CORN INSECTS

Insecticide Quick Reference Chart for: Corn, Sorghum

Other resources available through NDSU Extension Service:

Publications:
E493   Aphid Management in Small Grains, Corn and Sorghum (1993)
E631   Corn Insects in North Dakota (1990)
E830   The Armyworm and the Army Cutworm (2000)
E272   Grasshopper Management (1997)
E188   Wireworm Control (2001)

APHID

Corn Leaf and Greenbug

The greenbug and corn leaf aphid are the most common species causing problems with corn and sorghum. The greenbug is the most injurious because it injects a toxin with its saliva during feeding.

Threshold:

The critical period for injury by corn leaf aphid is during tassel emergence through pollination. Treatment is suggested only when 50% of the corn plants have 100+ aphids per plant during tassel emergence and plants are drought stressed.

Natural Controls:

Lady beetles, Aphid lions, Syrphid fly, and parasitic wasps play a major role in reducing aphid populations. When natural enemies are present in large numbers, and the crop is well developed, farmers are discouraged from spraying fields.

RUP - Restricted use pesticide

ARMYWORMS

Armyworm outbreaks in North Dakota can occur when large migrations of moths from southern states occur in late spring and early summer. Moths prefer to lay eggs in moist, shady areas where small grains or grasses have lodged or been damaged by hail or wind. Armyworms feed at night and hide under vegetation or in loose soil during the day. To scout for armyworms in grains, part the plants and inspect the soil for fecal pellets. If pellets or feeding damage are found, look for larvae under plant trash, soil clods or in soil cracks.

Threshold:

Treat when 25 to 30% of the plants have 2 or more worms or 75% of the plants have 1 worm.

Migrating Armyworms:

Treat a couple of swaths ahead of the infestation in the direction of movement to form a barrier strip.

RUP - Restricted use pesticide

CUTWORMS

Several cutworm species affect regional crops. The Dingy cutworm, Feltia jaculifera, overwinters as a partially grown larva and is one of the first cutworm species to cause problems during crop emergence from early to mid-May. The moth of the dingy cutworm is known to lay her eggs on sunflower heads from mid-July through September. Crops following sunflowers in rotation are at greatest risk of injury by this cutworm.. Other cutworms, the red-backed, Exoa ochregaster, and the darksided, Exoa messoria, overwinter as eggs which hatch in mid to late May. Eggs are laid in the fall and survive in weedy, wet, and reduced tillage areas. Feeding injury by these cutworms normally occurs in late May to early June.

Some criteria that may help predict cutworm problems are: 1) field history of cutworm damage; 2) surface crop residue from reduced or minimum tillage; 3) bottom land or low spots in field; 4) fair to poor drainage; 5) near shelterbelts with grassy ground cover. Because eggs of the important cutworms are laid during late summer in North Dakota. Soil moisture at this time is important for their winter survival. Growers should be cautious when planting corn following pasture, alfalfa, or clover sites; survival may be greater at these locations.

Thresholds:

Begin scouting for cutworms when corn is up to a stand and continue until mid-June. Treat when 3 to 6% of the plants are cut and small larvae (<3/4 inch) are present. Application rate of 15 to 20 gallons of water per acre by ground application is suggested.

RUP - Restricted use pesticide

CORN ROOTWORM LARVAE

Rootworm larvae injure the root system of the corn plant. Yield potential may be reduced and/or lodging of plants may occur. Annual crop rotation from corn should prevent serious damage and losses. Early planting of corn allows for better root development prior to the late June hatch of rootworm eggs.

Threshold:

The decision to rotate from corn or to use an insecticide may be based on field scouting for adult beetles during a three week period after pollination. Record the number of corn rootworm beetles on the foliage and silk of 100 plants. When the adult population averages 1 beetle per plant in continuous corn or 0.5 beetles per plant in first year corn fields, the potential for larval root damage the next summer is sufficient to rotate from corn or to apply an insecticide.

RUP - Restricted use pesticide

CORN ROOTWORM ADULTS

Rootworm beetles feed on the leaves, silk, and pollen of corn. Occasionally, the beetles congregate and feed on silks during early pollen shed. If silks are chewed back to the tips of ears (less than 1/2 inch of silks protruding) during the period of maximum pollen shed, poor pollination and grain set can occur. Adult injury very seldom occurs in North Dakota.

Threshold:

When an average of 5 or more beetles per silk mass are found during the first week of pollen shed, control may be necessary. Another management threshold uses silk clipping. When silk clipping is occurring on 25 to 50% of the plants during pollen shed, control would be justified.

RUP - Restricted use pesticide

Page 2, CORN INSECTS

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