Spongy moth caterpillars in a window box last week in Warner. (Dana Wormald | New Hampshire Bulletin)
Eating everything it can find, the spongy moth caterpillar – formerly known as the gypsy moth caterpillar – may not notice the defoliation it’s causing. But state officials and researchers do and are preparing to track the caterpillars’ population boom across several counties this spring.
The spongy month caterpillars, an invasive species, cause the large-scale loss of leaves, which can eventually cause the death of several types of trees, including oaks, apples, birches, poplars, and willows. (In 2021, the Entomological Society of America renamed the species due to the word gypsy being seen as an offensive term by the Romani people.)
The New Hampshire Division of Forests and Lands has done an annual survey for the past 75 years on the impacts of the spongy moth caterpillar in the state. This year’s survey is planned for late June or early July, when the caterpillars are at their largest size of 1.5 inches and have essentially finished eating.
“We’ve had pretty steady defoliation that just went away and then just last year for the first time in a long time popped up again,” said Kyle Lombard, forest health specialist with the Division of Forests and Lands.
Lombard said last year’s increase in spongy moth caterpillars was driven by the decline of a fungus called Entomophaga maimaiga, which was brought to the state 100 years ago specifically to control the moth population. The fungus has been affected in recent years by rising temperatures, but Lombard hopes the wet spring will help revitalize the fungus and limit the moth population this year and next. The moth population rises and falls every 10 to 15 years in a cyclical pattern.
Rachel Maccini, a University of New Hampshire entomologist who runs the university’s Pesticide Safety Education Program, has been hearing about spongy moth caterpillars invading several counties, including Merrimack and Hillsborough. The Seacoast region was hit hardest last year.
While pesticides have been used to cull the caterpillars’ population, Maccini advises against that practice unless a tree has particular sentimental value – and then the only pesticide that should be used is Bacillus thuringiensis subspecies kurstaki, which harms only the caterpillars. Their eggs can also be destroyed by scraping them off plants and leaving them in a bucket of soapy water, according to the New York Department of Conservation; they can also simply be hosed off houses and decks.
After eating the leaves of trees they favor, the caterpillars move on to eat a variety of available leaves from numerous types of plants, according to a University of New Hampshire fact sheet. The fact sheet advises against removing a plant defoliated over the summer unless it fails to leaf out the following spring. A tree is unlikely to die if it hasn’t been defoliated over multiple years. The public can report sightings of the spongy moth caterpillar through a UNH website, NH Bugs.
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