Statewide Surveys in Illinois Reveal Overall Low Numbers of Corn and Soybean Insects

Recently, statewide surveys of insects in corn and soybean fields were conducted in 28 counties across Illinois. The surveys were performed by sampling five corn and five soybean fields (randomly selected) per county during two periods (August 1 through 6, and August 14 through 16). In each cornfield, 20 consecutive plants were examined for western corn rootworm adults. In soybeans, 100 sweeps were taken (at least 12 rows from the field edge) per field. 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 August 1 to August 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 (August 14 to 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.

The number of western corn rootworm adults in soybean fields throughout the state was very low during the August 1 to 6 sampling period. The greatest number of beetles in soybeans occurred in Livingston County, 13.8 beetles per 100 sweeps, during this time frame. Most soybean fields in the other counties had fewer than 5 beetles per 100 sweeps (range = 0 to 7.8 per 100 sweeps). During the August 14 to 16 sampling period, the number of western corn rooworm 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-1990’s and early 2000’s), particularly in soybean fields.

Densities of other soybean insects during both sampling periods were very low and included: grasshoppers, green cloverworms, 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, especially during the August 14 and 16 sampling period (Ogle – 67.8 per 100 sweeps, Whiteside – 54.8 per 100 sweeps).No brown marmorated stink bugs were detected in any of the soybean or cornfields that were sampled. This is somewhat surprising since this stink bug species has been detected in several counties throughout the state.

More specifics on the results of this statewide survey will be shared at the Corn and Soybean Classics in January 2014. In many respects, the results of this year’s survey mirror those of 2011, that is — densities of many insect pests remain at very low levels across Illinois. Reasons for this include several environmental factors (e.g. wet springs, 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. An important point to remember — the goal of integrated pest management is to keep pest numbers below economic injury levels by the thoughtful integration of several management tactics. Near elimination of pest densities is not the objective. As the classic definition of IPM indicates — implementation of IPM 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.

I offer my thanks to Ron Estes, Principal Research Specialist in Agriculture, and Nick Tinsley, Research Specialist, both in the Department of Crop Sciences at the University of Illinois, for providing the leadership to ensure this survey was properly conducted.

Mike Gray