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Western Corn Rootworms: The Variant Continues Its Spread

November 1, 2002
The variant western corn rootworm continued its expansion in Illinois during the summers of 2001 (Figure 1) and 2002 (Figure 2). Surveys were conducted in late July and early August each year by Joe Spencer, Illinois Natural History Survey, and Scott Isard, Department of Geography. They confirmed the presence of western corn rootworm adults in soybean fields in 59 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). In issue no. 23 of the Bulletin, severe first-year corn rootworm larval damage was reported 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 (2002) 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.

Similar survey efforts the previous year (Figure 1) revealed only 0.5 adult per 100 sweeps in soybeans in Pike County. 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.

On-Farm Root Injury Evaluations

In addition to the sweep-net surveys of soybean fields, Jared Schroeder, a graduate research assistant in the Department of Crop Sciences, coordinated an on-farm root-injury evaluation of first-year cornfields (following soybeans) in August 2002. His methodologies were similar to those used by Kevin Steffey and Don Kuhlman in their survey of rootworm larval injury in first-year cornfields during the late 1980s. Their study was conducted to determine the incidence of first-year corn rootworm larval damage in fields affected by northern corn rootworms that were able to prolong their
diapause.

They determined that areas of the state characterized by intensive rotation of corn and soybeans had the greatest chance for first-year corn rootworm larval damage caused by northern corn rootworms. In 2002, Jared Schroeder, working closely with IPM and Crop Systems Extension Educators, determined the level of first-year corn rootworm larval injury in 32 Illinois counties (Table 3). 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.

Similar to the results obtained by Kevin Steffey and Don Kuhlman in the late 1980s (northern corn rootworm first-year corn injury), the greatest concentration of first-year corn rootworm larval injury occurred in east-central Illinois. The percentages of plants with root-injury ratings greater than or equal to 3.0 (some root pruning, never equivalent to one node) for east-central counties were as follows: Champaign--4%, Ford--30%, Grundy--35%, Iroquois--16%, Kankakee--11%, LaSalle--66%, Livingston--22%, McLean--53%, Vermilion--9%, and Will--20%.

The frequency of root injury at or above the economic injury index of 3.0 was less in central Illinois counties: Christian--0%, Logan--2%, Macon--32%, Marshall--10%, Mason--2%, Peoria--2%, Sangamon--0%, Stark--0%, Tazewell--6%, and Woodford--4%. 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.--Mike Gray

Author: Mike Gray


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

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