To spray or not to spray: Gypsy moth infestations reach critical levels

By Sara Kangas

With warmer weather here to stay, experts are anticipating a significant gypsy moth caterpillar infestation in 2017 that will no doubt jeopardize the health of forests across the eastern United States.

Gypsy moths were first introduced to the United States in 1869 when enterprising traders brought them to Boston in the hopes that the silk-producing caterpillars could outpace silk worms. A few of the insects escaped – and, as is the case with invasive species – the caterpillars thrived, dominating the landscape and expanding westward.

Gypsy moth caterpillars’ primary food of choice is oak leaves. With 600 – 800 caterpillars per egg mass, tree defoliation is quick to follow. The caterpillars are also covered in urticating hairs – fine hairs that make the larvae appear ‘furry,’ but cause extreme itching, skin irritation or rashes, and in some cases, respiratory irritation. These stinging hairs can fall out accidentally, but they also snap off as a defense mechanism when the caterpillar feels threatened.

The gypsy moth does have naturally occurring predators – a fungus, Entomophaga fungi, and a virus, Nucleopolyhedrosis, or NPV. “Both of those need moisture to proliferate and actually do their job,” said Rhode Island Association of Conservation Districts President Dick Went. The fungus and the virus, however, are both severely inhibited by drought conditions, so in low precipitation years like 2017, caterpillar populations increased, particularly between April and late June.

“Since 2014, we’ve been in a drought, and it came to a head last year where nothing touched the gypsy moths other than spraying,” Went said. “One year of defoliation is tolerable; healthy trees will make it through that without a problem. Two years and you’re starting to push it. And certainly, a third year will be definitely a problem,” he added.

Rhode Island Association of Conservation Districts President Dick Went.

States in the Northeast are at the highest risk, with Connecticut and southern Massachusetts seeing already the worst infestations since the last major gypsy moth outbreak of the 1980s. Illinois has also been hard hit this year, with Kendall, Kane, LaSalle, and Will counties under quarantine – meaning all lumber and nursery trees must be inspected before they are cleared for transport. In attempts to prevent gypsy moths from spreading, Pennsylvania, Wisconsin, and Illinois have all started aerial spraying programs in the last month.

Beginning at dawn, aerosolized BtK – a naturally-occurring bacterium found in healthy soils – is released from small airplanes. The bacteria stick to the leaves of forest canopies where egg masses and caterpillars are most often found, but the residue does not affect bees, pets, or children. “It’s an organic thing, and it does a good job, but it’s also very expensive,” Went explained. This aerial spraying is controversial, and not just because of a cultural aversion to chemicals.

“It’s very questionable about whether (spraying is) really needed or wanted,” Went said. “When there’s enough moisture, natural enemies will take care of (the gypsy moths)” and spraying can also “kill off all the natural enemies, because the natural enemies won’t have enough gypsy moths to feed on.” Some states leave the decision to spray up to individuals, so that they can determine each season whether natural predators are effective on their properties.

With state agencies making the decisions to spray or not, conservation districts and their state associations are primarily taking on the role of education. In Rhode Island, where the state decided against spraying for financial reasons, Went says “We’re telling (landowners) what they can spray that’s not dangerous to many other things. We’re doing the outreach to help people decide whether to spray or not to spray given the good side and bad side.”

From year to year, it can be almost impossible to predict how gypsy moth populations will fluctuate, or when an outbreak will occur. Often, landowners have to play a waiting game, Went said. “(This year) our forest service is saying ‘Hang in there, make it through, spray the ornamental trees around your property if you want to to make things a little bit easier for you, but be careful.’”

“I’m just hoping that we don’t have a problem with too much defoliation and that the trees will be healthy enough to make it through,” he said. “I’ve got my fingers crossed.”

For more information on gypsy moths, click here. For information on gypsy moth control,  visit the Rhode Island Department of Environmental Management or the Pennsylvania Department of Conservation and Natural Resources.


Sara Kangas is NACD’s communications and operations coordinator. She can be reached at sara-kangas[at]

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