Bacteria, Fungi May Be Crucial in Weed Biocontrol by Insects

By Kathryn Barry Stelljes
July 9, 1999

Microorganisms that live in the guts of insects and in the soil may be keys to enlisting Mother Nature against weeds, scientists with the Agricultural Research Service are reporting this week at an international meeting in Montana.

Many scientists around the world study and import natural insect enemies to combat invader weeds, such as leafy spurge or spotted knapweed. Since 1865, the approach has been used against at least 118 weeds in 55 countries.

But consistent success has been elusive. Microbes may be part of the reason, according to scientists with the ARS Northern Plains Agricultural Research Center in Sidney, Mont. They are reporting their findings this week at the "Biological Control of Weeds --X International Symposium," held July 4-9 at Montana State University in Bozeman.

ARS microbiologist Barbara A. Frederick has discovered that several insects used as biological control agents for leafy spurge and spotted knapweed harbor bacteria known as Wolbachia. The bacteria affect sperm in the insects, causing offspring to be up to 90 percent female. The skewed gender ratio may reduce the insects' ability to reproduce.

But other microbes are valuable insect allies. ARS plant physiologist Anthony J. Caesar has found that flea beetles imported to control leafy spurge have been most successful where naturally present soil-borne bacteria and fungi also feed on spurge. The microbes invade roots via wounds inflicted by insect larvae when they feed.

Both findings suggest useful tactics for mass laboratory rearing of insects that would be released to attack weeds. Scientists are working on artificial diets to increase the insects' populations. In the future, they may try including in these diets a compound to rid the insects of undesirable bacteria. Or, they might add beneficial microbes to a diet, so the insects would transport them directly to the weeds.

ARS is the USDA's chief scientific agency.