24 July 1998

New insecticide might end fears of Bt gene over-use

By Peter McGrath

FEARS about the control of insects which have become resistant to insecticides could be eased by the discovery of an alternative control method in Canada.

Concerns have been particularly great where the Bt gene from the bacterium Bacillus thuringiensis is used to produce an insect toxin in genetically modified crops.

Bt-cotton, maize and potatoes are all widely grown in the US. But there are fears that over-use could hit its effectiveness. Organic farmers are particularly concerned, as they rely on sprays of the live bacteria.

Now a team of researchers led by Richard ffrench-Constant, at the University of Wisconsin-Madison has found an alternative. Four genes encoding toxic proteins secreted by another bacterium, Photorhabdus luminescens, have proved effective in trials. The bacteria live in insect-eating nematodes, where they secrete so-called Pht-toxins. Those are active against a wide range of insect species, killing them in two days.

Although the nematodes have been used by organic gardeners for some time, they have a limited shelf-life. "What we have done is to clone the active ingredient," says Dr ffrench-Constant.

There is good news for organic farmers, too. "If you grow the bacteria in a flask, the culture broth itself can be used as a spray."

Research by collaborators at Dow AgroSciences has also shown that Bt-resistant Colorado beetles are susceptible to Pht-toxins, confirming the different modes of action. "The main selling point with Pht is that it is one of the first viable alternatives to Bt," says Dr ffrench-Constant. &#42