and egglaying interval make it more difficult to locate and evaluate the seriousness of these infestations. Yield loss can result from a variety of factors including physiological damage caused by larval tunneling, harvest losses represented by lodged stalks and ear droppage, and direct kernel feeding.

Kansas research demonstrated that corn plants do not suffer as much disruption in water (sap) movement as previously thought following significant damage to their water-conducting vessels. In other words, corn plants have significant ability to compensate for severed vascular bundles within a node by recombining their water transportation system components further up the plant. Bottlenecks to compensation appear to occur at leaf collar, leaf sheath, and ear shank locations. Other research in Kansas indicates that shank tunneling (one European corn borer larva/earshank) can cause significant ear droppage. Ear droppage for at least one variety accelerated if shank-infested plants remained unharvested three or more weeks following physiological maturity (black layer formation). Earlier harvesting of fields in which European corn borers infest a significant percentage of ear shanks is warranted.

Limited data indicate that shank tunneling may be as costly, or even more costly, in terms of physiological yield loss, as equal densities of larvae in the stalks. Some data indicate that weedy fields infested with grasses such as shattercane may become more heavily infested with European corn borer larvae. Grasses may serve as resting or mating sites and maintain higher humidity levels, all of which favor denser corn borer infestations. Therefore, more weed-free fields should support fewer corn borers. Removing weeds before second-generation moths deposit their eggs eliminates synergistic losses that might develop if both weeds and insects stress the corn throughout the growing season.

If second-generation control is considered, treatment should generally be applied where field inspection reveals an average of between 10 to 20 egg masses per 100 plants. Egg mass counts should include both hatched and unhatched egg masses. The sampling should be most intensive within 10 to 12 days after first eggs are laid. Typically, 70 to 85 percent of the eggs are laid on the seven leaves nearest the ear (ear leaf and three leaves above or below). Subsamples should be taken from several locations to obtain a representative sample. The threshold varies with the expected price of corn post-harvest consult the Threshold Calculation for European Corn Borer for assistance.

Applying treatments strictly on the basis of calendar dates or stage of corn development often results in unsatisfactory protection. Conduct field inspections at least once a week during the period of second generation activity (roughly mid-July through August). During periods of peak moth activity, inspect more frequently (two to three times per week), especially in fields of silking-stage corn. Treatments generally need to be applied within five to six days of when the majority of the infestation is composed of eggs. Treatments remain effective as long as a majority of the infestation is composed of small, exposed larvae in the area of the leaf sheaths. One exposed larva per plant on approximately 50 percent of the plants would justify treatment. Two applications are frequently necessary to suppress second-generation European corn borer larvae. Good coverage above, below, and in the ear zone is essential. However, many corn borer insecticides have the potential to increase mite problems. This must be a consideration in justifying corn borer controls and selecting insecticides. Be cautious in using broad spectrum insecticides that destroy natural enemies of spider mites, while leaving mite populations intact in areas where spider mites are likely to be a problem (this concern would include most of the pyrethroid insecticides except bifenthrin).

Please consult the most recent Corn Insect Management Guide for more information on corn borer management.


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