True Armyworms. Photo by OSU Extension.

Controlling corn and soybean pests

By James Hoorman, Hoorman Soil Health Services

Controlling pests of corn and soybeans can be difficult.  Most farmers rely on seed treatments and broad-spectrum insecticides which terminate the pests but also takes out the beneficial natural predators.  The most common Ohio pests in corn and soybeans fields with cover crops are wireworm, seed corn maggot, black cutworm, true armyworm, slugs, and grubs.

Wireworms have a five-year life cycle with adults (called click beetles) laying 100-200 eggs in the spring and early summer.  Larva live in the soil until they mature into adults. Wireworms are a copper color, long, and slender. Wireworms damage corn and soybean seeds and cause seedling roots damage.

Wireworms have many natural predators including centipedes, soldier beetles, wasp which infect their eggs, and parasitic nematodes.  Metarhizium fungi are a great wireworm predator; infecting the eggs, larva, and pupae and may give up to 95% control.  Metarhizium fungi infect up to 200 insect species in 50 families including root weevils, flies, gnats, thrips, locust, grasshoppers, grubs, borers, even mosquitoes.

Seed corn maggots have three cycles per years with the first cycle causing the most damage. Adult flies lay eggs in the soil where pupae overwinter.  In spring, they burrow into seeds which reduces germination and they infect young growing corn and soybean seedlings.  They generally cover an entire field with an estimated 1.2 million wire worms per acre possible.  The maggot is a pale yellowish white color. Seed corn maggots are more prevalent in cold wet springs so they may not be problematic this year.

Natural seed corn maggot predators include black ground beetles, predator nematodes and several fungal disease organisms including Metarhizium fungi.  Ground beetles (Carabidae) are called the “Lions of insect pests”; eating their weight daily in weed seed and insect eggs, larva, and pupae.  Ground beetles are highly susceptible  to most neo-nicotinoid insecticides used on seed treatments.  They only lay 10-20 eggs per year and they have a five-year life cycle, so it takes time for them to recover after a broad-spectrum insecticide is used to kill the main pest.

The black cutworm moth migrates in from the South and lays eggs in green fields starting in March-May.  Eggs hatch in May and the larva feeds on corn roots and leaves.  The black cutworm  larva (black striped) can be hard to scout because they are nocturnal. They often burrow in the soil around infected plants or in the whorl of leaves. They can circle young seedings and cut off the emerging vegetation.  For home gardeners; coffee grounds, egg shells, and corn meal can be natural deterrents.

Black cutworms have many predators including fireflies, paper wasp, soldier beetle, ground beetles, rove beetles, parasitic nematodes, spiders, and toads.  Fireflies live in the soil for 18 months consuming many insect eggs, larva and pupae.  When they emerge in June, they are looking for a mate to breed, lay their eggs, then they die rather quickly.  Fireflies are hurt by over use of insecticides and unfortunately night lights.  Night light illumination confuses the adults and prevents them from breeding as they search for a mate.

Another common pest found in green covers is true armyworm.  True armyworm moths migrate in from southern states  in April and May looking for green vegetation, especially grasses and dense weeds.  The moths lay 2000 eggs with 2-3 generations or waves spaced 3-4 weeks apart.  The larva generally migrates to grassy plants like corn and wheat in early May.  True armyworm seems to be especially attracted to cereal rye and corn planted into rye. The best time for rescue sprays is when caterpillars are 0.5 inches or smaller.  The most severe crop damage occurs when the caterpillar larva are 1.5 inches long but by then they are much harder to terminate.  Ground beetles, rove beetles, and 12 species of parasitic wasp and flies help control true armyworm.

Slug populations peak every 2-5 years and prefer lush vegetation and cold wet springs, so they should not be a major problem this year.  White grubs infect corn (come from May/June beetles, masked chafer or Japanese beetle eggs), or in soybean (Japanese beetle). May/June beetles and masked chafer prefer to lay their eggs in grassy vegetation. Ground beetles, fireflies, rove beetles and Metarhizium fungi are major predators to slugs and white grubs.  Farmers can reduce pest damage by scouting for all these pests, using pesticides only when needed, and looking for ways to maximize natural predation by beneficial organisms.

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