Gypsy moth egg masses were first observed to be hatching in a single location off US-202 in Belchertown, MA on 4/26/2017. That location seems to be a bit ahead of other locations being monitored by scouts in Acton, Boston, Boylston, Cape Cod, and Hanson. As of 4/26/17, scouts in those areas report that egg masses have not yet begun to hatch. On 4/27/17, egg hatch has been observed at a location in Hingham, MA as reported by the Elkinton Lab of the University of Massachusetts. Anyone monitoring gypsy moth egg hatch should check local egg masses, particularly in locations where Amelanchier (shadbush) are blooming and growing degree-day accumulations are at or above 90-100 GDD, using a developmental threshold of 50°F. Egg masses are approximately 1.5 inches in length and can be a tannish-brown color and spongey as they are covered in hairs from the female moth who laid the eggs in 2016. A single egg mass can hold on average, 600 eggs. (However, the number can range up to 1000 eggs per mass.) Tiny, hairy, dark-colored caterpillars may be observed on top of or crawling away from these egg masses. Since roughly 1989, Massachusetts has enjoyed a reprieve from the outbreaks of this insect largely due to the caterpillar-killing fungus, Entomophaga maimaiga. Recent drought conditions in 2015 and 2016, particularly in May and June, have obstructed the ability of the fungus to infect young gypsy moth caterpillars during those years and this insect’s population has greatly expanded. Gypsy moth was attributed to causing over 350,000 acres of defoliation across Massachusetts in 2016 according to the Massachusetts Department of Conservation and Recreation. Many communities suffered this defoliation in 2016 particularly but not limited to areas of Cape Cod, the Southeast Region, and southeastern Worcester, Franklin, Hampshire, and Hampden counties. Defoliation and the subsequent location of overwintering egg masses was sporadic across the state. It is possible to have heavy defoliation in one area of the same town where seemingly no defoliation from these caterpillars exists.
Gypsy moth was brought to Medford, Massachusetts around 1868 or 1869 by Etienne Leopold Trouvelot. Trouvelot was an artist, amateur entomologist, and eventually an award-winning astronomer. He purposefully brought gypsy moth from France to his home in Medford for the purpose of studying them in hopes of using gypsy moth caterpillars for silk production. Some of the larvae escaped and Trouvelot tried to notify the authorities, as he understood the potential degree of this accident. By 1889, the gypsy moth problem had spread and the local efforts to manage this insect began.
Gypsy moth is a non-native, invasive insect in North America. Gypsy moth will feed on the leaves of and defoliate oak (preferred), maple, birch, poplar, willow, apple, and other deciduous plants. When their preferred hosts have been defoliated and gypsy moth populations are high, they will eat the needles of conifers such as eastern white pine, spruce, and hemlock. Such behavior was observed in 2016. Overwintering egg masses begin hatching between 90-100 growing degree-days, using a developmental threshold of 50°F. For more information about using growing degree-days to predict insect development, please visit our Growing Degree Days fact sheet: https://ag.umass.edu/landscape/fact-sheets/growing-degree-days-for-management-of-insect-pests-in-landscape. Typically, this occurs around the first week in May in Massachusetts, or when serviceberry (Amelanchier) are blooming.
Gypsy moth caterpillars, following egg hatch, will continue to feed and grow throughout roughly the 3rd week of June at which time caterpillars typically form the cocoons within which they pupate. Following pupation, adult gypsy moth males and females will emerge around the end of June, early-July. Adult males are brown and can be seen flying to find and mate with white colored, flightless (although winged) females. The females will lay the egg masses (that they coat with the fuzzy, brownish-tan hairs from their bodies) that can contain 500-1000 eggs and will overwinter in that stage. The caterpillars are the only stage of the gypsy moth that feed.
Small mammals such as mice can have a large impact on gypsy moth populations when they are low. Certain species of native birds and insects may also act as predators of gypsy moth. Previously introduced parasitoids in the fight against gypsy moth including Ooencyrtus kuvanae, Cotesia melanoscela, and Compsilura concinnata can attack certain percentages of the gypsy moth population, but even together their impact is typically not significant enough to reduce an outbreak.
The most effective organisms at regulating gypsy moth outbreaks tend to be two pathogens- the fungus, Entomophaga maimaiga and the NPV virus. E. maimaiga is an insect killing fungus native to Japan that kills gypsy moth caterpillars. A first attempt to introduce it into North America was made in 1910 and 1911 for gypsy moth control, but it was thought that the attempt at that time failed. In 1989, the fungus was found to be killing large numbers of gypsy moth caterpillars in North America, perhaps from a separate introduction. Since 1989, E. maimaiga has been particularly successful in keeping gypsy moth populations out of an outbreak situation. Prior to that time, gypsy moth outbreaks occurred approximately every 10 years. The issues in Massachusetts seen in 2015, 2016, and 2017 are thought to be because of the very dry springs we have been experiencing, particularly in May and June. The fungus needs ample moisture at the right time in its lifecycle in order to effectively infect gypsy moth caterpillars. If enough rainfall is received, E. maimaiga is a density independent pathogen- meaning that typically, even in low populations of gypsy moth, this pathogen can be lethal to the caterpillars. The NPV virus, however, is density dependent- meaning that it requires a large population of gypsy moth in order to effectively infect and therefore reduce the population. The NPV virus tends to kill high populations of older caterpillars typically after they have done most of their feeding damage. It may also take a couple of years for the NPV virus to “catch up to” the exploding gypsy moth population due to its density-dependent relationship with the insect.
Management Options for Gypsy Moth Caterpillars and Hope for the Future:
A commonly used management strategy for gypsy moth on valued, ornamental landscape trees and shrubs would be to apply Bacillus thuringiensis Kurstaki (Btk) on the foliage of the host plants when young gypsy moth caterpillars are actively feeding (shortly after egg hatch, when caterpillars are roughly between ¼ and ¾ inch in length). Older caterpillars are not as susceptible to Btk. Btk is practically non-toxic to birds, fish, and other aquatic organisms. It is non-toxic to people and pets. Btk is also practically non-toxic to non-target insects including pollinators such as honeybees. This particular strain of Bacillus thuringiensis is toxic only to the Lepidoptera (moths and butterflies); greatly reducing the number of different insect species it could potentially affect. It is a naturally occurring, soil-dwelling bacterium that must be ingested by the caterpillars to be effective. It eventually destroys their gut and causes the caterpillars to stop feeding and they die. Btk products have been listed as suitable products for organic food production.
Another active ingredient, spinosad, may also be used against gypsy moth caterpillars. This is also derived from naturally occurring, soil-dwelling bacterium. Spinosad is toxic to gypsy moth through both contact and ingestion, and can be effective against slightly older caterpillars. It will typically cause the death of the insect in 1-2 days (roughly) due to improper nerve functioning. Spinosad should not be applied to flowering host plants, as it is highly toxic to pollinators, such as bees, before it dries. Once it dries, it is practically non-toxic to these non-target organisms. Spinosad products have been listed as suitable products for organic food production.
Insecticides containing other active ingredients are also labelled for their use against gypsy moth caterpillars, but the different risks associated with these active ingredients should be fully understood and taken into consideration prior to using products that contain them. Always completely read and understand the insecticide label and apply the product according to label instructions only. These active ingredients include but are not limited to acephate, acetamiprid, azadirachtin, Bacillus thuringiensis subsp. Aizawai, carbaryl, emamectin benzoate, insecticidal soaps, permethrin, and tebufenozide. Some of the products associated with some of those active ingredients are restricted use products. Homeowners looking for assistance in managing gypsy moth caterpillars should contact a Massachusetts licensed pesticide applicator and arborist for help.
Non-chemical options include banding trees for gypsy moth caterpillars and removing egg masses prior to hatch. Banding trees for gypsy moth caterpillars will not be 100% effective and may not yield the desired results. While the sticky band may catch some caterpillars, many others may still make it to the canopy of the tree to feed. Young, newly hatched caterpillars can also disperse through ballooning, or using a silken thread to catch the wind and blow to a new location. This can allow young caterpillars to move from tree canopy to tree canopy of hosts and never encounter these tree bands, early on in their life cycle. Sticky materials or petroleum jelly or other such products used with these bands should not be applied directly to the bark of the host tree to avoid further injury to the plant. Scraping the brownish-tan, fuzzy gypsy moth egg masses into a can of soapy water can be labor intensive and time consuming, and not all egg masses will be found or be able to be reached. Some may be hidden from view or on the undersides of branches that are out of the reach of the homeowner. Once egg hatch begins in your area and tiny caterpillars begin to disperse, it is too late to scrape egg masses for the 2017 season.
Another option for gypsy moth management is to do nothing and wait for the population to collapse. Historically, widespread gypsy moth outbreaks would occur approximately every 10 years or so throughout the 20th century, with population collapses in between. This might not provide the best protection for valued ornamental plants, but may be part of a management option chosen in largely forested areas. Ornamental plants suffering from multiple, consecutive years of gypsy moth defoliation may become weakened and more susceptible to secondary invaders, or other organisms such as wood boring beetles and decay fungi that are attracted to otherwise weakened hosts. Add drought stress to that equation and such plants may be even further compromised.
For seasonal updates about gypsy moth activity and where the most significant defoliation is being seen in Massachusetts, please visit the Landscape Message from UMass Extension: http://ag.umass.edu/landscape/landscape-message . For gypsy moth updates, look under “Woody Ornamentals” and “Insects” within this message. Please subscribe to the e-mail list to get seasonal updates regarding pest insect, disease, and weed activity across the state along with environmental data and phenological indicators for pest activity. You can also “like” and “follow” us on Facebook @UMassExtLandscape for gypsy moth and other updates.
A fact sheet with more information about gypsy moth is available here: http://ag.umass.edu/landscape/fact-sheets/gypsy-moth.
Information from the Massachusetts Department of Conservation and Recreation as well as the Massachusetts Department of Agricultural Resources regarding predictions for gypsy moth defoliation in 2017 may be found here, along with a map of where defoliation occurred in 2016: http://www.mass.gov/eea/agencies/dcr/pr-2017/another-year-of-defoliation-from-gypsy-moth-in-2017.html.
An article discussing the history of gypsy moth in Massachusetts, early attempts at biological control, population fluctuations, and the outbreak in 2016 by Dr. Joseph Elkinton and Jeff Boettner of the University of Massachusetts may be found here: http://www.mass.gov/eea/agencies/dfg/dfw/publications/gypsy-moth-outbreak-of-2016.html.
Written by Tawny Simisky, Extension Entomologist, UMass Extension Landscape, Nursery, and Urban Forestry Program