A spongy moth caterpillar. (Feliciano Moya Lopez / Pixabay)

The Illinois Department of Agriculture will spray to control the invasive spongy moth (Lymantria dispar, formerly known as gypsy moth) in late June at several sites in northeastern Illinois, including Waterfall Glen, Des Plaines Riverway, Hidden Lake and Wood Ridge forest preserves, officials announced.

The moth’s caterpillars are voracious eaters of foliage, showing a preference for oaks. During large outbreaks, trees can be entirely stripped of their leaves within days. These weakened trees are more susceptible to other diseases and other pests, which affects their chance for survival, according to the Department of Agriculture.

The spray treatment, using what’s known as a “pheromone mating disruptor,” will be applied by a low-flying airplane, conducting multiple passes. Officials said the treatment is not harmful to people, pets, other wildlife, plants, cars or structures. Signage will be posted on treatment days, expected to be June 27 and 28.

Spongy moth was introduced to the U.S. in the 1860s and after escaping captivity in the Boston area has been migrating outward ever since. Northeastern Illinois is now on the leading western edge of the management zone to contain the pest. 

There are two common methods of containment, explained Scott Schirmer, of the Illinois Department of Agriculture's Bureau of Environmental Programs. 

One is Btk, a bacterial insecticide that targets feeding caterpillars and kills them. It's applied in April or May, depending on spongy moth caterpillar emergence and tree foliage development, in places where there's a large population of moths in a small area.

"It's like taking a bucket of water and pouring it on a campfire," Schirmer said. "You're putting out the fire before it spreads."

Mating disruption is the other option, widely used when there's a lower level of infestation over a larger area.

Here’s how it works: Synthetic sex pheromones are applied in waxy droplets, and the scent overload overwhelms the males.

"Everything smells like their favorite cologne. They can't pinpoint the females," Schirmer said. "It's not lethal. It's just out there to confuse them."

Thinking that females are everywhere, males can’t find an actual one to mate with; no mating equals no offspring. The treatment only affects the spongy moth, not other species of moths or butterflies, according to Schirmer. 

The combination of the two treatments has proven highly effective. Illinois has been able to hold the line on the spread of the spongy moth and has even occasionally driven it back over the last 30 years, he said.

The department monitors close to 6,000 traps in the northern part of the state, with the U.S. Department of Agriculture trapping the southern third. Traps are set at the end of May and checked at the end of summer. The number of moths caught in the snares determines the next year's course of treatment, according to Schirmer.

"The program is called 'Slow the Spread.' We know we're not going to be able to eradicate it," he said. "But we can stop it from jumping from, say, Kane County to Rock Island."

This story was originally published on June 10. It has been updated.

Contact Patty Wetli: @pattywetli | (773) 509-5623 |  [email protected]