“What are all those caterpillars that are chewing the leaves of my trees and shrubs?” That’s a question I have been getting asked a lot of late. It is not unusual to see caterpillars on woody plants in spring. Eastern tent caterpillars and forest tent caterpillars are quite common, but rarely do they get out of hand to the extent that these caterpillars have this year. In case you haven’t figured it out, I am referring to gypsy moth caterpillars.

From 2015 to 2019, gypsy moth caterpillar infestations, and the related defoliation of woody plants, had been confined to the eastern part of the state. However, the problem declined considerably beginning in 2019 and is no longer of much concern in that region.

The primary reason for the decline was the presence of a soil-borne fungus, which goes by the name of Entomophaga maimaiga. Yes, I know, it would be easier to pronounce if we could just call the fungus “Bob”, or “Joe”. Nevertheless, it is this naturally occurring fungus that infects and kills the caterpillar.

Why it didn’t get the job done in 2015, and the following few years, was due to spring drought conditions that reduced the effectiveness of the fungus. With the return of moist spring weather, the fungus thrived and brought down the caterpillar numbers. I assume it is the spring drought of last year and, at times, this spring that explains the seemingly sudden increase in gypsy moth numbers here in the Berkshires and neighboring New York counties.

After that explanation of who these hairy crawlers are and why they are here, the next obvious question is, what we do about them. You could hand-pick them (wear gloves), but that’s tedious and it is easy to overlook the smallest caterpillars. Actually, the answer is much simpler: spray. Of course, you can do this to control the caterpillars that are defoliating shrubs and small trees. On larger trees, such as oaks, a favorite host of gypsy moth caterpillars, that would be beyond the capability of a hand-held sprayer. If the damage to large trees is of concern, then it would be a task for a certified arborist who is a licensed pesticide applicator.

As for home gardeners, the best material to spray onto bushes and small trees is the biological pesticide Bacillus thurigiensis kurstaki (e.g. Monterey B.t.) applied directly to the leaves. This product is available under several trade names and is readily available at local garden centers. A friend of mine said he had success spraying with neem oil, another organic product. Usually, these sprays work best on the younger stages of the caterpillar, but I am seeing gypsy moth caterpillars at many stages of development. As such, I would not yet hesitate to apply the B.t.

Since both these products could disrupt pollinating insects, the best time to spray is early morning or late evening. Repeat applications may be necessary. Always read and follow label directions when using any pesticide, organic or not.


Well, if feeling stressed by gypsy moths, here are some relaxing gardening tasks:

  • Weed! With the heat and recent showers, weeds are growing rapidly in vegetable and flower gardens. The problem with weeds, of course, is that they compete with our crops and flowers for moisture, nutrients and root space. Weeding … a relaxing task? Yes, it is if you use the Zen method for weeding, that is, crawling on your hands and knees and pulling up weeds. As a mindless task, it allows time for meditation.
  • Side dress long-season vegetable crops. No, side dressing is not a current fashion trend. It is the application of fertilizer around individual plants or along rows of plants. The fertilizer applied should contain nitrogen, since it is the nutrient most likely to leach from the soil. Granular fertilizers may be used, but most often we’ll apply water-soluble fertilizer. Granular fertilizer should be applied about once per month around the season-long crops, while liquid fertilizers may be needed a little more often.
  •  Don’t rest on your laurels, nor your carrots, kale, beets and other early spring-planted crops that you may be harvesting now. Sow seed of these same crops in vacated garden space for a late-season harvest. Also, sow seeds of summer squash, pumpkins and acorn squash in pots to use as replacements for those plants that may be ruined by squash vibe borers.
  • Go ahead and harvest some young garlic plants any time there is a need for garlic. These young plants can be harvested and used the same as one would use scallions, i.e. chopped up and added to dishes requiring a garlic flavor. If bulbs are needed, it’s okay to pull up a few plants now as they have begun to form bulbs. However, bulbs that will be stored for winter must be harvested later in summer after the one third of the plant leaves have turned brown.
  • Keep newly planted trees and shrubs watered during hot, dry spells. Applying a layer of wood chips, pine bark or other organic mulch over the ground around the plants will help prevent moisture stress. Heat and water stress weaken these plants and make them prone to diseases and dieback problems.
  • Take cuttings from sage and rosemary. Root the cuttings by sticking the bases an inch deep into a moist mixture of peat moss and sand.
  • Shear Cranesbill (geranium) after it has completed its initial burst of bloom. It will bloom again sporadically through the rest of the summer. Because of its sprawling habit, I like to plant this hardy geranium as a groundcover near the base of upright shrubs. Also, shear catmint and early-blooming phlox.

Garden Journal columnist Ron Kujawski began gardening at an early age on his family's onion farm in upstate New York. Although now retired, he spent most of his career teaching at the UMass Extension Service.