Last year, Iowa State University's Aaron Gassmann confirmed populations of western corn rootworms that are resistant to the Cry3Bb1 protein. At today's University of Illinois' Agronomy Day, U of I agronomist Mike Gray announced that he'd confirmed the exact same thing in Illinois.
Gray says a variety of factors account for the rapid adaptation of western corn rootworm populations to the Cry3Bb1 protein. Released in 2003, it was the first Bt protein on the marketplace. Rootworm populations have been grazing on Bt-protected roots for almost 10 years now.
Second, like other rootworm Bt proteins, Cry3Bb1 is a low-to-moderate-dose event. Gray says this differs from the use of Bt proteins in protecting against European corn borer. Though these hybrids have been in the marketplace since 1996, Gray says the high-dose event has helped protect against resistance evolution in European corn borer.
Next, it's a poorly-kept secret that refuge compliance is not 100%, Gray says. Adding to that, Gray notes many growers are disregarding the tenants of integrated pest management, and have been for years. An over-reliance on Cry3Bb1 technology, and only Cry3Bb1 technology, has been a primary cause for this technology's failure in parts of Iowa and Illinois.
Gray notes the potential loss of Cry3Bb1 technology has implications for Bt-protection against corn rootworm across the Cornbelt. Today, farmers can choose between three Bt traits: Cry3Bb1, mCry3A and Cry34/35Ab1. Gray notes mCry3A is very similar to Cry3Bb1. In many cases, growers who've lost Cry3Bb1 as an effective option may be down to only Cry34/35Ab1.
Gray says the lowering of refuge requirements from 20% to 5% could be a major problem. If Cry34/35Ab1 is the only effective Bt trait in parts of Iowa and Illinois, the selection pressure is significantly magnified in these 5% refuge-in-a-bag situations. Gray, along with 21 other university entomologists, authored a letter to EPA pointing this out in March. The EPA has yet to respond.