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Adult Corn Rootworm Suppression Programs: Some Comments

July 27, 2001
Earlier this week (July 23) a joint team of entomologists from Purdue University and the University of Illinois dug and evaluated roots (72 producers' fields) from the USDA-ARS­sponsored areawide corn rootworm suppression experiment (16 square miles), located just south of Sheldon, Illinois (Iroquois County, Illinois; Benton County, Indiana). The roots were rated for injury to assess the performance of an adulticidal bait (Invite + Sevin XLR) that was applied in 2000. Results from this multistate and multiyear study continue to be generated by many other investigators (Iowa, Kansas, South Dakota, Texas). One of the basic goals of the experiment is to suppress the overall corn rootworm population across an area to reduce the amount of insecticide that may be required to manage this pest in subsequent years. Although some soil insecticides provide adequate root protection against corn rootworm larvae, these products do not lower the overall population of this pest. Consequently, many producers continue to use soil insecticides each year. Successful implementation of an areawide management approach for corn rootworms may result in overall significant reductions in insecticide use. Final results from this investigation will be analyzed following the 2002 growing season.

For producers who are interested in implementing a corn rootworm adult suppression program, scouting efforts should have begun in earnest no later than mid-July. For continuous corn production systems, determine the average number of adults per plant by counting beetles on two plants selected at random in each of 25 areas of a field. Count all western and northern corn rootworm beetles each time. The counts take about 45 minutes in a 40-acre field. As you approach a plant, move quietly to avoid disturbing the beetles. Count the beetles on the entire plant, including ear tip, tassel, leaf surface, and behind leaf axils. Record the number of beetles you find per plant. If the average is greater than 0.75 beetle per plant in corn after corn or 0.5 beetle per plant (first-year corn) for any sampling date, consider the following options: (1) plan to rotate away from corn, (2) be prepared to use a soil insecticide at planting in 2002, or (3) initiate a program for the prevention of egg laying. If densities do not exceed these thresholds for any sampling date, rootworm larvae should not cause economic root damage to corn next year. By no later than mid-July, you should be committed to scouting for corn rootworm adults at least once each week through early September.

Producers involved in a beetle-suppression program may find an insecticide application warranted if beetle thresholds are reached and 10% of the females are gravid (with eggs). If more than 10% of the females within a field are gravid, significant egg laying may have occurred already. Late attempts (significant oviposition has occurred already) to reduce beetle densities to decrease root injury in 2002 may prove less than satisfactory.

Adult suppression programs should be conducted by professional consultants or by trained and experienced producers who are very familiar with this pest management system.--Mike Gray

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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