Efforts to control the moth Lymantria dispar received a boost more than 30 years ago when the fungus Entomophaga maimaiga, native to Japan, was discovered in the United States. Since then, the fungus has played a significant role in curtailing the moth larvae’s attempts to devour large chunks of forest. A new article published in July in Environmental Entomology chronicles the spread of the fungus, both naturally and with the aid of scientists and natural resource managers. Shown here are L. dispar larvae killed by the E. maimaiga fungus. (Photo by Steven Katovich, Bugwood.org)

By Paige Embry

Paige Embry

The larvae of the moth Lymantria dispar* can defoliate a million or more acres of susceptible trees in a few months. Since its arrival in North America in 1869, the moth has spread fairly slowly, currently being found in 20 eastern and midwestern U.S. states and five Canadian provinces. Part of that slow spread has to do with the life cycle of the moth, as the adult females of L. dispar can’t fly. Instead, the work of species expansion falls to the youngest larvae—the tiny, hairy, first instar caterpillars—which move from their birth spot via ballooning. They spin a silken thread and dangle, waiting for a bit of wind to waft them away. If they land on a plant they like, they stay. If not, it’s time for another ballooning trip.

Efforts to control the moth received a boost more than 30 years ago when the fungus Entomophaga maimaiga, native to Japan, was discovered in the United States. Since then, the fungus has played a significant role in curtailing the moth larvae’s attempts to devour large chunks of forest. A new article published in July in Environmental Entomology chronicles the spread of the fungus, both naturally and with the aid of scientists and natural resource managers.

Ann Hajek, Ph.D.

Often, biological control agents are raised in a lab and released during a pest outbreak. They die when the outbreak ends and need to be purchased and re-released with each new outbreak. Entomophaga maimaiga is “exceptionally difficult” to mass produce, so resource managers have gotten creative. They obtain spore-infested soil or infected dead larvae that they seed at the base of trees in areas that have moths but no known fungal presence. Once the fungus is seeded, it’s left to reproduce and deal with the moths on its own—which it does quite successfully. The authors write, “Where it has become established, this acute pathogen has become the dominant natural enemy and has exerted considerable influence in reducing [L. dispar] damage.”

Since 1989, scientists have conducted various studies on the fungus’s impact on the moths, but they’ve also investigated the best ways to spread the spores. For example, fungus-killed larvae have to be collected shortly after dying, while they are still on the trees, but are light and chock full of spores. Conversely, soil can be collected later but is heavy and expensive to ship, requires testing to know the spore density, and may contain other unwanted organisms. Experiments also showed that infection levels were higher when the fungus-containing soils were watered—or when it rained during May.

After infecting and killing a Lymantria dispar larva, the fungus Entomophaga maimaiga forms a type of spores known as conidia that appear as white granules on the caterpillar’s hairs (setae), as shown here. (Photo by Mark Guthmiller, originally published in Hajek et al 2021, Environmental Entomology)

Ann Hajek, Ph.D., a professor at Cornell University, is the lead author on the paper and has studied L. dispar and E. maimaiga for decades. She explains how a soil-dwelling fungus can cause so much infection and death to an insect that mostly lives on trees. “The early instars—the first and second and thirds—once they’re on food that they like, they mostly stay up in the tree canopy and eat. But when they get to be the later instars, every day they walk down the tree trunk and go into the leaf litter at the base of the tree and they spend the daylight hours there. This,” Hajek says, “is unusual behavior for caterpillars, but it definitely fits right in with the fungus infecting it.”

Spores can survive in the soil for years, providing on-going biological control. After a fungus-caused epizootic (insect epidemic), the spore reservoir in the soil will be large, but it will diminish over time.  Since the fungus thrives in wet weather, the combo of a dry spring and a long time since the last epizootic means conditions will be ripe for a large L. dispar outbreak. New York, where Hajek is located, was in that position this spring and experienced its first big L. dispar outbreak in 30 years. Hajek says that after a dryish early spring, there was a spate of thunderstorms and the fungus really got going. On July 1, Hajek says, “One of my technicians was out in the woods yesterday and he said the trees are just covered with dead insects. So, the fungus came back. It was enough. … It’s definitely still around, that’s for sure. … Now we’re going to be building up another resting spore reservoir.”

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.

*On July 7, 2021, the Entomological Society of America announced the removal of the existing common name for Lymantria dispar, “gypsy moth,” from its Common Names of Insects and Related Organisms List, as part of a new ESA program to review and replace insect common names that may be inappropriate or offensive. The Environmental Entomology article linked above was accepted prior to this change; articles submitted to ESA journals after July 7 will no longer use the previous common name for L. dispar. Learn more.