According to Tabashnik, Cry3Bb1 is effective enough to be economically useful, but not effective enough to meet the so-called high dose standard, the ability to kill at least 99.99 percent of susceptible pests and also nearly all of the hybrid pests that are produced when resistant pests mate with susceptible pests.
"When Bt crops meet the high dose standard, resistant individuals are extremely rare, and smaller refuges work fine, because you might have one resistant insect in a million. In this case, a 20 percent refuge provides enough susceptible individuals to dilute that rare resistance."
But plants with Cry3Bb1 allow survival of 1 to 6 percent of pests, which is expected to quickly select for resistance.
"A single farm can have millions of these beetles," Tabashnik explained. "If 1 to 6 percent survive on Bt corn, you have tens of thousands of potentially resistant insects and the refuge needs to be much bigger."
Tabashnik's research has shown that in Arizona, Bt cotton meets the high dose standard against pink bollworm and the small refuge strategy has prevented resistance for more than a decade.
On the other hand, Tabashnik pointed to a case in Puerto Rico, where adequate refuges were not planted. Within a few years, the pests evolved resistance and devoured the Bt corn plants. The biotechnology companies voluntary stopped selling Bt corn seed there, but five years later, the insects remain resistant to the toxin.
Although biotech companies recently starting selling some varieties of Bt corn that produce combinations of Bt toxins, Tabashnik said, the resistance to one toxin still raises concerns.
"You can think of the multi-Bt toxin approach as a pyramid: The base has to be stable. If one of your building blocks, which is susceptibility to Cry3Bb1, is crumbling, you have a problem.
|Contact: Daniel Stolte|
University of Arizona