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Cutworms showing up in area corn and soybeans

Posted Monday, June 13, 2011, at 2:42 PM

During the past 10 days many reports of black cutworm damage to seedling corn plants and soybean have been received according to Wayne Bailey, University of Missouri entomologist.

Damage across the state on seedling corn and soybean plants has resulted from larvae produced from late moth flights. Normally black cutworm, low populations of black cutworm larval in soybeans goes unnoticed as surviving plants often compensate for lost plants. However, moderate to heavy feeding on soybean seedlings may substantially reduce crop yields if high seedling mortality occurs.

A good economic threshold for black cutworm feeding on soybean seedlings is lacking, 20% or more cutting has been used in past years. However with the higher commodity prices for soybeans, this economic threshold is probably too conservative. As commodity prices go higher, producers can benefit by treating pest infestations at lower thresholds. With this in mind, Bailey has recommended an economic threshold of 10% or more cutting as a more reasonable economic threshold in soybeans.

Bailey recommends that soybean producers monitor stands at least twice per week during the next month. Seed applied insecticide treatments on soybeans should help reduce black cutworm larval numbers, but may not prevent economic damage in situations where larval feeding is severe. In corn, these seed treatments often control about 50% of black cutworm under heavy infestation, according to Bailey. There are several insecticides available for rescue application. Be sure to follow label directions concerning whether the insecticide selected for rescue or replant needs to be incorporated or simply broadcast over the soil surface.

True armyworm larvae are occurring in some grass pastures in south central and southwestern regions of the state and in wheat and corn in scattered fields throughout Missouri. True armyworm larvae do not feed on legumes, only grasses. Larvae of true armyworms are often active at night or on cloudy days as they avoid light. To determine the presence of small larvae, scout plant debris on the ground and for feeding damage on lower plant foliage. As larvae increase in size, they will feed during both night and day periods and move upward on host plants as they consume foliage. Larger larvae tend to remain on the upper regions of host plants.

Economic thresholds vary with the crop:

1. Tall fescue and grass pastures -- Treat when an average of 4 or more half-grown or larger worms (1/2 inch to 1 inch) larvae per square foot are present during late spring.

2. Wheat -- Treat when an average of 4 or more half-grown or larger worms per square foot are present during late spring and before more than 2% to 3% of seed heads are cut. Wheat should be monitored several times per week after heading as true armyworm larvae can cut most heads from plants in a 2-3 day period once they begin cutting heads.

3. Corn-Treat corn seedlings when 25% or more of plants are being damaged. Control is justified after pollen shed if leaves above ear zone are being consumed by larvae.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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