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Scouting for Black Cutworms

Ellen Phillips, Crop Systems Educator, Countryside Extension Center, 708-352-0109, ephillps@uiuc.edu

There is no way to make an early prediction of the impact of black cutworms this year. Scouting is the only sure method to deal with this unpredictable pest and be ready to apply a rescue treatment when necessary.

Several cutworms can be found in cornfields. Correctly identifying what insect is present is important since not all cutworms cause economic damage. The claybacked cutworm is more likely to cause economic losses in fields that were previously planted to clover or alfalfa. Dingy and variegated cutworms are also commonly found in fields last planted to clover or alfalfa. They are normally leaf feeders and do not usually cause significant economic damage. Sandhill cutworms survive best in sandy soils and can overwinter in Illinois. They usually cut seedlings below the growing point so a reduced stand may be the only indicator of their presence.

Normally, the black cutworm does not overwinter in the Midwest. During April and May, adults migrate from the Gulf states, arriving in the Midwest within 2 to 4 nights. You can monitor the flights of adult black cutworms with black cutworm pheromone traps. Traps need to be monitored on a regular basis. For more information on how to use pheromone traps contact your local Extension office or view http://www.ag.uiuc.edu/cespubs/pest/articles/v983b.html. An intense capture occurs when nine or more moths are caught during a 1- to 2-day period. Once an intense capture occurs, daily heat units (base 50°F) are tracked. Upon the accumulation of 300 heat units (beyond an intense capture), it is possible to have black cutworm larvae cutting plants. Let us know if you are monitoring traps and we will report your findings in the Pest Management and Development Newsletter. To check the status of black cutworm moths in your regions, read the Pest Management and Crop Development Newsletter http://www.ag.uiuc.edu/cespubs/pest/.

Weed-free fields of early-planted corn have the lowest potential for black cutworm damage. Females deposit eggs on grasses in pastures or in fence rows as well as densely growing weeds such as curled dock, yellow rocket, velvetleaf, and rough pigweed. Debris such as soybean residue in low-lying fields that have not been tilled also attract adult females. Late planted, weedy fields offer an invitation to this pest.

Corn plants at the 4-leaf stage of development are most susceptible to cutting and economic losses. Black cutworm larvae are able to cut plants once they reach their fourth-larval instar stage. Earlier instars feed on leaves, creating small holes on the leaf surface.

To scout your fields, check plants for leaf feeding, cut, wilting or missing plants. Look at 50 plants in five different locations throughout the field, especially those in low areas or with higher rates of residue. If damaged plants are found, dig around the plant to find at least 10 live cutworm larvae. Verify that they are black cutworms. Determine their instar stage by comparing the size of the larvae head capsule with the chart below.

This chart is also available in University of Illinois Extension, Black Cutworm Factsheet available from your local Extension office or at http://www.ipm.uiuc.edu/vegetables/insects/black_cutworm/

If the larvae are between instars 1 and 3, inspect the fields again in 24-48 hours. A large number of 4 and 5 instars are a greater concern than if they are at instar 7 and have completed most of their leaf feeding.

Consider a postemergence rescue treatment when 3 percentor more of the plants are cut and larvae are present. There are several insecticide rescue treatments to chose from. Soil moisture is critical for these to be effective. Detailed information on insecticide rescue treatments is available in the 2002 Illinois Agricultural Pest Management Handbook. To purchase a copy call 1-800-345-6087.