Pesticides derived from the bacterium Bacillus thuringiensis — widely known as Bt — have been important to farmers since the 1920s.

Sixteen years ago, transgenic seed that produces insecticidal Bt proteins became available. Use of these transgenic Bt crops in the U.S. has reached 75 percent of cotton and 65 percent of corn acreage, while Bt pesticides are the most important insect control method available to organic farmers.

The Bt crop technology has saved producers millions of dollars by increasing yields and greatly reducing applications of broad-spectrum chemical pesticides. These factors make Bt important to both agriculture and human health.

Yet entomologists know that constant exposure creates evolutionary pressure for insect pests to become resistant to a pesticide. That’s why Bt plants are required to produce a high dose of insecticidal toxin and producers adopting Bt crops have been mandated to plant non-Bt refuges to bolster populations of susceptible insects in their fields. Susceptible insects emerging from these refuges mate with and dilute any resistant populations.

However, resistance to Bt crops has already emerged in India, China and Puerto Rico, where damage by resistant fall armyworms to a Bt corn variety resulted, for the first time in the U.S., in withdrawal of this variety from the market.

Although cold temperatures prevent their northward movement, Bt-resistant fall armyworms from Puerto Rico are believed to have migrated into Florida, representing a risk to southern growers and organic farmers.