A CONFLUENCE of factors expected to cause heavy corn rootworm (CRW) damage in parts of the Corn Belt could help answer questions about transgenic CRW hybrids that arose last year.

Early planting, a cool spring that delayed larvae development, followed by drought and an unusually large rootworm larvae hatch in parts of the Corn Belt could put all CRW control technologies to the test this year. This combination of factors, which was prevalent across east-central Illinois through mid June, already has resulted in reports of aggressive feeding on corn roots, even in fields treated with CRW insecticides.

The challenging CRW control environment showed up in University of Illinois field plots in mid June. “Right now, the only things in our CRW trials that aren't showing moisture stress are YieldGard Rootworm hybrids,” Kevin Steffey, University of Illinois entomologist, said in late June.

Unexpected root damage

Questions about CRW hybrid performance were raised last year when a small number of fields — reportedly about 20 out of thousands of fields planted to YieldGard Rootworm and YieldGard Plus hybrids by 20,000 customers — experienced unexpected root damage.

One of the affected fields was a University of Illinois CRW test plot, which placed entomologists from the university in the spotlight as they considered possible reasons for the performance glitch. The spotlight refocused on Illinois entomologist Michael Gray this spring when he readdressed transgenic hybrid performance questions in the May 6 edition of the university's online agronomy newsletter The Bulletin (www.ipm.uiuc.edu/bulletin).

Writing in response to Monsanto research published in the journal Crop Science in late March, Gray highlighted revelations that expression of the Bt protein in YieldGard Rootworm and YieldGard Plus declined as corn plants matured from the V4 to V9 corn growth stages. The decline averaged about 37% across the five hybrids in the Monsanto test, dropping from 69.8 parts per million (ppm) to 44.0 ppm on average. Despite the decline in the expression of the Bt protein, the root protection afforded by each transgenic hybrid was excellent.

In an interview, Gray said the decline could explain late-season feeding on roots of the CRW hybrid in the Illinois plot last year, which was planted in early April.

“Planters are rolling in many years in very early April,” he said. “Whether you plant a transgenic hybrid or use an insecticide, you put more pressure on these products to perform. Expression declines, combined with early planting, could cause some concerns.”

Scientists also have noted that ingestion of the Bt protein by CRW larvae delays their maturation. Depending on planting dates and environmental conditions during the growing season, this maturation delay could have negative consequences late in the season, Gray said.

Consistent protection

For its part, Monsanto scientists point out that its transgenic CRW technology has performed well over millions of acres.

“Our technology has provided 94% consistent performance in university trials, compared to 63 to 69% consistency for the top CRW insecticides,” says John Goette, the transgenic CRW technology lead for Monsanto. “This is a great story for consistent performance. But this also says that 6% of the time there wasn't stellar performance. Nothing is bulletproof, but this is the most consistent protection you can buy, bar none. This might be the year when YieldGard Rootworm really shows its colors if rootworm insecticides stumble.”

Goette doubts that early planting will prove to be a challenge to Monsanto's transgenic CRW technology, since it has performed well on early- and late-planted fields the past two years and in earlier field trials. “Based on two seasons of use and a doubling of acres this year (to almost 5 million acres), we feel very confident,” he says. “Despite the decline in Bt protein expression, the protection is excellent.”

Currently, YieldGard Rootworm and YieldGard Plus hybrids are the only transgenic CRW hybrids on the market. But in 2006, pending regulatory approval, Pioneer Hi-Bred International and Dow AgroSciences plan to market transgenic CRW hybrids under the Herculex RW and Herculex Xtra brands. Scientific studies on Bt protein expression from the Pioneer/Dow AgroSciences CRW hybrids have not been published. But Dow AgroSciences scientists say there has been no evidence of a decline in expression of the Bt protein in studies to date.