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A cocktail of insecticides containing a plant protein and a common insecticide may be more lethal to crop pests than either ingredient used alone, according to biologists. The one-two punch also inhibits the insects' growth rate and reduces their chance of developing resistance.

"We found a synergistic effect, where the two insecticides together decreased the growth rate of caterpillars more than either one did alone," said Dawn Luthe, professor of plant stress biology at Penn State. "The insect mortality rate was also much greater than the sum of mortality rates when only one insecticide was used."

One of those insecticides, Bt, is commonly used around the world. When insects feed on plants genetically modified to produce Bt, the toxin binds to chemical receptors lining the insects' midgut. This disrupts the receptors function, eventually killing the insects. But researchers say some insects always survive the ordeal and over time subsequent populations could develop resistance to the toxin.

Luthe and her colleagues Srinidhi Mohan and Peter W. K. Ma, post-doctoral student and professor at Mississippi State University, and W. Paul Williams, research geneticist at the U.S Department of Agriculture studied a unique plant-based insecticide known as Mir1-CP.

Their goal is to see if Mir1-CP, when used in tandem with other biological pesticides, such as the Bt toxin, can prevent pests from developing resistance and make the toxin more effective.

"This protein, which we developed from certain strains of corn from Antigua, breaks down other proteins and peptides in the insects' gut," said Luthe, whose findings appear in the current issue of PLoS ONE.

Unlike Bt, Mir1-CP breaks down proteins in a protective membrane covering the midgut. This membrane acts as a barrier that protects the caterpillar from toxins in the diet, and cycles nutrients to the midgut.

"It is the caterpillar's first line of defense agai
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