January 21, 2020

Working for safer pest control> Natural virus showing promise against budworm

For the first time since the late 1980s, state officials have seen an increase in the number of spruce budworm moths trapped at monitoring stations along the Quebec border.

In response to environmental and health concerns about man-made pesticides, scientists are trying to develop nonchemical methods to control the destructive pest that feeds on the needles of evergreens. Researchers in Quebec have reported good results from a naturally occuring virus that kills the caterpillars.

Henry Trial Jr., an entomologist with the Maine Forest Service, said stations in western Maine trapped four to 12 moths during June and July 1996, compared with just one or two in the recent past.

“We’ve had very low populations since 1988,” said Trial, “but we continue to monitor it at about 40 sites.”

The slight increase in Maine corresponds with rising populations of the budworm in Quebec.

“We will step up the number of traps we put out this year,” said Trial.

The spruce budworm killed millions of Maine fir and spruce trees when it last appeared in a decadelong outbreak that began in the late 1970s. A native of North America, budworm populations follow cycles of explosive increases and dramatic declines.

Trial isn’t willing to predict another budworm outbreak is imminent. In the first place, scientists look for a three-year trend of population increases, and the 1996 data could be just an aberration.

Second, the budworm prefers to feed on mature fir trees. Most of the state’s older firs either were killed in the last outbreak or harvested in the heavy cutting that began in the budworm era and has continued thereafter.

“We’ve got a lot of young, vigorous fir out there, and budworm usually gets started on mature and overmature trees,” said Trial.

Even so, the entomologist said there was a less-severe outbreak in the 1940s in which the budworm fed on younger trees that had grown after a major epidemic 20 years earlier.

“That’s what I expect, another mild outbreak on vigorous young trees,” said Trial. “But that probably won’t happen until the early 2000s or 2010.”

Any outbreak before about 2025 could devastate the forest products industry. Maine stocks of fir and spruce are at extremely low levels, with shortages predicted until the second or third decade of the next century.

During the last budworm outbreak, millions of acres in Maine were sprayed with man-made pesticides. The program cut losses from the insect, but it also galvanized Maine’s environmental community. Some of the activists now prominent in the debate over forest practices got their start by opposing the aerial application of pesticides.

Biological controls offer a way to limit the damage from budworm without spraying chemicals that are harmful to humans and wildlife. A bacterium called Bacillus thuringiensis was used with some success late in the last outbreak. Trial said Bt costs a little more than chemical pesticides, but newer formulations of the bacteria are even more effective.

When ingested by caterpillars, Bt produces a toxin that kills the insect. While the bacterium affects many moths and butterflies other than budworm, it poses no danger to other insects, birds, amphibians, fish or mammals, according to Stephen Woods, assistant professor of entomology at the University of Maine.

Forest managers, however, need to have more than one way to fight the budworm. Researchers recently have noted that some moths can grow resistant to Bt in just two or three generations.

The next wave in budworm control may have been discovered already in Quebec, where vast fir forests provide a refuge for the insect. Scientists at the Ministry of Natural Resources and the Institut Armand-Frappier say they have discovered a naturally occurring virus called Granulosis virus Bonaventure that dramatically increases the mortality of budworm larvae.

Jean Cabana, a research forester for the ministry, and Claude Guertin, a virology researcher at the institute, isolated the virus in some budworms in the Gaspe Peninsula.

“In 1992, we went to the Gaspe to the last spot where the budworm [outbreak] was still active,” said Cabana. “We found larvae that were sick, took them back to the lab and screened them for a disease. After we isolated the virus, we gave it to populations of spruce budwarm larvae [in the lab] and found it was very active against the insect.”

Field tests conducted in 1995 and 1996 suggest the virus can increase budworm mortality by 300 percent, according to Cabana. Also, the virus affects only budworm — it has no effect on other creatures.

“The larvae with the virus act very sick,” he said. “They move slowly and they don’t eat. Gradually, they bulge, liquefy and die.”

Cabana said the researchers are working to develop a formula for wide-scale aerial applications of the virus. They are experimenting with substances that make the virus stick longer to needles and encourage the budworm to eat it.

Cabana said a small aerial spray program will be conducted this year, and he hopes to have the product registered for commercial use in two years.

That should be just in time to combat a major increase in budworm populations in Quebec. Cabana said an estimated 15,000 acres of forest will be severely infested this year, compared to just 1,250 acres in 1993.

Most of the problem is occurring along the Gatineau River north of Ottawa, but outbreaks have also been reported closer to Maine: near Drummondville and north of Quebec City.

Woods said it has proved difficult to develop effective pesticides from viruses. Large-scale production is particularly difficult, because the virus must be grown inside living caterpillars. The insects are ground up and sprayed to disperse it.

Even so, Woods thinks viruses are a nearly ideal way to control insect pests. They have a protective coat that allows them to survive in the environment for years or decades, he said.

“You release them, and they spread through the population,” said Woods.

Cabana said the persistence of the Bonaventure virus has yet to be demonstrated.

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