This summer, you may have noticed that many trees and shrubs are being defoliated by gypsy moth (Lymantria dispar) caterpillars. We are experiencing an unusually heavy infestation this year, as we did in 2016, and the effect on our trees has been devastating in some areas, with whole swaths of forest denuded by the voracious critters.

According to Joe Elkinton, professor of entomology at the University of Massachusetts, Amherst, the reason for the surge in the gypsy moth population is that recent drought has limited the growth of an airborne fungus, Entomophaga maimaiga, that lives in the soil and kills the caterpillars before they can do their damage.

Gypsy moths are native to Europe and are a classic invasive species. But unlike many pests that arrive on our shores by accident, they were deliberately imported to Medford in 1868 by an entomologist named Etienne Leopold Trouvelot. He was interested in cross-breeding them with silk worms for silk production, a major industry in Holyoke and other cities in the late 19th century. Unfortunately for Trouvelot, the breeding experiment was a failure; unfortunately for New England and the entire eastern region of the country, the moths adapted very happily to our forests of oak, poplar, birch and maple, and have been a destructive pest ever since.

The life cycle of gypsy moths begins when eggs hatch in early May, producing thousands of tiny black caterpillars, only 3 mm or so long. These creatures climb into trees and begin munching their way through the leaf canopy until they mature in late June, at which point they are 2 to 3 inches long.

When the small caterpillars climb high into the trees, they spin a single filament from which they dangle until a strong burst of wind breaks the thread and carries them to another tree. This wind-surfing habit is critical because female gypsy moths cannot fly and thus cannot spread their egg-laying around.

Not surprisingly, given the amount of foliage they consume, they also generate large quantities of excrement, called frass. In areas where infestation rates are extreme, the dropping of frass sounds like raindrops. Yuck! The only good thing I can say about frass is that it’s a slam dunk on the Scrabble board.

Once the caterpillars have matured, they go into their second stage, pupation, when they wrap themselves in a cocoon, from which they emerge 10 to 15 days later as moths. Only the male moths fly, and they spend their short lives impregnating the females, who put all their energy into egg production.

The females aren’t choosy about where they lay their egg masses. A single mass can contain between 100 and 1,000 eggs. They can be found on trees, cars, storage containers and lawn furniture; they appear as tawny brown tubes, approximately 1½ inches long, firm to the touch and without holes. Be on the lookout for them in August and September and destroy them. Unless the egg masses are destroyed, they will over winter and hatch in early May, beginning the life cycle over again.

Elkinton says the last serious gypsy moth infestation happened in 1981. There are a number of naturally occurring predators that generally keep the gypsy moth population under control. One is the fungus mentioned above. The nucleopolyhedrosis (NPV) virus also kills the caterpillars. White-footed mice, who depend on significant acorn harvests to thrive, prey on the pupae. Copious acorn harvests tend to correspond to fewer gypsy moths the following spring.

A biological pesticide called BtK (Bacillus thuringiensis var. kurstaki) is an effective agent against gypsy moth caterpillars. It is applied to plants on which caterpillars are actively feeding. It is not harmful to humans or pets or to birds or other animals that may eat the infected caterpillars. BtK is available at hardware stores and some plant nurseries. It should be applied twice to affected plants, about 10 days after the caterpillars hatch and again two weeks later, when the caterpillars are ⅜ inches long. It’s too late to use BtK this year, but keep it in mind if we are plagued by gypsy moths again next year.

Most healthy trees will survive one defoliation event. Successive years of insect feeding, however, can weaken trees and make them susceptible to other stressors such as drought. This can lead to long-term damage and even death. If you have affected trees or shrubs, make sure they have plenty of water this summer. Some fertilizer and mulch (but not too much!) also will help their recovery.

The good news, sort of, is that this year’s defoliation is almost at an end. Soon the pupation phase will begin and we can say goodbye to the hungry caterpillars for another season. As for next year, Elkinton says, “it’s a race between the gypsy moths and the fungus.” Fingers crossed for a wet spring next year and a bumper crop of acorns.

Visit the Hawley Bog

If you haven’t explored Hawley Bog in the high Berkshires yet, don’t miss this opportunity to tour the bog with plant ecologist Glenn Motzkin. Saturday, 8:30 a.m. to noon. More common to our north, bogs are typically high-acid, nutrient-poor habitats. Bog plants possess a variety of fascinating adaptations to help them thrive under these conditions. You’ll see insect-eating plants, the plant that makes peat moss, and several native orchids.

The tour will be in the open hot sun for a portion of the trip so sun protection is suggested. Directions to the meeting location will be provided upon registration. Members: $20; nonmembers: $30. For more information and to register, go to: hitchcockcenter.org.

Explore the natural history of the Connecticut River

On July 15, you can join John and Wendy Sinton, authors of “The Connecticut River Boating Guide: Source to Sea,” for a guided paddle of the Connecticut River. Even if you’ve paddled on the river before, you’ll see or learn something new on this exciting journey along the shores of Northampton and Hadley.

Bring your own kayak or canoe, or you may rent a boat from Sportsman’s Marina in Hadley the morning of the outing. Space is limited to 15 participants.

The group will meet at the Elwell Recreational Area next to the Coolidge Bridge in Northampton at 8:30 a.m. for a two-hour tour. The tour is free; boat rental is extra. Go to: kestreltrust.org for more information and to register.

Photography workshop

If you’re looking to hone your landscape photography skills Tower Hill Botanic Garden in Boylston is offering an intermediate level class July 15 from 9 a.m. to 2 p.m. The instructor is Steve McGrath, a professional freelance photographer and photography teacher, who has been creating images for 20 years.

McGrath will show the various camera settings appropriate for landscape photography, using shutter and aperture settings for the best exposure. The class will also cover composition, focal length, depth of field, filters and the use of natural light. Tripods are a plus but not mandatory.

After a presentation, the group will head out into the gardens to photograph. The cost is $60 for members; $75 for non-members. For more information and to register, go to: towerhillbg.org

Sale season at Andrew’s Greenhouse

Now’s the perfect time to fill in empty spots in the garden. Andrew’s Greenhouse in South Amherst is having its summer sale now on perennials and annuals. There are lots of great plants to choose from.

Mickey Rathbun can be reached at foxglover8@gmail.com.

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