Saving forests the natural way.

SAVING FORESTS THE NATURAL WAY

Biological control is at least 1,700 years old. As early as the third century A.D., citrus growers in China used predatory ants to forage for insect pests. Today, as they did then, the growers place ant nests in a few of the trees and connect the tree crowns with long bamboo strips, which the brigades of hungry ants use as aerial bridges.

Biological control has come a long way since the third century, thanks partly to the efforts of legions of Forest Service scientists - some of whom have even subjected themselves to personal physical risk while pursuing their research.

The balance of nature turns on the struggle of all organisms against natural enemies ranging from parasites and predators to diseases. We humans have learned to influence the balance by manipulating those enemies to control pest insects and plants.

Biological control had its first important success in North America a century ago. In 1889 entomologists from the U.S. Department of Agriculture (USDA) chose the vedalia lady beetle, an Australian species, to control another Australian import, cottonycushion scale, accidentally introduced into California citrus orchards. The beetle was so effective that the scale nearly disappeared within two years.

The earliest work in the U.S. on biological control of a forest insect pest focused on the gypsy moth, a European and Asian insect that was introduced into Massachusetts from southern France. By 1904 the moth had defoliated nearly 1.5 million acres of hardwood forests. To combat the problem, USDA entomologists decided to try a biological solution.

They worked closely with colleagues in Europe, Japan, and Russia to collect and ship parasites of the gypsy moth to the U.S. Ultimately, nine species of beneficial parasitic wasps and flies became established in the Northeast.

At the Forest Service Center for Biological Control of Forest Insects and Diseases in Hamden, Connecticut, scientists continue to study the biology of gypsy moth parasites. Since 1981, the scientists have made numerous trips to the Soviet Union and People's Republic of China to learn about predators of the gypsy moth in those countries and to make arrangements for the importation of parasites.

In the early 1930s, at the same time that USDA entomologist Phillip Dowden was obtaining foreign parasites to release against the gypsy moth, he was also initiating a program to control the larch casebearer, another European pest that presumably arrived in the United States on nursery stock and threatened our native eastern larch. Dowden enlisted the aid of entomologists in Austria, Holland, and England, who sent him parasitized casebearers.

One of these parasites was the Agathis wasp, which reduced the damage by casebearers so successfully that only low numbers of the pest still exist in the once heavily infested stands of eastern larch.

But the Agathis wasp could not prevent the casebearers from spreading. The first western infestation was discovered in Idaho by 1957. Because the Agathis wasp had proved so effective in the East, it was the logical species to do battle in the western larch forests. In 1960 Robert Denton, an entomologist with the Forest Service's Inter-mountain Research Station, received his first shipments of Agathis wasps from Dowden. Within a few years, the diminutive wasp was destroying 50 to 70 percent of the casebearers within 10 miles of the release sites.

When Denton made the first release near St. Maries, Idaho, an overzealous newspaper ran the story of a lady who reported that her small son had been stung by one of the wasps. The Agathis wasp - less than one-eighth inch long - is known to attack only the casebearer, but the newspaper story illustrates the lack of understanding that accompanied this new tiny hero on the western scene.

When the pest first appeared in Oregon in 1970, Roger Ryan of the Forest Service's Pacific Northwest Research Station in Corvallis became interested. Ryan's efforts to use biological control against the casebearer spanned 14 years. During this time he mass-reared the Agathis wasps and released them in infested stands in Oregon, Washington, Idaho, and Montana. As populations of the wasps became established, the casebearer was finally reduced to the status of a trivial pest in western forests.

Viruses are another effective tool for biological control. The alarming spread of the gypsy moth - despite the activity of introduced parasites - encouraged Forest Service scientists to develop other promising biological methods for controlling this pest. Research on a virus disease began in the mid-1960s when Dowden and Frank Lewis, a colleague of Dowden's at the Forest Service's laboratory in New Haven, Connecticut, began collecting diseased larvae for study.

As Lewis describes the work, "We would collect larvae, put them in water in gallon jugs, and let them rot down until they were insect soup." The "soup" was then centrifuged and filtered, and cooperators were given vials of the filtered, viral material to spray on gypsy moth caterpillars.

Lewis and Dowden continued to collaborate on identification of the most potent viral strains. Later, with funding from the USDA, Lewis was able to coordinate a diverse array of virus studies by colleges, universities, industry, and state and federal agencies. The work resulted in registration of Gypchek, a virus formulation for controlling gypsy moth.

Another Forest Service research effort to use a virus to control an insect pest began in the 1960s. The target this time was the tussock moth, which was damaging and killing, Douglas-fir and true firs in the West. Again, special USDA funds accelerated the research. At the research station in Corvallis, Clarence "Hank" Thompson, an insect pathologist, and Mauro Martignoni, a microbiologist, pioneered work leading to the successful development of a virus-based insecticide for use against the tussok moth.

"Back in early July of 1965," says Thompson, "Mauro and I planned the first aerial application of the virus on a few hundred four-foot-tall Douglas-fir trees in pots. We wanted to spray the trees with the virus, then let the tussock moth caterpillars feed on them so we could observe the amount of virus infection."

At the time of this test, microbial control in pest management was not new, but the technology of propagating viruses was relatively undeveloped. The earliest virus formulations - Martignoni calls them "potions" - were slurries made from thousands of virus-killed caterpillars mixed in water and corn syrup.

Also still primitive was the technology of applying a virus from aircraft. On that morning in early July, the two scientists tested the practicability of spraying the virus from a helicopter. Thompson describes the experimental spray apparatus they used as being "just a jumble of metal tubes and rubber hoses under the helicopter."

At the same time, the two wanted to demonstrate that the virus is not toxic to humans. So that morning they deliberately allowed themselves to be sprayed with the virus.

Martignoni points out, "We were pretty sure it wouldn't have any effect on humans. But we couldn't very well have asked someone else to be a guinea pig, so Hank and I just stood there and got squirted right along with the trees."

Today the Forest Service maintains a production facility for the tussock moth virus at the Corvallis lab, where about 200,000 diseased larvae are reared each month. Anita Hutchins, a microbiologist and manager of the facility, says the unit currently has a stockpile of freeze-dried virus material sufficient to treat 300,000 acres.

Because of public concern about environmental problems associated with chemical insecticides, researchers view biological control as a more acceptable means of dealing with forest insect pests. Scientists with the Forest Service are therefore concentrating on fine-tuning the technology and searching for new ways to use biological control to combat damaging insects.

Current projects include efforts to use insect pheromones (synthetic copies of naturally produced chemicals) as sex attractants to confuse male insects in search of females and thus prevent normal mating. Another method is to sterilize the insect by irradiation. When sterilized insects are released into the natural population, the eggs that are produced are infertile.

New microbial materials, including strains of genetically engineered bacteria such as Bacillus thuringiensis (B.t.), and European strains of microsporidians are also being tested against major forest insect pests.

Biological control research will continue to be a part of Forest Service efforts, according to James Stewart, director of forest insect and disease research. "The key role of biological control in effective pest management," says Stewart," obligates us to continue a strong research effort in this area... within the limits of our resources."

In other words, given adequate funding, the Forest Service will continue the search for bigger and better beneficial bugs. AF