Dr. Joseph Elkinton of the University of Massachusetts and the Department of Environmental Conservation has extensively studied the non-native insect, the gypsy moth (Lymantria dispar). He was one of the guest speakers at the Invasive Insect Certification Program for Landscape, Nursery, and Urban Forest Pests provided by UMass Extension in late February.
It was a surprise to see them in large populations in Massachusetts during the growing seasons of 2015 and 2016; the last big outbreak was in 1981. A possible reason for this population surge are the spring-time droughts the state has experienced in the months of May and June in these years. These drought conditions do not allow for infection of the gypsy moth caterpillars with an insect-killing fungus, Entomophaga maimaiga, which has been largely responsible for the relief from large gypsy moth populations. This fungus needs ample moisture at the correct time to be more effective at killing gypsy moth caterpillars.
The egg stage of this moth is what does the overwintering. Adult female moths are flightless and the males (which are good fliers) will seek them out during the summer to mate. Females will lay brown, spongey egg masses which can have 500-1,000 eggs each on host plants but also inanimate objects such as lawn furniture, firewood piles, parked trailers, and homes. These eggs will hatch approximately the first week in May in Massachusetts. Caterpillars will disperse and feed, capable of completely defoliating trees, throughout approximately the third week in June at which point they pupate and transform into the adults. Adults are present by late June/early July and do not feed.
They were brought into Massachusetts in the late 1860’s by an amateur entomologist seeking to study the insects for silk production. In subsequent years, the gypsy moth became a major pest in Massachusetts and New England; however, it took time in the beginning for them to spread. “It took 50 years for them to cross Massachusetts because their females don’t fly. Over 135 years later, they’re still spreading into the Midwest,” said Elkinton.
During the earlier days of attempts at managing gypsy moth, extreme measures (which are not recommended by today’s standards) were undertaken. Flamethrowers were aimed at trees, and dozens of men climbed to the tops of trees scraping off the egg masses. In many cases, very harsh and environmentally unsafe insecticides were used to manage this insect. “Lead and arsenic were brought in to treat them in Boston — residues from these chemicals may still be present in soils in that area.” said Elkinton.
The most susceptible trees include oak, but gypsy moths will defoliate poplar, apple, hawthorn, gray and white birch, basswood, serviceberry, larch, sweetgum and willows. This insect does not preferentially feed on tree species such as ash, yellow poplar, sycamore and dogwood. In years with high populations, conifers like eastern white pine, hemlock, and spruce may be defoliated when preferred hosts have been stripped clean.
Early in the gypsy moth control effort, at least 10 different parasitoids were brought in as biocontrols for this insect. Unfortunately, these biological control organisms were not alone able to reduce gypsy moth populations. DDT was historically used to manage gypsy moth, until it was banned in 1972 following research suggesting its adverse effects on eagle eggs and people. The chemical insecticides which replaced DDT in some cases were carbaryl and diflubenzuron, plus the biorational insecticides Btk (Bacillus thuringiensis kurstaki) and spinosad.
Elkinton cautions not use pheromone-based traps for gypsy moth management. “The pheromone traps that you could buy were completely useless. Bands on tree trunks also do not work,” he said. Spraying insecticides, such as biorational insecticides like Btk and spinosad, which have less negative implications for human health and the environment, do work and can help prevent defoliation.
Gypsy moth populations, when low, can be kept in check by small mammals, which will eat their larvae and pupae. “Small mammals are able to eat gypsy moths and keep it in check,” he said, in low gypsy moth density areas. A 1992 study in the Quabbin Reservoir proved when the mice population declined, moths increased.
There is a nucleopolyhedrosis virus, NPV, which is visible at times when dead caterpillars drip down trees, hanging in an inverted V-shape. However, while the virus fosters the collapse of outbreaks of gypsy moth populations when caterpillars are older and in high populations, it is the fungus Entomophaga maimaiga which wipes out gypsy moth even when at low densities.
“The fungus completely changed the ecology of the gypsy moth,” he said.
The fungus was first introduced to gypsy moths in Boston around 1910 with no evidence it became established. It is thought just prior to 1989, a strain better adapted to North America was accidentally introduced in New England and finally took hold.
“The fungus disperses in the wind. Only if a fungal spore lands on the caterpillar — if the caterpillar is wet — will it infect the [gypsy moth] caterpillar. The whole thing is linked to rainfall.”
The drought conditions in May of 2015 and May and June of 2016 made it too dry for the fungus to work against gypsy moths. “During that time, the population builds up. We have a lot of egg masses out there currently; we’re going to have a bad year even with normal rainfall,” said Elkinton.