Natural fungi may help rein in important insect pests of vegetable crops like broccoli. Agricultural Research Service entomologist John D. Vandenberg has been focusing on whether two parasitic fungi – Beauveria bassiana and Paecilomyces fumosoroseus – could become biological controls for diamondback moths.

The diamondback moth is a worldwide pest of cabbage, broccoli, and other crucifers. Each year, growers world-wide spend more than one billion dollars to control it – primarily with chemical insecticides. But in many areas, the moth has become resistant to conventional insecticides as well as natural bacterial controls such as Bacillus thuringiensis.

Vandenberg has conducted laboratory and field tests that show the moth succumbs to both fungi. But only Beauveria had a consistent effect in the field. Vandenberg works at the U. S. Plant, Soil, and Nutrition Laboratory operated in Ithaca, New York by the Agricultural Research Service, the chief research agency of the U. S. Department of Agriculture. Vandenberg and colleagues were the first to field-test Mycotrol, a commercial formulation of B. bassiana, against the diamondback moth. Weekly or twice-weekly applications significantly reduced insect populations and damage to seedlings, compared to chemical controls. Mycotrol was first developed to combat silverleaf whiteflies through a cooperative research and development agreement between ARS and Mycotech Corporation of Butte, Montana.

Research Proves Successful
At least ten other insects, including Indian meal moths, have developed some resistance to Bt, especially where growers have used it extensively. But fungi are promising alternatives, according to Vandenberg.

"With P. fumosoroseus," Vandenberg says, "field studies on application rates and residual effects on subsequent populations are needed to select appropriate strains. We also need to better understand and manage the fungal infection process." In laboratory studies, Vandenberg and Cornell University graduate student Jennifer Altre compared eight strains of P. fumosoroseus for the ability to infect and kill the diamondback moth. "We found dramatic differences among these strains in their relative virulence, spore size, germination speed, and ability of spores to attach to the surface of the insect," Vandenberg says. "The highly virulent strains have larger spores that attach easily to the insect's cuticle and germinate quickly. A strain with smaller spores doesn't attach well, germinates slowly, and isn't able to infect the moth."

In laboratory experiments with B. bassiana, all larval stages of the moth were susceptible to infection. But larvae exposed to the spores shortly before molting avoided infection, because they quickly shed their cuticle. "Larvae died more quickly at moderate temperatures and when exposed to higher doses of B. bassiana spores," he says. "These studies will help us to predict the success of timely applications of field-applied fungi."

In related research, Vandenberg and Anthony M. Shelton, a Cornell University entomologist, investigated B. bassiana for diamondback moth control on greenhouse-grown cabbage seedlings. Growers who grow cabbage rely on these seedlings, which they transplant and grow in their fields. "Commercial crucifer seedlings can be contaminated with insecticide-resistant diamondback moths," Vandenberg says. "By the time these seedlings become established in the field, diamondback moth larval populations can be high enough to require control.