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Gypsy Moth Defoliation Becoming More Apparent in Some Areas of the State

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A gypsy moth caterpillar measuring 2 inches in length viewed on 6/20/2018 in Amherst, MA. (Photo: T. Simisky)
June 22, 2018

A gypsy moth caterpillar that has succumbed to the caterpillar killing fungus known as Entomophaga maimaiga viewed on 6/20/2018 in Amherst, MA. Note the vertical orientation of the caterpillar that is now shriveled and dried. (Photo: T. Simisky)Gypsy Moth: (Lymantria dispar) caterpillars continue to feed and grow in size. Caterpillars observed on 6/20/18 in Amherst, MA are variable in size, but some are 2 inches in length at this time. 5th instar caterpillars will be present in most locations at this time. However, caterpillars may be of different sizes, depending upon your location in Massachusetts. Caterpillars have the red and blue pairs of warts or raised bumps recognizable in older gypsy moth caterpillars. They also have yellow coloration to their head capsule. Historically, gypsy moth caterpillars that have developed the yellow coloration on their head and those that are greater than ¾ inch in length are not as responsive to Btk (Bacillus thuringiensis Kurstaki) applications. For other management options, see our fact sheet at Host plants include, but are certainly not limited to, oak (favored), maple, birch, poplar, and many others.

While scouting at a location in Amherst, MA on 6/20/18, some caterpillar mortality due to the fungus Entomophaga maimaiga was detected. Shriveled, dried, and spore covered caterpillars were seen stuck hanging vertically from trees at this one location. One juicy caterpillar hanging in an inverted-V shape (typical of a NPV virus-killed caterpillar) was also observed at this site. However, the number of dead caterpillars stuck to the sides of trees (as of 6/20/18) was not as great as what was observed last year.

As of this date at this particular location, the sound of falling gypsy moth caterpillar frass (excrement) could be heard falling from tree canopies, a sound analogous to that of a soft rain. Frass could be seen on understory leaves, paved pathways, and vehicles. At this location in Amherst, shredded and partially fed upon leaves also littered pathways beneath the feeding caterpillars. Nearly completely defoliated oaks could be seen at the intersection of Bay Road and West Street (Rt. 116) in Amherst as of 6/20/18. Most of the calls/reports regarding gypsy moth defoliation, raining frass, and shredded leaves have come in to Extension from Amherst, Belchertown, Bolton, Boylston, Gloucester, Hanson, Ware, and Worcester (multiple), MA so far in 2018. Shredded leaves and partial defoliation of trees was reported in Ware, MA and it was noted that the activity there, although certainly noticeable, is not as bad as it was by this time last year. Raining frass was heard in Hanson, MA and some completely defoliated trees have been reported in Boylston, MA. Defoliation has also been reported in Gloucester, MA. Thanks to the Entomophaga maimaiga activity in 2017, hopefully we will not see entire hillsides of defoliation.

Despite the fungal outbreak that swept through the 2017 caterpillar population, some lucky caterpillars survived to pupate and emerge as adult moths. However, adults were present in 2017 in far fewer numbers than would have existed without the fungus. While it is very difficult to predict how much defoliation Massachusetts will see in 2018 due to gypsy moth caterpillar feeding, we can be certain that in areas where many egg masses were seen overwintering, pockets of defoliation could still occur in certain areas of the state this year. Thanks to the gypsy moth caterpillar-killing fungus Entomophaga maimaiga, however, the population should be on the decline, but we cannot expect the caterpillars to disappear completely from Massachusetts landscapes this season.

Reported by Tawny Simisky, UMass Extension Entomologist

Commercial Horticulture