New B.t. Corn Exempted from Tolerance

In November, the EPA exempted the Cry3A protein developed by Syngenta Seeds from the tenebrionis strain of B.t.  The protein produced within the plants is coded for by genetic material inserted in event MIR604, so the lines carrying it are called MIR604 CRW (for corn rootworm).  It will have the brand name Agrisure® RW.  It is also stacked with other traits, such as herbicide (glufosinate, glyphosate) resistance, or corn borer protection.  The conditional registration expires on 9/30/10. 

The insecticidal activity of the protein occurs in the insect’s gut.  At a specific gut pH, which varies among insect groups, the protein breaks down the gut lining, followed by a breakdown of other body parts and collapse.  Even at very high doses, the Cry3A protein had no effects on mice.  The toxin is quickly broken down in simulated gastric juice, and is not glycosylated when expressed in corn, factors which both minimize risk of allergenicity.

The EPA expects Cry3A, as the third B.t.-based rootworm protectant, to lower seed prices and increase adoption of plant-incorporated protectants.  In the U.S. corn rootworm protection is the single largest use of conventional insecticides, many of which are restricted.  The Agency also stated that it found no significant risk of Cry3A-producing genetical material to wild plants or soil microbes.  The registration also specifies that at least 20 percent of planted acreage must be non-Cry3A corn to reduce potential resistance.  Syngenta must also study the fate of the protein in the soil and its effects on non-target invertebrates.  (Pesticide & Toxic Chemical News, 11/6/06).