June 30, 1998 MADISON - By isolating and characterizing the biochemical properties of a new-found natural insecticide, scientists have taken an important step toward augmenting the sparse armamentarium of biological pest control.
Writing today (Friday, June 26) in the journal Science, a team of scientists from the University of Wisconsin-Madison describe the properties of a family of insecticidal toxins produced by Photorhabdus luminescens, a bacterium that, in nature, infects and kills insects with the help of a tiny worm or nematode.
The toxins produced by Photorhabdus are active against a wide range of insects and are at least as potent as the insect-killing poisons produced by Bacillus thuringiensis or Bt, the reigning king of natural insecticides, according to Richard ffrench-Constant, a UW-Madison professor of toxicology in the department of entomology and the principal author of the new study.
"These new toxins are highly efficient killers of insects and they hold for the future the same promise first revealed in Bt more than 30 years ago," said ffrench-Constant.
Widely used for decades in the home, in forests and on farms, Bt is also a bacterium and is considered to be a safe, effective and environmentally benign weapon in the war on insect pests. Moreover, in the last few years the genes that govern the production of the Bt toxin have been moved from the bacterium into crop plants, which this year account for 20 percent of the U.S. cotton crop and nearly 10 million acres of transgenic corn, mostly in the Midwest.
As a form of biological pest control, Bt is the only bacterium from which widespread commercial insecticidal applications have been possible, giving it, in effect, a microbial monopoly on insect control worth hundreds of millions of dollars.
But the development of new, naturally occurring insecticides has taken on new urgency in recent years as resistance to Bt has been reported in some populations of insect pests.
"Potential resistance to Bt is now a big issue," said ffrench-Constant. "Developing new biological agents for the control of insect pests is therefore essential."
Photorhabdus, ffrench-Constant suggests, may become an important alternative to Bt, or could be deployed in concert with Bt to prolong the effective life of both biological insecticides by delaying the evolution of resistant strains of insect pests. He described the deployment of Bt transgenic crops as the biggest experiment in natural selection for insecticide resistance since the introduction of chemical pesticides 50 years ago.
"What we have with Photorhabdus and other bacteria is a natural source, almost an infinite variety" of toxic molecules, says ffrench-Constant. "We can't afford to hook ourselves to a single bacterium or a single toxin."
In nature, Photorhabdus bacteria live inside the guts of nematodes that invade insects. Once inside an insect host, the bacteria are released from the nematode, kill the insect, and set up rounds of bacterial and nematode reproduction that turns the insect into a "protein soup," food for large numbers of nematodes.
Moreover, the insect corpses left behind glow in the dark as the microbe produces luminescent proteins in addition to potent insecticides.
Previous studies have shown that, in concentrated doses, the toxin can be used as a spray or fed directly to insects. The greatest potential application, however, lies in transferring the toxin-producing genes from the bacteria to crop plants.
The incentive to confer crop plants with their own insecticides is huge. Farmers now spend more than $575 million annually on chemical pesticides to protect corn alone.
In addition to ffrench-Constant, co-authors of the Wisconsin study include David Bowen, Thomas A. Rocheleau, Michael Blackburn, Olga Andreev, Elena Golubeva and Rohit Bhartia.
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