Michael E. Gray Professor and Associate Head, Department of Crop Sciences Phone: (217) 333-6652 E-mail: m-gray4@illinois.edu

The summer of 2002 will be remembered by many producers as one that proved challenging regarding the management of two beetle species in their corn and soybean fields. These two insect species, Japanese beetles (Popillia japonica Newman) and the western corn rootworm (Diabrotica virgifera virgifera LeConte) were both "featured" many times this past season in our Pest Management and Crop Development Bulletin. Many farmers, particularly those in east central Illinois, indicated that they couldn't recall a summer in which densities of Japanese beetles had reached the sobering levels experienced in 2002.

How many Illinois counties support infestations of Japanese beetles in their field crops? Joe Spencer of the Center for Economic Entomology (Illinois Natural History Survey) and Scott Isard of the Department of Geography used sweep nets to sample soybean fields in each Illinois county during 2002. Results from their survey revealed that Japanese beetles could be found in soybean fields in 59 counties. Densities were greatest in east central Illinois—ironically, the area of the state infamous for a failed eradication attempt in the mid to late 1950s by departments of agriculture at federal and state levels.

Japanese beetles cause injury to corn plants by feeding on root hairs as grubs and on silks as adults. By feeding on root hairs, the grubs may interfere with efficient uptake of water and nutrients, especially phosphorous. Nutrient deficiencies (phosphorous) in seedling corn plants may result in a purpling of stems. Severely infested fields may have plant stands significantly reduced and warrant replanting. Silk feeding by adults may interfere with the pollination process, especially in drought years. Much of the Japanese adult beetle feeding (defoliation) in soybeans occurs during the reproductive stages of development. Although the grubs will feed on soybean roots, the injury is generally not of economic importance.

Preventing injury to roots (grubs) and silks (adults) caused by Japanese beetles has proved to be a frustrating experience for many entomologists and producers. In 1981, Steve Briggs and Don Kuhlman, entomologists with the Illinois Natural History Survey, indicated that both Counter 15G (two pounds of active ingredient per acre) and Lorsban 15G (one pound of active ingredient per acre) provided only an 18 percent reduction (evaluated on May 26, 1981) in the number of Japanese beetle grubs in an insecticide efficacy trial in Iroquois County. Each product was applied in a 7-inch band over the row and incorporated at planting on May 7, 1981. The following year (1982), they reported that infestations of Japanese beetles were very high in east central Illinois. They offered the following comments regarding an experiment performed to determine the effect of silk clipping on yield: "Tests conducted during 1982 with no, three, five, and ten beetles caged per ear tip at silk emergence were inconclusive in determining the number required to affect pollination. Ten beetles per ear tip did not reduce ear weight. It should be pointed out, however, that the beetles remained caged on the ear tips for only five days." Clearly, more work is warranted with respect to improving our economic thresholds for Japanese beetles and also increasing our understanding of the efficacy for various insecticides that are targeted at the grubs.

Our current economic threshold for Japanese beetles in corn is as follows: "Treat during the silking period to protect silks if there are three or more beetles per ear and pollination is not complete." To prevent economic damage in soybeans, we recommend that producers consider a rescue treatment when "defoliation reaches 30 percent before bloom and 20 percent between bloom and pod fill." Successfully predicting the severity of future infestations of Japanese beetles will most likely prove to be a frustrating experience. However, we can offer some general expectations regarding the population fluctuations of this insect. For instance, producers should anticipate economic densities of Japanese beetles after mild winters, followed by early planting (first two weeks of April).

Western Corn Rootworms: The Variant Continues its Spread

The variant western corn rootworm continued its expansion in Illinois during the summer of 2002. Surveys were conducted in late July and early August by Joe Spencer and Scott Isard. They confirmed the presence of western corn rootworm adults in soybean fields in 58 counties of the state. The greatest densities occurred in east central Illinois, an area with the largest concentration of rotated corn acres. In 2002, western corn rootworms also were common inhabitants of soybean fields in northeastern counties (Lake County). Severe first-year corn rootworm larval damage occurred in Lake County in corn following soybeans as well as in corn planted after wheat. Although western corn rootworms were found far less often in soybean fields in western and northwestern counties, 11 adults per 100 sweeps were collected from soybeans in Pike County, an ominous early-warning signal that crop rotation may begin to fail as a pest management strategy for this insect species in western Illinois. Farmers, even in western and northwestern counties, are encouraged to use Pherocon AM traps (yellow-sticky traps) to monitor their soybean fields for variant western corn rootworm adults. If densities begin to approach five adults per trap per day in soybean fields, producers are encouraged to consider the use of a soil insecticide on rotated corn acres.

We initiated an on-farm root injury evaluation of first-year cornfields (following soybeans) in August of 2002. Jared Schroeder, a graduate student in the Department of Crop Sciences, worked closely with IPM and Crop Systems Extension Educators and determined the level of first-year corn rootworm larval injury in 32 Illinois counties. In each county, 10 rotated cornfields were selected at random. In each field, 10 plants were selected at random, and the roots were washed and rated for injury on the Iowa State 1 to 6 injury scale. The greatest concentration of first-year corn rootworm larval injury occurred in east central Illinois. The percentage of plants with root injury ratings greater than or equal to 3.0 (some root pruning, never equivalent to 1 node) for east central counties was as follows:

The frequency of root injury at or above the economic injury index of 3.0 was less in central Illinois counties:

Two northern Illinois counties, DeKalb and Lee, had 6% and 14%, respectively, of roots with injury ratings at or above 3.0. None of the roots from 10 counties located in the western region of the state had been pruned. These counties included Adams, Brown, Fulton, Hancock, Knox, McDonough, Mercer, Pike, Schuyler, and Warren. Care must be exercised in the interpretation of these root rating data. For instance, root injury ratings would undoubtedly have been greater in east central Illinois if it were not for the common practice of using soil insecticides on first-year corn. These data also seem to suggest that although western corn rootworm adults are beginning to appear in some soybean fields of western Illinois, egg-laying in these fields is likely below economic levels at this point in time. Again, we advise producers to use Pherocon AM traps (even in western counties) to make more informed management decisions.

There is considerable anticipation on the part of producers regarding the potential commercialization of transgenic hybrids for corn rootworm management. In addition, interest in the reliability of seed treatments for rootworm management continues. During the next five years, a transition most likely will occur as producers lean more toward seed treatment and transgenic technologies as the "backbone" of their rootworm management programs. Obviously, much has to occur before this transition begins to take shape. Most notably, the U.S. Environmental Protection Agency will need to approve the use of transgenic hybrids for corn rootworm control. In addition, to date, seed treatments have not shown that they provide consistent root protection against heavy corn rootworm infestations. I think that systemic seed treatments have great potential; however, we still have much to learn.

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