Fungi fight maple menace.

Tiny critters from the soil are being harnessed to prevent other tiny critters from defoliating vast areas of northeastern hardwood forests.

THROUGH A SPOTTING SCOPE, Dr. Margaret Skinner eyes a maple bud in the canopy 60 feet above her head. In late April, the bud is barely in the green-tip stage, no bigger than an almond M&M.

If you held that tightly closed bud in your hand, you'd see a tiny insect scurrying along it, searching for an opening, an insect hardly bigger than a flea on a dachsund. This is a pear thrips, the insect that took the Northeast by surprise in 1988, defoliating a half-million acres of Vermont sugar maples and nearly three million acres in the northern U.S. and Canada.

The minuscule insect had likely existed in the Northeast for several years, but because of its tiny size was little known or recognized until it stripped much of the maple canopy bare and left other leaves in tatters. The thrips' knack for slipping into the thin crack of a just-emerging bud has made it tough to control.

But a fungus in the forest soil that colors its insect victims a delicate pink is showing promise as a biological method of controlling pear thrips. And its success as an insect killer is in the beginning of something more significant--tapping the forest environment for other fungi useful as biological insecticides. Using fungi as a weapon against damaging insects is an effort that's just getting underway in the field of biological control. It's a crucial part of Integrated Pest Management (IPM) in the forest and elsewhere, and adds to the range of nonchemical options available.

Use of insect-killing fungi is just developing in the U.S.--as yet, none is on the marketplace here. Commercial production in the late 1970s withered when companies failed to find a way to mass-produce them in a form that would work consistently.

Skinner, University of Vermont insect pathologist Michael Brownbridge, and University of Vermont entomologist Bruce Parker are narrowing in on a strain of the fungus Verticillium lecanii similar to one commercially produced in Europe for use in greenhouses against whitefly, another pest. The sweet potato whitefly, a field pest that feeds on agricultural crops in the Southwest, has recently turned up as a greenhouse pest in Vermont, feeding on the undersides of the leaves. A new whitefly--the European cabbage whitefly--first found in 1992 feeding on a wildflower in a Vermont maple forest; several have since been found feeding on cabbage and broccoli.

"It's not so unique that we found it here but that we found it in the forests here and are testing it," says Skinner, who did her doctoral dissertation on pear thrips. "And it's unique that we're looking in a forest environment that's an undertapped resource for fungal pathogens. Pear thrips initiated the whole thing, but now we're looking more and more at the forest environment for pathogens that can be used against other insects."

Skinner and Parker, the project's leader, began seeking an enemy for pear thrips in 1988. That's when Vermont sugarmakers, fearing the loss of maples, asked for help and donated funding for research, as did the U.S. Forest Service and the Vermont Department of Forests and Parks.

Carbaryl, which is no longer recommended for use in the sugarbush, was the only insecticide available six years ago. But since it had to be applied by airplane, sugarmakers considered it expensive, and its effectiveness was suspect.

When Parker, Skinner, and later Brownbridge began their search, one of the first things they found was that dead thrips larvae, colored a pale pink, had been infected by a fungus, Verticillium lecanii. Other dead larvae that had different symptoms, such as a dried-up shell, were infected with other types of fungi.

If the V. lecanii and two other promising fungi prove they can do the job, they won't be available commercially until at least 1995, which is actually a relatively fast turnaround.

One help for sugarmakers first available this past fall was a simple test that tells if thrips levels in the soil are high enough to cause damage the following spring.

Although damage amounted to 68,000 acres in Vermont alone, thrips damage to Northeast forests had been relatively light in the last six years, due mainly to deterring weather conditions. To cause damage, adult thrips must emerge from the soil at the same time trees begin to bud; both events depend on the weather.

Adults thrips spend the winter underground and emerge the following spring. The University of Vermont team found it could estimate the number of thrips in the spring from soil samples taken in December before the ground froze. Samples are taken with a bulb planter, placed in a paper cup with a sticky lid, and kept at room temperature. Convinced it's spring, the thrips emerge.

"It's simple, it's inexpensive, and it's something sugarmakers can do to predict damage in their stands," Skinner says. "If I were a sugarmaker with high populations in the soil, I'd know that I might get damage and I'd want to tap my trees conservatively the next spring. Why put extra stress on my trees?"

When the team set out to find an effective fungus against the thrips, it first had to choose which organisms to test. It took them not only from dead thrips larvae but from infected hemlock looper pupae, and wax moth larvae (which easily catch diseases) set out on the forest soil as bait. In 1991, Brownbridge and Skinner went to Europe to seek other fungi fatal to thrips.

As a result, when they began work, they had more than a hundred strains of fungi to test. They finally narrowed it to V. lecanii and two other fungal pathogens found in the forest soil, Beauveria bassiana and Metarhizium anisopliae. Those seemed to work best against the thrips, and it was thought their forest origins could help them better survive when transplanted to the forest.

Now the team must test the fungi to make sure they work outside the laboratory, and it must assemble a lot of other data. Results of the first year's testing of the fungal pathogens showed Verticillium lecanii to be more effective than the other pathogens. However, untreated control plots revealed populations of thrips nearly as low as the treated plots. Rainy weather during the time thrips larvae drop from the trees may have naturally kept soil populations low, Skinner said. And it points up some of the frustrations of studying thrips--not only are they tiny and furtive, they are highly susceptible to the effects of weather.

The team also is turning the fungi loose on a species of ant and a species of beetle common in maple forests. That will help them determine whether adding more to the number of fungi already in the forest soil could hurt beneficial insects.

"That's one of the things we like about developing a fungus that occurs naturally and locally--we reduce the possibility of adding something that's foreign," Skinner says.

They'll also have to discover the best place, time, and method to apply the fungus. In early spring, adult thrips leave the forest soil where they've spent the winter, fly up to budding leaves and flowers, and feed. That's when they cause the most damage. But killing thrips at that point is difficult because they are feeding within the protection of the buds.

The adults then lay eggs; the larvae hatch, feed on the open leaves, then drop down onto the forest floor. Eventually they work their way down into the soil where they form cells and become adults in October and November.

Larvae pick up the sticky spores of the fungus from leaf litter and become diseased soon after entering the ground. That may mean spraying the fungus on the forest leaves would do little good against the thrips. The most likely bet is enclosing it in a granule that can be scattered on the forest floor in the spring when larvae are burrowing. One critical factor for a biological control is that it not only kills its first insect victim, but survives to grow out of it and move on to infect others. And the high relative humidity of the forest soil--nearly 100 percent--ensures just that.

"The beauty of it is the larvae spend 10 months of the year in the soil, and that's a more friendly environment for the fungi," Brownbridge says.

Two private firms are trying to come up with a formula that can be sold to and used by consumers. Once developed, the formulas must be registered by the Environmental Protection Agency.

Fungi were applied by hand in tests done on two sugarbushes during the summer of 1992, and the fungi is being tested now to see how it fared. By 1995, the team hopes to start aerial applications to 10 sugarbushes in Vermont and Connecticut.

The UVM entomologists are also studying using the fungi against gypsy moth and maple leaf cutter.

It's only a matter of time before fungi like these are routinely used as insecticides, especially those in the forest soil. Rich in organic matter, these soils are filled with fungi.

"It's a different approach to pest management--we can take the classic biological control approach of establishing the disease within a population to suppress the pest over time, or we can use the fungi more as biological insecticides to get a more rapid knockdown of a damaging infestation," Brownbridge says. "It's more difficult to use, for sure, but the advantages outweigh the disadvantages. It's cheaper, safer, and has fewer long-term effects on the environment."

Susan J. Harlow, managing editor of Turf magazine, does freelance writing from her home in Montpelier, Vermont.