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Scouting for corn pests helps protect yields
Indianapolis, Indiana
April 21, 2005

Source: Mycogen Agronomy Update

Producing a successful crop is challenging enough without the threat of yield-robbing pests. Whether it's an insect pest that has been encountered for years or the threat of something new, growers should scout their fields early and often this year to ensure their crop is protected. Though pests observed will vary according to geography, the following tips from Jeff Housman, District Agronomist, Mycogen Seeds, will help growers make the most out of their insect scouting trips this season.

Black cutworm

"Scouting for black cutworms should be done at least once a week for a three- to four-week period after the crop emerges," says Housman. "Black cutworm feeding may be present above and below the soil surface, so growers and their crop consultants should look for areas where plants have not emerged, plants that have been clipped off and are lying beside their stump, or plants that have died within the row. Sometimes they may have been clipped below the soil surface."

Black cutworms migrate in spring, laying eggs on winter annuals. Once eggs are laid, cutworms will start cutting approximately 300 heat units later. The worm has four dark tubercles of unequal size on top of each abdominal segment, and mature larvae can be up to 1½ inches long. Total feeding time for the larvae ranges from 20 to 31 days, and those in the 4- to 5-instar stage cause the most damage.

If cutworms average less than ¾ inch in length, an insecticide should be considered if 2 to 3 percent of the plants are wilted or cut. If cutworms are longer than 1 inch, treatment should be applied if 5 percent of the plants are cut. If the field has a plant population of 20,000 or less, these treatment thresholds should be lowered.

Before deciding on a control treatment, Housman advises growers to dig around the cut plants and assess the size of the cutworm. Treatment is not an economically viable option if the cutworms are about to finish their larval feeding cycle. Even if a seed-applied insecticide is used, growers should still be aware of the level of cutworm feeding in their fields. These products have activity on cutworms, but are not fail-safe. The Herculex® I Insect Protection trait will also control black cutworms, although minimal feeding may appear before the cutworm is controlled.

Corn rootworm

"Corn rootworm larvae can cause significant damage by feeding on plant roots, leaving plants vulnerable to lodging during storms with heavy rains and high winds," says Housman. "To avoid this, growers should be diligent in checking for root feeding and identifying common signs of corn rootworm, including brown scars on the root surface, tunnels within the roots and various degrees of root pruning."

The corn rootworm lays eggs over winter in the soil. The larvae hatch during late spring and summer, feeding on corn roots for nearly three weeks. From June until the first week of September, they pupate from larvae into adults. When the corn is silking during mid-to-late July until September, crop scouts and consultants should scout the fields to look for pregnant or gravid female beetles, and identify fields in need of treatment.

Though today's rootworm Bt varieties are an economically viable option to help control heavy infestations of larvae, growers should still monitor for corn rootworm presence. One way to do this is to place pheromone sticky traps in fields and observe how many corn rootworm beetles are captured in the trap over time. "August is the prime time to monitor the flights of adult corn rootworm beetles," says Housman. "The usual treatment threshold is five adult beetles in the trap per day. At this point, aerial or ground spray-on insecticides can provide very consistent control, but for greatest effect they should be sprayed before the beetles lay their eggs."

European corn borer

When scouting fields for corn borer, growers should look for masses of 15 to 35 eggs on the underside of leaves. Egg masses are usually found near midrib, and are laid in an overlapping pattern. Larvae are cream-colored and range in length from .04 to 1.18 inches long, depending on maturity.

More mature corn plants (usually taller than 18 inches) are susceptible to corn borer injury, and early planted corn is most susceptible to larval feeding by the first generation of borers. The larvae feed on all parts of the corn plant except the roots.

Typical early feeding signs of European corn borer include shotholes in the leaves as they emerge from the whorls. Damage from this pest is usually caused by the early larval stage where pests chew on the leaves. In later larval stages, or second generation, the European corn borer tunnels into stalks, ears and ear shanks. Late season signs of severe corn borer feeding include broken stalks and tassels, poor ear development and dropped ears. "Though in-plant Bt traits have minimized the impact of the corn borer, it is still important for growers to scout susceptible fields early and often," advises Housman.

Western bean cutworm

Originally found in Colorado and other Western states, western bean cutworm (WBCW) now has been found feeding in Nebraska, South Dakota, Kansas, Iowa, Minnesota, Missouri, Wisconsin and the western edges of Illinois. This pest can cause substantial yield loss and reduced grain quality if left unchecked.

"Growers should scout for WBCW larvae mid- to late season," says Housman. "Look for larvae that are dark brown in color, with a faint diamond shape marking on their back. As they mature, they become a light, gray-pinkish brown color." Mature larvae may be up to 1½ inches long, and have three dark stripes on the segment immediately behind the head.

WBCW larvae feed on pollen in the developing tassel, or on silks. As the ear develops, the larvae feed primarily on developing kernels. The treatment threshold is reached when 8 percent of corn plants have egg masses or small larvae. Once larvae move into the ears, they are protected by the corn husks, making insecticide treatments virtually ineffective. Housman suggests that growers consider planting hybrids with the Herculex I trait, the only in-plant trait on the market that offers protection against WBCW.

Resources

Growers who need assistance in identifying a pest or in determining a treatment threshold should consult their local agronomist or university. These local advisers are familiar with insects of particular concern in a local area, and will be able to help devise an effective control plan.


For further information, or to speak with an agronomist in your area, please contact Stephen Smith, Agronomy Services Manager with Mycogen Seeds, at (317) 337-4662 or via e-mail at sesmith@dow.com.

®™Mycogen and the Mycogen M Logo are trademarks of Mycogen Corporation.
®Herculex is a registered trademark of Dow AgroSciences LLC.
©2005 Mycogen Seeds. Mycogen Seeds is an affiliate of Dow AgroSciences LLC.
Herculex Insect Protection technology by Dow AgroSciences and Pioneer Hi-Bred.

Source: Mycogen Agronomy Update

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