Scout for black cutworms

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.   

Looking back at my Field Crop CAT Alert articles from 2008, I feel like I’m repeating myself…which I am. At this time last year, I was stumbling around in newly planted corn fields covered with heavy mats of dying chickweed, warning folks about black cutworm. This year is similar, except in many cases the corn still isn’t planted. The longer the delay in weed kill, the greater chance of black cutworms reaching Michigan from the south.

Adult black cutworm moths migrate into Michigan in early spring (now). Before the crop emerges, adult moths lay eggs on vegetation in field margins and ditches, on cover crops within the field, and especially on low, densely growing weeds. When the weeds or cover crops are killed by herbicide, larvae move onto the nearest green plants, often the crop. Small larvae initially feed above-ground, making small pinholes in the leaf or chewing on the leaf edges. Bigger larvae feed near or below the ground, cutting off plants at the base. Cut plants are wilted or simply dead.

Effective and timely weed control is key to avoiding or reducing a black cutworm infestation. Controlling low lying, densely growing weeds such as chickweed, deadnettle and mustard reduces the habitat for egg laying. If weeds or a cover crop can be killed a week or two before crop emergence, most cutworm larvae will starve.

Over the last few years, there have been spotty cutworm infestations in corn, soybeans and sugar beets, perhaps because of earlier planting or changes in production practices to no-till. Also with the use of herbicide-resistant crops, fields may be weedier early in the season before glyphosate is applied. This season, wet weather has delayed getting into field, so there has been additional time for weeds to grow.

Scout for cutworms at seedling emergence. Look for wilted or cut plants, and dig around the base of nearby seedlings to find the larvae. If you can’t find cutworms near a damaged plant, move down the row to the next apparently undamaged plant. Larvae feed at night and hide during the day, so the best time to look for larvae is in the early morning.

A general threshold is five percent or more of plants showing cutworm damage. Rescue (foliar) insecticide treatments are the preferred way to manage cutworm because not every field will have a significant problem. Insecticides are most effective if sprayed in the evening when the cutworms are active. In crops like corn, beans and alfalfa, pyrethroids such as Ambush, Baythroid, Mustang, Pounce, and Warrior are particularly effective against cutworms (check labels for specific crop registrations and rates). For sugarbeets, options include Asana, Declare, Mustang EW, and Lorsban. These insecticides should tank-mix well with Round Up, but be careful when adding them to microrate herbicide applications.

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