No. 18 Article 1/August 5, 2011

Survey of Illinois Cornfields Reveals Exceedingly Low Densities of Western Corn Rootworm Adults

In an earlier article (July 15, "Western Corn Rootworm Rebounds in Some Illinois Locations"), I reported that root pruning by corn rootworm larvae was above 2010 levels in some of our trials at University of Illinois research and education centers. Given these root injury evaluations, I was surprised by the results of a survey last week of western corn rootworm adults in 24 Illinois counties. Five fields were selected at random in each county (Figure 1); in each field, 20 plants were inspected for western corn rootworm adults. Plants were assessed from the ground up by careful examination of the stalk, leaves, leaf axils, and ear tip. Whole-plant-count averages were calculated for each county (n = 100).


Figure 1. Sampling locations for an Illinois survey for western corn rootworms conducted July 27 and 28.

Averages for the great majority of counties were very low (Figure 2). In general, an average of 0.75 to 1.0 beetle per plant is suggested as a density that could lead to economic infestations of larvae the following season. Producers with a density of 0.75 to 1.0 beetle per plant in a field are encouraged to rotate to soybeans the following season or consider using a Bt hybrid or a soil insecticide labeled for corn rootworms at planting. The whole-plant-count threshold should be dropped to 0.5 beetle per plant in first-year corn because more of the adults are females and likely to lay eggs in the sampled field. I recognize that most producers no longer use these thresholds to determine the likelihood of an economic infestation the following season. Consequently, inputs such as Bt hybrids and soil insecticides are used in a prophylactic fashion. Ultimately, the overuse of these inputs has consequences, such as resistance development.


Figure 2. County per-plant averages for the Illinois survey for western corn rootworms conducted July 27 and 28.

In the most recent survey, only two counties had densities that exceeded the 0.75 beetle-per-plant threshold (McLean, 0.81 beetle per plant) or approached it (Lee, 0.64 beetle per plant). No western corn rootworm adults were found in the cornfields sampled in southern Illinois, and densities were also very low throughout central Illinois. In Knox and Fulton counties, no western corn rootworm adults were discovered. These counties are historically known as "good" candidates for economic rootworm injury. Where have the western corn rootworm adults gone? I've mentioned potential factors previously; they include saturated soils in late May and early June in the springs of 2009 and 2010; increasing popularity of Bt rootworm hybrids; and intense use of fungicide/insecticide tank-mixes broadcast in both corn and soybean fields.

We will conduct a second round of surveys in mid-August. My observations in Bureau, Fulton, Knox, Lee, Warren, and Whiteside counties revealed very low densities of other insect species as well as of western corn rootworm adults. Aside from some Japanese beetles, most plants had no insects on them, including beneficial insects. We intend to conduct surveys again next year to determine if these surprising results are repeated.

Thanks to Ron Estes, Andy Morehouse, and Nick Tinsley, Department of Crop Sciences, for leading survey teams throughout the state.--Mike Gray

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