Corn Insect Pests

corn field


The 2022 La Nina weather pattern is expected to bring above average precipitation to Ohio and the Great Lakes Region along with stronger and more hurricanes. Since many insect pests are carried by hurricanes northward, farmers can expect higher insect infestations this year. Some insect pests over winter, and they are also around in higher numbers. A cool, wet spring slows down crop growth and insects love to feed on slow weak growing plants.

Black Cutworm and armyworm are larvae that damage crops. Several species of moths lay eggs that form cutworms and armyworms in Ohio. They generally have 1-2 generations per year depending on the species. Adult flights occur at different times through the growing season. They have 4 stages in their life cycles: eggs, larvae or caterpillars which damage crops, pupae, and adults. Some overwinter as partially grown caterpillars, others as pupae in soil, while others migrate as moths from the south. Paper wasp, ground beetles, and lighting bugs are natural predators with several insecticides labeled for cutworm control.

The Asiatic Garden Beetle (AGB) has been moving west into Central USA. A white, C-shaped grub with a brown head, it’s very aggressive and feeds on corn, soybeans, and potatoes, and marestail (weed). Controlling marestail helps control over wintering AGB. AGB are prevalent in sandy or loamy soils, but affects all soil types. AGB feeds on corn roots making plants stunted and purple (P deficient). AGB has only one generation, feeding on early planted emerging corn in April/May with 40% losses possible. Adults (Red-orange) beetle emerge in July-August and lay eggs that hatch and burrow into the soil to emerge next spring. Seed treatments, even at high rates and even tillage do not seem to control AGB. Chlorethoxyfos, the active ingredient in Fortress, Index, and Smart Choice; is the most effective chemical control for AGB. Natural predators include birds, ground beetles, and lightning bugs which eat soft bodied eggs and pupae. Seed corn maggots are an early season pest that thrives under moist conditions, especially fields high in soil organic matter or manure.

Seed corn maggots come from flies. Adults emerge from a pupal case in 7-14 days, mate, and start a new cycle. Maggots are 1/3 to ¼ inch long, small, yellowish white, larvae with no legs, pointed head, and blunt rear. Adults look like small black house flies. The entire life cycle is no more than 21 days and seed corn maggots can have 3 or more generations per year. Later generations do not effect more mature corn plants. Corn that grows fast and vigorously has less damage. There is no control for seed corn maggot once corn is growing, so replanting is the only option. Most neonicotinoids seed treatments (Cruiser, Poncho, Goucho) have poor to fair seed corn maggot control. Force (Tefluthrin) helps controls seed corn maggot, cutworms, and most grubs.

Other late growing corn pests include the western bean cutworm, European corn borer, corn ear worm, and western bean cutworm. Most of these pests are now becoming resistant to Bt corn, which was a gene that was inserted into most GMO corn for pest control. These are major pests to sweet corn, but now some field corn fields.

The western bean cutworm has 1 generation per year starting in Mid-June to Late July when moths lay 25-100 eggs on pre-tassling corn, usually the two top leaves. The eggs turn from white to pink to purple when they will hatch in 24-48 hours and infect the corn silks. Bt strains are still effective.

European corn borers are a drab gray caterpillar with dark brown almost black head that includes an early and late borer (two generations) that burrow into corn stalks, causing corn lodging. The early pupates or caterpillars come out in May-June, the second generation in Late July to early August. Eggs are hard to see and are deposited on corn silks. Corn ear worm moths start flying July-September, migrating from the south, especially after major hurricanes. Corn ear worms are several colors from orange, yellow, to bright green. Most corn ear worm damage occurs in sweet corn but some field corn is now showing signs of Bt resistance.

The VIP strain is currently the only effective Bt for corn ear worm and European Corn borer. Corn ear worms and European corn borer are difficult to treat because these pests live inside the corn silks and/or inside the corn plant. The corn refuge requirement may soon be increased from less than 5% of seeds planted to 10% to keep Bt genes viable. Keeping soils healthy with healthy predators and fast growth is always a good option against pests.