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Surveys: Corn, soybean insect numbers low

Published: Wednesday, Sept. 4, 2013 5:30 a.m. CST

URBANA – Recent statewide surveys of insects in corn and soybean fields conducted by University of Illinois researchers in 28 counties across Illinois reveal densities of many insect pests remain at very low levels across the state, said Michael Gray, an Illinois 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 to 6 and Aug. 14 to 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.

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 to 6 time frame in 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) counties. 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.

Gray said the 2013 results in many respects mirror the survey results from 2011, when insect density levels also were 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 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,” Gary said. “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.”

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