They have ravaged foliage, covered homes in droppings, and even delayed flights.
It all sounds like a bad B-movie, but gypsy moths are actually one of New England’s most destructive pests. 2016 is shaping up to be the second worst year on record, with gypsy moth larvae chewing through more than 100,000 acres of forest in Massachusetts alone.
But there may be solutions, and not all of them require breaking out the pesticide.
The gypsy moth’s spread through New England began in 1868 or 1869, when a few escaped from French entomologist Étienne Léopold Trouvelot’s home in Medford, Mass. The species flourished in the absence of native predators, and the first outbreak occurred just two decades later. Prior to that fateful mistake, these insects lived only in Europe and Asia.
The caterpillars, rather than the metamorphosed moths, are the main problem. Insect predators – even those that typically prey on caterpillars – tend to avoid these spine-covered larvae. The 1989 introduction of a Japanese fungus called Entomophaga maimaga, which destroys larvae from the inside, successfully reduced outbreaks. But unseasonably dry springs in recent years have kept the fungus from germinating.
As a result, gypsy moth caterpillars can eat freely and defecate excessively. The black droppings, known scientifically as frass, pile high on rooftops and porches. But the problems are not just aesthetic.
Since 1980, gypsy moths have defoliated one million acres of forest every year. In 1989, they stripped over 12 million acres. While most hardwoods can bounce back after a major defoliation, they are left weakened and susceptible to disease.
“In some cases, these trees have been attacked by other insects first,” says Joseph Elkington, an environmental conservation professor at the University of Massachusetts, Amherst. “When you have multiple stressors and multiple defoliations, that’s going to cause tree mortality.”
Luckily, the larval season is almost over and most of these caterpillars have already transformed into moths. But the troubles continue. Some passengers experienced flight delays at Boston's Logan Airport on Monday, CBS Boston reported. The cause? Swarming moths.
Searching for solutions
Scientists have offered solutions to stop future infestations. Pesticide treatments can preserve trees while killing off larvae, arborist Mark Bezreh told CBS Boston, but they come with high collateral damage. Most pesticides aren’t sufficiently targeted, and can kill butterflies and benign moth species as well as the intended pest. Most states discontinued region-wide pesticide treatments, which were popular in the ’80s, although small-scale pesticide use is considered safe.
“Back in those days we would apply pesticides, sometimes by air,” says Dr. Elkington, who has researched gypsy moth population dynamics for decades. “I could be wrong, but I don’t think we’ll ever do that again. But shade trees, which are worth a lot of money to homeowners, are worth the money to protect. Pesticides are a lot safer now. But it’s too late to do anything this year.”
Some suggest that natural predation is the solution. Parasitic wasps and other predatory insects have been known to target gypsy moths, for example. The USDA Forest Service notes that “approximately 15 species of woodland mammals,” including the white-footed mouse, are also known predators. Some cuckoos even have a specific adaptation allowing them to consume gypsy moth larvae, Massachusetts NPR station WCAI reports. After having its fill of caterpillar, a cuckoo can expel the spines from its stomach in the form of a pellet.
“The world has been scoured for gypsy moth predators,” Elkington says. “They play a role in keeping populations in check but they do not prevent outbreaks. And we can’t just introduce parasites willy-nilly like we did in the ’70s. We have to be sure they’ll only attack the target.”
Genetically modified pest control?
In 2015, a British bioengineering firm developed a genetic solution to control a different pest. An invasive population of medflies was destroying fruit crops in Australia, so Oxitec created genetically modified medflies with a self-limiting gene, the Monitor reported. These modified individuals, once released, could mate with the wild population and pass on the gene, which would prevent any female offspring from reaching adulthood and reproducing – a “non-toxic and pesticide-free” method. Could GMO pest control stop gypsy moth infestation?
“The limitation... is that a gene like that will be rapidly selected against,” Elkington says. “And it’s extremely expensive to do. It requires that you overflood the population ten-to-one. That can only be achieved in isolated populations. It’s completely out of the question in places like New England where populations are in the hundreds of thousands.”
So what more can New Englanders do to prevent future outbreaks? Nothing at all, if you ask Dr. Elkington.
“I think, from a policy standpoint, we have learned to let the gypsy moth outbreaks run their course,” he says. “Some trees will die, but the forests will continue.”
“We’ve got the fungus – just pray for rain next year,” he laughs.