IPM For IRM: Monitoring Corn Rootworm Abundance
and Root Injury In Illinois

Joseph L. Spencer Assistant Professional Scientist Center for Economic Entomology Illinois Natural History Survey (217) 244-6851; spencer1@illinois.edu

Jared B. Schroeder Graduate Research Assistant Department of Crop Sciences (217) 244-2687; jbschroe@illinois.edu

Figure 1: Severe lodging in La Salle County first-year corn, 2002.

Before the mid 1990s, annual corn-soybean rotation provided excellent control of rootworm injury in first-year corn. In 1995, root injury in first-year corn was widespread in east-central Illinois due to western corn rootworm egglaying in soybean fields. Circumvention of crop rotation by the western corn rootworm is now a perennial concern across an expanding area in northern and east-central Illinois

and surrounding states. To date, pest management recommendations have primarily relied upon grower use of Pherocon AM traps to monitor western corn rootworm beetles in fields rotated with corn.

Farmers are encouraged to deploy 12 traps in soybean fields for a four-week period beginning in late July. If western corn rootworm adult captures exceed five beetles per trap per day, farmers are urged to consider soil insecticide use during planting the following spring. The protocol may also be used by farmers considering transgenic corn hybrids that are protected against larval feeding. Beetle monitoring and judicious use of either management option may significantly reduce unnecessary soil insecticide use. It is the responsibility of producers choosing transgenic corn technology to follow the 20 percent refuge requirements when planting. Refuges help preserve transgenic corn technology by providing susceptible beetles that will dilute the impact of any rare, resistant beetles that emerge from transgenic cornfields.

Figure 2. Illinois on-farm survey of western corn rootworm larval damage in rotated corn.

Sampling beetle populations in Illinois soybean fields indicates that the area affected by rotation-resistant western corn rootworms is expanding into northern and north-central Illinois. Beetle monitoring in soybeans is especially important on the fringe of the rotationresistance epicenter. Failure to recognize the rootworm threat could lead to serious losses in first-year corn (Figure 1). On-farm surveys in 2002 and 2003 determined the distribution of rotation-resistant western corn rootworms by evaluating root injury in first-year corn. The study evaluated 1,600 root systems annually from 32 central Illinois counties using the 1.0-to-6.0 Iowa State scale. Root ratings above 3.0 indicate that economic injury may result from larval root feeding. In 2002, root injury above the economic injury level occurred in 17 counties, indicating that rotation-resistant beetle populations in these areas must have exceeded the five beetles/trap/day economic threshold the previous summer. These survey data help alert growers of the need to monitor adult western corn rootworms in fields that will be rotated to corn the following year.

The 2002 survey suggests that soil insecticides are still effective for managing WCR injury in first-year corn. For example, the survey found that rootworm adults and their eggs were abundant in Champaign and La Salle County soybean fields. However, the average root injury ratings in Champaign Co., where awareness of rotation resistance is high, were below the economic threshold, while root ratings for La Salle Co., on the fringe of problem in 2001, averaged above 3.0. (In La Salle Co., 66 percent of plants had root ratings of greater than or equal to 3.0, compared to only 4 percent in Champaign Co.) (Figure 2). We attribute the lower root injury ratings in Champaign Co. to greater grower awareness of the rotationresistance problem and management actions in response to beetle monitoring information. Monitoring western corn rootworm populations in rotated soybeans is a prerequisite to making wise management decisions about using soil insecticides or transgenic corn.

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