Is the bollworm growing resistant to the last defense of Bt corn and cotton?

Entomologists in southern states are reporting that earworm/bollworm caterpillars are feeding on genetically modified Bt corn and Bt cotton designed to kill them, and surviving.

These crops express the Vip3A protein, developed by Syngenta to combat increased pest resistance to older Bt Cry proteins, found in three-gene cotton varieties (Bollgard 3, TwinLink Plus) and pyramided corn hybrids (Viptera, Leptra).

“It doesn’t bode well for the longevity of three-gene cotton,” said University of Arkansas Extension’s Gus Lorenz. “There is just so much pressure on [Vip3A] technology, especially in corn, which is the preferred host for Helicoverpa zea.”

To complicate matters, corn earworms lay eggs, hatch caterpillars, then adult moths move on to Bt cotton fields and continue laying eggs. Without Cry proteins, Vip3A is left to fight the battle all alone.

Although it’s too early to tell if the feeding on VipA3 corn and cotton indicates actual resistance, more testing is needed. Syngenta claims the Vip3A trait offers “excellent control” of corn earworm and cotton bollworm, and that no “performance issues” have been reported. It suggests growers implement Best Management Practices to slow down insect resistance to Bt traits.

Source: DTN Progressive Farmer

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