Insect Update for Late May 2010

Published: June 2, 2010

In general insect problems have been low in number during the past two weeks. Insect problems reported during this time period have often been limited to small areas of the state with most fields not requiring rescue applications of insecticides. In general, bean leaf beetles are present in many early planted soybean fields where feeding damage is seen as small oval to round hole in foliage. This damage is generally minor if plants are actively growing and insect numbers remain below the economic threshold of 5 or more beetles present per foot of row and seedling plant mortality is not occurring. In corn, several minor pests have been found in higher numbers than most years. These include the southern corn rootworm, true armyworm, and the variegated cutworm. Southern corn rootworms (also called the spotted cucumber beetle) have been reported in relative high numbers in the southern third of the state. Although this beetle can feed on corn, it prefers to feed on a wide variety of vegetable crops. True armyworm can be a major problem in Missouri in some years with most damage occurring to fescue, wheat, and corn in this order. True armyworm is a relative rare pest of field corn in Missouri, although defoliation can be severe if true armyworm numbers are elevated in a field. Larvae of true armyworm are generally greenish-brown in color with a thin line running down the back and two orange lines running along each side of the body. A similar pest is variegated cutworm, a somewhat rare pest of seedling corn in the state. Variegated cutworm larvae are more common pests of alfalfa and white clover. Large larvae remaining in plant litter on the soil surface will devour newly emerging plant foliage of alfalfa and clover, often limiting plant regrowth and allowing for the establishment of broadleaf weed species. When they do attack corn seedling, they can cause severe defoliation and even cut plants similar to black cutworm. The larvae of variegated cutworm are brownish-gray to black with the identifying characteristic of 4 or more circular to diamond shaped yellowish-white spots running down the center of the back. Cool, wet spring weather favors the development of both true armyworm and variegated cutworm populations and may explain why they are present in higher than normal numbers this year.