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Issue No. 19, Article 1/July 31, 2009

Root Injury Ratings for Corn Rootworm Trials Have Begun--Overall Western Corn Rootworm Numbers Are Down

Our annual "root digs" have been under way the past week to evaluate the efficacy of various treatments aimed at protecting the root systems of corn plants from corn rootworm larval injury. So far we have completed the root injury ratings in our standard corn rootworm trials at University of Illinois Research and Education Centers located near Monmouth, Perry, and Urbana.

Overall, rootworm "pressure" was low to moderate in Monmouth and Perry. Injury in the check treatments was greater in Urbana. On July 29 our crew led by Ron Estes, senior research specialist, is traveling to the Research and Education Center near DeKalb to dig roots from our standard experiment and return them to Urbana for washing and rating. I look forward to sharing the results of these evaluations later this fall and winter with clientele throughout the state.

A general observation shared by many entomologists this season has been the overall low density of several insect pests, including western corn rootworms. Joe Spencer, a research entomologist with the Illinois Natural History Survey, indicates that exceedingly low numbers of western corn rootworm adults are being caught on Pherocon AM traps in east-central Illinois production fields. The numbers of beetles being trapped are considerably below economic thresholds, suggesting that many corn fields in 2010 will have low corn rootworm pressure. Yet most producers are not monitoring for western corn rootworm adults and will automatically decide to use either a Bt hybrid, a soil insecticide, or both next year. Not exactly a fundamentally sound IPM approach. This accelerated use of insect management inputs, regardless of insect abundance, will have unintended consequences at some point. This has been the history of western corn rootworms to date.

Why are we seeing such low numbers of western corn rootworm adults? One of the weak links in the life cycle of corn rootworms occurs when larval hatch takes place in saturated soils--precisely the scenario that unfolded in many areas of the state this year. Another potential explanation is the increasing use of Bt stacked hybrids targeted at the lepidopteran complex and corn rootworms. Despite the heavy use of soil insecticides in continuous corn throughout the 1960s, '70s, '80s, and mid-'90s across the northern third of Illinois, western corn rootworm populations flourished. Why? Recall that the granular soil insecticides were applied in a band (7-inch) or in the seed furrow. Rootworm larvae outside of this soil insecticide zone, in essence between corn rows, were not exposed to these products. Consequently, adult emergence was typically plentiful each season, setting the stage for more corn rootworm challenges the following year. Soil insecticides applied in this fashion did their "job"--that is, they protected the root system near the base of the plant and prevented lodging. In addition, producers were unwittingly utilizing a refuge strategy each season by banding soil insecticides and leaving larvae unexposed to toxins between corn rows. This explains why resistance to these soil insecticide products has not emerged despite decades of heavy use. Where insecticide resistance has occurred, such as in Nebraska, the explanation was repeated use of broadcast insecticide treatments to suppress egg laying by adult western corn rootworms.

Western corn rootworm adults from Urbana experiment, July 23, 2009.

Severe root pruning in check, corn rootworm trial, Urbana, IL, July 28, 2009.

As the usage of Bt stacked hybrids accelerates and refuge size is reduced, I anticipate overall reductions in corn rootworm populations across the state. Unlike the granular soil insecticides applied in a band, or in-furrow, Cry proteins are expressed throughout the root systems of Bt plants and therefore expose corn rootworm larvae both in the row and between corn rows. In future years, several Cry proteins will be aimed at corn rootworms within the same root system of corn plants. This should result in less root injury and adult emergence. We have witnessed the historically low densities of European corn borers across Illinois and some nearby states that are now believed to be linked to the widespread adoption of Bt corn hybrids. Will we see a similar phenomenon unfold with western corn rootworms? I suspect we might be headed down this road. Will western corn rootworms adapt as they have repeatedly done so in the past? If we don't integrate management tactics, we could have the answer sooner than we would like.--Mike Gray

Mike Gray

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