URBANA, Ill. — The western corn rootworm (WCR) season is progressing at an unprecedented pace, reported Mike Gray, University of Illinois professor of entomology and crop sciences extension coordinator.

Reports of severe injury to Bt corn that expresses the Cry3Bb1 protein targeted against corn rootworms have come in already this season, from western Cass County. In 2011, similar reports of injury to Bt hybrids expressing this protein surfaced in other North-Central states, particularly in Iowa.

On Aug. 16, 2011, Gray verified severe corn rootworm pruning on some Bt hybrids that express the Cry3Bb1 protein in Henry and Whiteside counties, located in northwestern Illinois. The fields were in continuous corn production systems for many years, and the producers had relied upon Bt hybrids that expressed the Cry3Bb1 protein as their primary protection against WCR injury.

“Thus far (in 2012), resistance to this protein in Illinois has not been confirmed,” Gray said.

However, large numbers of WCR adult beetles are visible in the whorls of plants in Cass County. Seed industry representatives confirmed that beetles have been observed nearly a full month earlier than when they are usually first reported, around the Fourth of July. The plants were under intense moisture stress, the leaves were rolled tightly and beetles were feeding on the epidermis of corn leaves.

“This type of injury often occurs when beetle emergence is early and plants have not yet begun to shed pollen or produce silks,” Gray said. The plants in the fields visited were not at this reproductive stage of development.

Roots were removed randomly from this field and from a nearby field that also had been planted to a Bt hybrid expressing the Cry3Bb1 protein. Roots from both fields exhibited feeding injury, with more injury in the second field, although fewer beetles were observed there.

Because adults were not as noticeable in the second field and emergence was just beginning in the first field, root feeding is expected to continue in both fields. No lodging was observed, probably because the plants were short and the soil was hard and dry.

“However, as root feeding continues and plants become taller, lodging should be expected, especially if storms with rain and winds materialize,” said Gray.

On June 8, the plants that had been evaluated for root injury were checked for the expression of the Cry3Bb1 protein at the University of Illinois laboratory. All tested positive. This does not mean a resistant WCR population has been confirmed in Illinois. The registrant of this technology has been notified and will conduct some follow-up investigations in these fields.

At this point, it is not clear why performance challenges of some Bt hybrids expressing this protein are continuing; however, experts say producers should remain vigilant during this growing season and report any performance issues with corn rootworm injury on their Bt hybrids.

Like the fields that had Bt performance issues last year, the fields in Cass County had been in continuous corn for many years (at least 10 consecutive). Moreover, the same trait (Cry3Bb1) had been used since 2007 six growing seasons. Under these conditions, the selection pressure for resistance development is increased markedly.

“It remains to be seen how the rest of this growing season will play out,” said Gray. “For now, it makes sense to monitor this unfolding situation carefully and pay attention to the performance of Bt hybrids this season. As reports come in, we will share them, but we just can’t predict what will happen. I wish I knew what to expect.”

Longer-term, it will be increasingly important to integrate management practices such as: the rotation of corn with other crops, rotation of Bt traits from season to season, the use of a non-Bt hybrid along with a soil insecticide at planting and use of refuges when planting a Bt hybrid.

More information can be found online at http://bulletin.ipm.illinois.edu