Spongy moths (Lymantria dispar) deposit their eggs on trees and other surfaces in sponge-like masses, which overwinter before larvae emerge in the spring. This egg stage lasts as long as 10 months, and a new study evaluating pesticide effects on spongy moth egg masses shows the potential value in targeting the invasive pest during its dormant wintertime stage, before hungry larvae emerge. (Photo by Milan Zubrik, Forest Research Institute – Slovakia, Bugwood.org)

By Paige Embry

Paige Embry

Larvae of the spongy moth (Lymantria dispar) are voracious and indiscriminate eaters—the plant world is their buffet. Spongy moth larvae consume more than 300 species of trees and shrubs, although they do have favorites, including oak, aspen, birch, cottonwood, willow, and fruit trees.⁠ Their numbers tend to increase and decrease cyclically over a period of years, and during an upswing they can cause massive defoliation in yards, parks, and forests. As an added insult, the stripped landscape is littered with hairy caterpillars and their excretions.

Some homeowners have gone to extremes to eradicate the pest. “We’ve seen people make flame throwers from aerosol pesticides, paint their houses with kerosene, and shoot the larvae with BB or bigger guns,” says Kevin Chase, a research entomologist with Bartlett Tree Research Laboratories. Bartlett Labs provides research and technical support to Bartlett Tree Experts, a commercial tree-care company.

Kevin Chase

Spongy moths aren’t native to North America. They arrived in the 19th century and can be found in at least 20 states and four Canadian provinces⁠. Chase is one of a group of scientists that investigated ways of reducing the number of moths when they are in the egg stage—before they start chowing down on the landscape. Results of this investigation were published in June in the Journal of Economic Entomology.

The scientists conducted three experiments. The first experiment investigated four pre-emergent pesticides, in both the field and the lab, to see if any could prevent egg hatch. The four pesticides were Golden Pest Spray Oil (GPSO, active ingredient 93 percent soybean oil), RTSA Horticultural Oil (active ingredient 98.8 percent mineral oil), Triact 70 (active ingredient 70 percent neem oil) and Acelypryn (active ingredient 18 percent chlorantraniliprole). The second experiment tested different concentrations of GPSO for L. dispar. The label-recommended rate of GPSO to water is 1:1, but the researchers also tried concentrations of 1:2, 1:5 and 1:10.  The third experiment was conducted in 2022 at a widespread, ongoing outbreak in Wisconsin in areas where significant defoliation was deemed “unacceptable” by state park and regulatory managers. In addition to scheduled spraying with Bacillus thuringiensis kurstaki (Btk), they tried one of two additional treatments: GPSO sprayed on egg masses (using a boom lift to reach the mid and upper tree canopy) or application of Lepitect (97.4 percent acephate) to the soil at the base of trees.

Larvae of the spongy moth (Lymantria dispar) are voracious and indiscriminate eaters. They consume more than 300 species of trees and shrubs, although they do have favorites, including oak, aspen, birch, cottonwood, willow, and fruit trees.⁠ (Photo by Karla Salp, Washington State Department of Agriculture, Bugwood.org)

For experiment 1, GPSO was the clear winner; no caterpillars emerged in either the field or the lab. Both Acelepryn and Triact 70 reduced emergence somewhat (varying between 60 percent and 90 percent emergence) but the RTSA results were the big surprise with zero emergence in the lab and 100 percent in the field. The authors offer two potential explanations. The first is that the eggs in the lab (stored in petri dishes) were saturated with the oil for longer periods than those on the tree. The second is that the oil held up better in the fridge where the eggs were stored than out in the field. The disparate results between field and lab suggests that RTSA may still be a useful treatment option with a few tweaks.

“We think it is worth pursuing higher rates of RTSA or other horticultural oils,” Chase says. “As plant healthcare practitioners, we want as many tools in the toolbox as possible because there may be situations where certain materials are practical or usable and other situations where other materials are needed.”

For experiment 2, the 1:2 rate was equivalent to the 1:1 on the first day the researchers checked emergence (20 percent for both), but significantly worse several weeks later (10 percent for the 1:1 and 50 percent for the 1:2). Both of the more diluted solutions had emergences higher than 50 percent. For experiment 3, the GPSO didn’t help as an adjunct treatment to Btk spraying, possibly due to egg masses left unsaturated, even though they used a boom to reach higher levels. Some trees treated with Lepitect did show less defoliation.

In a lab experiment, spongy moth (Lymantria dispar) egg masses were treated with a range of pesticides and then observed for the rate at which larvae did or did not emerge. (Photo by Kevin Chase)

The end result of this array of experiments is that spraying spongy moth egg masses to reduce or prevent the hatching of ravenous larvae shows promise, and additional research could prove helpful. Importantly, spraying egg masses during the dormant season means the treatment window is large—all winter—and winter-time spraying lessens the impact on beneficials.

“GPSO should be incorporated into a spongy moth IPM program for homeowners living in outbreak areas,” Chase says. “On an isolated, small tree you can get great control. But, more importantly, it gives the homeowner a sense of having some direct control. They are less likely to take direct actions that can be dangerous for themselves or others.”

Hopefully, it would lessen the impulse to shoot caterpillars or hurl pesticides at trees.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.