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Cutworms including the Black Cutworm, Agrotis ipsilon, Lepidoptera: Noctuidae

Description

Cutworm larvae are dark gray to black caterpillars that can generally be found just below the soil surface feeding on seedling grain sorghum. Although larvae are minute, <1/8 inch long, at hatching, they are not likely to be detected until they are at least ½ inch long. At maturity larvae are almost 1.5 inch in length (Photo 4). The caterpillars have three pair of true legs on the thorax and five pair of fleshy “prolegs” on the abdomen.

Cutworms occur throughout the U.S. and throughout Arkansas. Recent surveys have detected large populations near Stuttgart and Des Arc. Larvae feed on emerging seedlings and often cut off the plant near or below the soil surface. Several adjacent plants within the drill line can be killed by a single cutworm. Also, rough or cloddy soil appears to harbor larger cutworm populations.

Life History

Cutworms generally are capable of overwintering as pupae in soil in Arkansas especially in southern counties. In addition to adults emerging in late winter from the overwintered pupae, adult moths fly into Arkansas from more southern states. Moths lay eggs on many weed hosts and crops including grain sorghum. Newly hatched larvae can produce “shot holes” in grain sorghum foliage. Larger larvae often cut the seedling and feed below the soil surface. Where damage occurs, larvae can be detected by removing the upper layer of soil near a damaged plant. The number of generations per year varies from 1 to 3 depending on cutworm species.

The first step in cutworm management is proper crop rotation. Grain sorghum that follows grain sorghum or planting grain sorghum in recently turned pastures tend to have more damage from cutworms. Also, adults are attracted to fields with weeds on which eggs are laid. Thus, early seedbed preparation prior to planting reduces the likelihood of cutworm damage. Providing a minimum of two weeks of host free time prior to planting should reduce the attractiveness of the field to cutworm adults. No or reduced till fields are more susceptible to economic losses from cutworms. Also, seed beds prepared during wet conditions are often cloddy. This may later harbor increased numbers of cutworms. Bed knockers or flat rolling beds can reduce the chances of developing damaging cutworm populations. In areas with histories of cutworm problems, treated seed or soil insecticides applied at planting may be justified. Foliar insecticide sprays may be used to reduce cutworm populations but early scouting for damage is critical. Foliar insecticide sprays should only be used when the damage levels exceed the threshold of 6-8% of the seedlings with cutworm damage above the surface of the ground or 2-4% of the plants cut below the surface. Finally, insecticide success may be reduced when late stage larvae are targeted as most of their time is spent underground.

See Tables 1 and 2 for insecticide recommendations current at time of publishing. Current updated recommendations can be found in the Cooperative Extension Service publication MP-144 and on the world wide web at WWW.cdms.net/manuf/default.asp. Always follow instructions on pesticide labels.