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Weeds, Wet Weather and Cutworms

May 4, 2016

Armyworm catch is increasing in the Purdue University trapping network, and Elkhart County (just across the border from Michigan) had a significant black cutworm catch by April 27, 2016. Traps on the Michigan State University campus just picked up black cutworm and armyworm. The risk of both of these pests increases with weediness and wet fields.

Black cutworms lays eggs on grassy weeds or cover crops, low-growing winter annuals and no-till crop residue like dead stems or stalks. This spring is a banner year for winter annuals, particularly chickweed and purple deadnettle, so moths will find plenty of egglaying habitat. Small larvae feed above-ground on weeds or seedlings, making small pinholes. Older larvae feed below ground, cutting off plants at the base (before V6) or boring through the stalk at the soil line (after V6); cut or damaged plants wilt and die.

Armyworms lay eggs in areas with dense vegetation, such as weedy corn fields, but also cover crops, grassy alfalfa stands and wheat fields. Weedy corn fields, reduced and no-till corn, and corn planted after alfalfa are at greater risk, so are portions of corn fields along ditch banks, fencerows and small grain fields. Larvae climb plants and feed above-ground, giving corn leaves a tattered appearance. In severe infestations of corn, only the leaf midrib is left. In wheat, larvae feed on the leaves and also may clip heads. As small grains mature and dry down, larvae can “march” en masse into neighboring corn fields. Larvae feed at night; during the day, they hide near the base of plants or down in corn whorls. Besides damage, a give-away of armyworms during the day is to look for large, cylindrical frass pellets in the whorl or on the ground.

At this point, we are in the midst of flight and egglaying. Scouting for black cutworm cutting on corn starts 300 degree-days after a significant flight in an area. “Significant” is defined as nine or more moths in a pheromone trap over two nights of trapping (thus far I captured six moths over all of last week). For armyworms in corn, trap numbers do not necessarily correlate with the size of infestations in fields.

For both insects, even if moth flight is heavy, crop damage depends largely on the timing and effectiveness of weed control. Plowing, or early herbicide applications to kill weeds or terminate a cover crop, removes egglaying sites. Herbicide applications made at or just after planting can remove the alternate food source, forcing larvae onto the crop. This is when damage occurs and why it differs field-to-field. This is also why weather plays an important role in the story. Weather fronts can move moths from south to north, while wet conditions delay planting, crop emergence, termination of cover crops and herbicide applications. These delays can synch the larval life cycle with crop emergence, potentially exposing crops to more larvae, for a longer damage period.

If you have time to scout corn fields in May and early June, concentrate on two things: Non-Bt corn fields, since most Bt hybrids contain at least one trait that controls armyworms and cutworms, and fields where annual weed pressure was heaviest or where a cover crop was terminated late. Note that even Bt corn can suffer damage by these insects under heavy pressure. This happens because small larvae start out on weeds and cover crops, and only move to Bt corn much later in life. Large larvae need to eat to die, plus larger larvae tend to be less susceptible to Bt toxins.

For armyworms on wheat, the story is different and not tied to weed control because egglaying occurs directly in small grains in the spring. Armyworm outbreaks in wheat tend to be on a wider scale, with most or all fields in a neighborhood infested. High trap captures are an alert to scout for leaf feeding so that populations can be managed quickly across an area before head clipping occurs. MSU Extension wheat educator Martin Nagelkirk, based in Sanilac County, is coordinating armyworm trapping this spring, so watch for updates from his network.

For a free bulletin on armyworms in corn or wheat, see “March of the Armyworms Part 1: Wheat” and “March of the Armyworms Part 2: Corn.”

Source: Michigan State University