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Statewide surveys in Illinois show overall low numbers of corn, soybean insects

Published August 23, 2013
late summer corn

URBANA, Ill. - Recent statewide surveys of insects in corn and soybean fields conducted by University of Illinois researchers in 28 counties across Illinois reveal that densities of many insect pests remain at very low levels across the state, said Michael Gray, a U of I professor of crop sciences.

The surveys were performed by sampling five randomly selected corn and five randomly selected soybean fields per county during two periods (Aug. 1-6, and Aug. 14-16). In each cornfield, 20 consecutive plants were examined for western corn rootworm adults. In soybeans, 100 sweeps per field were taken at least 12 rows from the field edge, Gray said.

Densities of western corn rootworm adults exceeded the 0.75 per plant (continuous corn) or 0.5 per plant (first-year corn) beetle thresholds during the Aug. 1-6 time frame in the following counties: Christian (0.91 adults per plant), Kankakee (0.98), Livingston (1.16), McDonough (0.47), McLean (0.63), Piatt (1.07), Ogle (0.84), and Whiteside (1.06). During the second sampling period (Aug. 14-16), only Kankakee (0.84) and Livingston (1.75) counties had averages that exceeded the per-plant western corn rootworm adult thresholds.

“Densities that reach or surpass the thresholds suggest that a producer should rotate away from corn in 2014 or consider the use of a Bt rootworm hybrid or apply a planting-time soil insecticide,” Gray said. 

The number of western corn rootworm adults in soybean fields throughout the state was very low during the Aug. 1-6 sampling period. The greatest number of beetles in soybeans occurred in Livingston County, with 13.8 beetles per 100 sweeps during this time frame. Gray said most soybean fields in the other counties had fewer than 5 beetles per 100 sweeps, with a range of 0 to 7.8 per 100 sweeps.

During the Aug. 14-16 sampling period, the number of western corn rootworm adults in soybean fields increased. The largest densities occurred in LaSalle (30.2 beetles per 100 sweeps) and Livingston (28.6) counties. The range in beetle numbers apart from these two counties was 0 to 11.8 per 100 sweeps.

“Although densities of western corn rootworm adults were somewhat greater in 2013 as compared with the most recent surveys conducted in 2011, they remain low by historic standards (mid-1990s and early 2000s), particularly in soybean fields,” Gray said.

The surveys also reveal that densities of other soybean insects during both sampling periods were very low and included: grasshoppers, green clover worms, soybean loopers, brown and green stink bugs, bean leaf beetles, and Japanese beetles. “Even though Japanese beetle densities were low in most counties, some counties in northwestern Illinois had impressive numbers (Ogle with 67.8 per 100 sweeps and Whiteside with 54.8 per 100 sweeps), especially during the August 14-16 sampling period,” Gray said.

No brown marmorated stink bugs were detected in any of the soybean or cornfields that were sampled. “This is somewhat surprising because this stink bug species has been detected in several counties throughout the state,” he added. 

Gray said that the 2013 results in many respects mirror the survey results from 2011, when insect density levels were also low across the state. “Reasons for this include several environmental factors such as wet springs, the record drought of 2012, extensive use of Bt hybrids, and the widespread broadcast applications of pyrethroid insecticides (tank mixed with fungicides) to corn and soybean fields in recent years,” he said.

Gray stressed that the goal of integrated pest management (IPM) is to keep pest numbers below economic injury levels by the thoughtful integration of several management tactics and that near elimination of pest densities is not the objective.

“As the classic definition of IPM indicates, implementation will help promote favorable economic, environmental, and sociological consequences. Excessive use of inputs, used primarily as an insurance approach, will hasten the onset of resistance and shorten the longevity of some very useful management tools,” he said.

More specifics on the results of this statewide survey will be shared at the Corn and Soybean Classics in January 2014.

Gray added that Ron Estes, a principal research specialist, and Nick Tinsley, a research specialist, both in the Department of Crop Sciences at U of I, provided leadership in conducting the surveys.