Illinois Natural History Survey - University of Illinois

Effectiveness of Crop Rotation on Corn Rootworms

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Since the early 1990s, a behavioral adaptation to crop rotation by the western corn rootworm has jeopardized crop rotation as a management practice for this pest. Beetles that once stayed in corn to feed and lay their eggs now leave cornfields to deposit eggs in fields of soybean and other crops. By laying their eggs in other areas, female beetles assure that some of their eggs will hatch where corn will be grown the following spring (larvae can only survive on the roots of corn and a few grassy weeds). This ability of the western corn rootworm to circumvent crop rotation has forced farmers in east-central Illinois, northern Indiana, southwest Michigan, and western Ohio to apply soil insecticides to their first-year (rotated) corn.

To monitor the magnitude of adult western corn rootworm movement from corn into soybean fields and to track changes over time, we have established beetle traps in pairs of adjacent corn and soybean fields at five sites in Illinois. Since 1998, traps have been established at a site near Urbana (Champaign County) in east-central Illinois where western corn rootworms are a problem in first-year corn, and at four remote University of Illinois Field Research Centers located outside the area where western corn rootworms are a problem in rotated corn. These remote sites are located near Shabbona (DeKalb County), Monmouth (Warren County), Perry (Pike County), and Dixon Springs (Pope County). On-site cooperators at each Field Research Center deploy traps by early July in a pair of adjacent corn and soybean fields. Traps contain an insert coated with insecticide and powdered squash, a feeding arrestant. Rootworm beetles enter the traps, feed on the squash powder, and in the process ingest a lethal dose of the insecticide. At each site, a line of 10 equally spaced (10-30 meters) traps are deployed perpendicular to the interface of the corn and soybean fields. Traps in soybeans are attached to posts at canopy level while traps in corn are attached directly to stalks at ear height. Beetles are collected from the traps and the inserts are replaced weekly through early September.

In 1998, although western corn rootworm adults were trapped in corn at all sites, they were scarce only in soybean field traps at the four remote sites. In Champaign County, beetles were initially detected in corn and remained most abundant there for three weeks. Thereafter, western corn rootworm population densities increased in soybeans and rapidly exceeded those in corn. Western corn rootworm movement into soybeans occurred at fairly low beetle densities and even while fresh corn silks were available. The DeKalb County site differed from the other remote sites in 1998 in that the soybean border trap caught nearly as many beetles as traps in corn. The 1999 results were similar to those from 1998 except in DeKalb County, where total western corn rootworm capture in soybeans equaled that in corn. In 2000 and 2001, the pattern of very few western corn rootworms being captured in soybeans in Warren, Pike, and Pope counties was again observed. As was the case in 1999, total beetle capture in soybeans was about the same as in corn at DeKalb County. Capture of western corn rootworms in soybeans again exceeded that captured in corn at Champaign County in 2000 and 2001.

High western corn rootworm beetle abundance in soybean fields continues to distinguish problem from nonproblem areas. The western corn rootworm strain that lays eggs outside of cornfields is spreading very slowly from its origin in east-central Illinois to the north and west. Adult western corn rootworm movement patterns from corn into soybeans in DeKalb County now matches those of problem areas like Champaign County. Warren, Pike, and Pope counties are currently unaffected by the new strain; crop rotation there remains an effective pest management practice for limiting western corn rootworm injury to corn roots.

This project was funded in part by the Illinois Council on Food and Agricultural Research and the Illinois Soybean Program Operating Board.

Eli Levine, Joseph L. Spencer, Timothy R. Mabry, and Scott A. Isard, Center for Economic Entomology

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