Issue No. 15, Article 1/July 3, 2008
Still Waiting on Corn Rootworm Adults
Although we are now in July, we had not yet received any reports of western corn rootworm adults in cornfields. By now we could have expected observations of western corn rootworm adults in southern Illinois, but with the continued, excessive rainfall this spring, rootworm eggs and larvae may not have survived in the saturated or ponded fields. We have wondered how the excess moisture in central Illinois might have affected corn rootworms, and we finally have insight from one field in Champaign County. Joe Spencer, research entomologist with the Illinois Natural History Survey, thoroughly inspected one of his research trials just south of Champaign on July 29 and 30 and reported some interesting observations.
Joe and his crew found mostly third instar (fully grown) larvae, several in the prepupal stage, and about one pupa per plant, suggesting that initial emergence of the adults (males first, followed by females) is imminent. He also observed significant larval injury to some of the root systems from a plot of corn (not a rootworm Bt hybrid) planted after soybeans, with no protection against rootworms. On the 0-to-3 node-injury rating scale developed at Iowa State University, the injury to the roots in the photo would likely rate as equal to or greater than 1 (at least an entire node of roots pruned).
Corn rootworm larval injury to roots of corn planted after soybeans, Champaign County, June 29, 2008 (photo courtesy of Joe Spencer, Illinois Natural History Survey).
Joe indicated that the field where significant root injury was observed was planted on May 5 and did not have any long-term standing water. He was on his way to check another trial area northeast of Urbana that was flooded during some of June. Comparison of his observations in that field with his observations in a nonflooded area should give us some relative sense of the mortality of rootworm larvae in flooded soils.
Before we turn all of our attention to the adults soon (their emergence during pollination will threaten some fields), it might be prudent to check out the level of injury (or lack of injury) caused by corn rootworm larvae, getting an early look at the performance (or lack of performance) of rootworm control products. We hope to begin evaluating our rootworm management trials during the week of July 14, delaying our assessments to accommodate the slower development of rootworms this year.
We'll discuss management of corn rootworm adults in pollinating cornfields in next week's issue of the Bulletin.--Kevin Steffey