One-two punch insecticide combo more lethal to crop pests: Study

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Washington, Mar 12 : In a new study, biologists have cited that a one-two punch combination of insecticides containing a plant protein and a common insecticide may prove to be more fatal to crop pests than either ingredient used alone.

This new strategy used by Dawn Luthe, professor of plant stress biology at Penn State and colleagues, also inhibits the insects' growth rate and reduces their chance of developing resistance.

One of the two insecticides used in this one-two punch approach is Bt, which is commonly used around the world, while the other is a unique plant-based insecticide known as Mir1-CP.

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

When insects feed on plants genetically modified to produce Bt, the toxin binds to chemical receptors lining the insects' midgut. This disrupts the receptor's function, eventually killing the insects. However, the researchers said that some insects always survive this attack and over time subsequent populations could develop resistance to the toxin.

The researchers then the plant-based insecticide known as Mir1-CP with an aim to see if Mir1-CP, when used in combination 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,

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

"It is the caterpillar's first line of defense against toxins and chemicals in its diet," said Luthe.

Insects were then given a sub-lethal dose of the two insecticides to test the effectiveness of both insecticides on the pests. It was found that when used alone, a concentration of Bt at five parts per billion killed 4pct of all corn earworms and 5pct of tobacco budworms. Mir1-CP, when used at a concentration of 60 parts per billion, killed 8pct of the corn earworms and 3pct of the tobacco budworms.

However, when the two insecticides were added together, the mixture killed 61pct of corn earworms and 57pct of tobacco budworms, which is more than 10 times better than either by itself.

When the insecticides were used at slightly different strengths, similar results were seen against the fall armyworm and the southwest corn borer. Also, the study indicates a significant decrease in the growth rate of the survivors, in addition to a high mortality rate among the insects.

"We think that Mir1-CP is making holes in the membrane, which in turn is making it easier for the Bt toxins to reach the insects' midgut," explained Luthe.

Luthe said that the findings have important implications for agriculture because each year insects cause major losses to farmers. Almost 20 pct of major crops worldwide are lost to insects.

The findings of this study appear in the current issue of PLoS ONE.

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