Gypsy Moth FAQ
Q: When will gypsy moth egg masses begin hatching? What plants will they affect?
A: Gypsy moth (Lymantria dispar) 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, go to the UMass Extension fact sheet "Growing Degree Days for Management of Insect Pests in the Landscape". Typically, this occurs around the first week in May in Massachusetts, or when serviceberry (Amelanchier) are blooming. 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 eastern white pine, spruce, and hemlock. (The 2016 population of gypsy moth did feed on conifers in certain areas.)
Q: How long will the gypsy moth caterpillars feed?
A: Gypsy moth caterpillars, following hatch in the 1st week of May, will continue to feed and grow through 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.
Q: What can I do to manage gypsy moth on ornamental plants?
A: A common 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. THis is 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). 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 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 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.
Insecticides containing other active ingredients are also labelled for 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. 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 these 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 as they crawl up the tree trunk. 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 the tree bands early on in their life cycle. 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 reachable. Some may be hidden from view or on the undersides of branches that are out of the reach of the homeowner.
Another option for gypsy moth management is to do nothing and wait for the population to collapse. (see below: Are there other organisms that will affect gypsy moth populations?) This might not provide the best protection for valued ornamental plants, but may be a feasible management option to choose 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.
Q: Are there other organisms that will affect gypsy moth populations?
A: Yes. 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 melanoscelus, and Compsilura concinnata can attack certain percentages of the gypsy moth population, but even together their impact is not significant enough to reduce an outbreak.
The most effective organisms at regulating gypsy moth outbreaks are 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 problems in Massachusetts seen in 2015 and 2016 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 their feeding damage.
Q: Where did gypsy moth come from?
A: 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 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.
Q: Where can I find more information?
A: Check the UMass Extension Landscape Message for regular updates about gypsy moth and other insect pest activity throughout 2017. Information about egg hatch and defoliation seen in Massachusetts by this invasive pest will be shared under the Woody Ornamentals and Insects section. Further information about gypsy moth can be found in the UMass Extension Gypsy Moth Fact Sheet.
Tawny Simisky, UMass Extension Entomologist