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Gypsy Moths in 2018: What Happened, What Does the Future Hold

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This article comes from the NOFA/Massachusetts 2018 April Issue Newsletter

By Ellen Anderson, Contributor

Gypsy Moth Caterpillar Mortality (Courtesy Tawny Simisky)

Gypsy Moth (Lymantria dispar) Order : Lepidoptera Family : Erebidae

My first encounter with the hungry, hairy offspring of Lymantria dispar was on a hot sunny day in 1981 when my newborn daughter and I came home from the hospital under an umbrella to protect us from a hailstorm of caterpillars and their digestive products. We ran from the car to the house passing under our mostly defoliated old sugar maple. Some of the caterpillars ballooned over our heads on silky threads. Others rained poop as they digested the leaves of our poor tree.

These hairy creatures were new to me but they were really nothing new in Massachusetts. In fact, they had been appearing about every ten years following an introduction into the United States (from France). In the eighteen-sixties they escaped from the home of an amateur entomologist in Medford, MA. Arborists and homeowners fought back using powerful, broad-spectrum pesticides like DDT until it was banned in the late 1960’s and 1970’s. However, in 1989 an entomopathogenic (i.e. insect killing) fungus known as Entomophaga maimaiga became naturally established in Massachusetts and began successfully reducing gypsy moth populations below damaging levels.

Previously, the nucleopolyhedrosis (NPV) virus was also used to reduce gypsy moth population numbers; however, the NPV virus is most effective when gypsy moth populations are highest, while the fungus, under the right environmental conditions, can be highly effective even when gypsy moth population numbers are low. Both the fungus and virus are able to overwinter in the soil in Massachusetts. The fungus spreads to gypsy moth caterpillars most successfully when we experience wet springs with ample rainfall.

Fast-forward thirty-five years to 2016 and my new granddaughter’s arrival was accompanied by the greatest gypsy moth infestation since 1981. Over 350,000 acres in Massachusetts were defoliated according to the state Department of Conservation and Recreation. When the first eggs of 2017 were seen hatching in Hingham and Pelham on April 26th and 27th, observers predicted that the damage could intensify.

What happened and what does the future hold? According to entomologists at the University of Massachusetts, the severe drought plaguing Massachusetts during 2015, 2016 and early 2017 reduced moisture levels and impeded the life cycle of Entomophaga maimaiga. When the moths arrived their enemies (particularly this fungal pathogen) were unprepared - to put it mildly. Had May and June 2017 been as dry as the previous two years the fungus would have had a more difficult time infecting gypsy moth caterpillars. But the rains did come, activating the fungus in time to reduce populations of caterpillars prior to the development of breeding female moths. The Elkinton Lab at the University of Massachusetts reported “as of June 28, 2017 that, with the exception of one of the research sites in Massachusetts, 80-90% of the sampled caterpillars have perished and will not pupate.” 

But what of that other 10%, given that each female moth can potentially lay hundreds of eggs on tree trunks in fuzzy, tannish-brown masses? I asked Tawny Simisky, Entomology Specialist with UMass Extension, to comment on what to expect in Massachusetts in 2018. She said that it depends upon local conditions. “The public/property owners need to look for the egg masses in their specific location,” said Simisky. “If they are counting many of them, particularly on individual, valuable specimen trees that they wish to protect, they may need to consider management and decide what option is best for them. However, if there are few egg masses in their area, or if the trees are otherwise healthy and were not defoliated last year, they can let ‘nature take its course’ and allow the fungus to continue to do its job. A perfectly acceptable management option is to ‘do nothing’ and let the natural enemies of gypsy moth continue to collapse the population. That will/is happening, but it is difficult to predict exactly when the caterpillar population will decrease to below noticeable levels in all areas of MA.”

In my community in central Massachusetts, people are reporting numerous egg clusters, many of which can be expected to hatch during late April or early May.

According to Simisky, “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 are currently seen overwintering, pockets of defoliation could still occur in certain areas of the state this year. Thanks to the gypsy moth caterpillar-killing fungus, the population should be on the decline, but we cannot expect the caterpillars to disappear completely from Massachusetts landscapes this season.”

So what can we do? On a landscape scale the most likely answer is “be patient and pray for rain”. Farmers and orchardists working in areas where many gypsy moth egg masses are overwintering may need to protect their most important crops and trees. For organic farmers and gardeners with masses of eggs on their trees, the best treatment is Bacillus thuringiensis Kurstaki (BtK) applied very early just after the eggs are hatching, but when the young caterpillars are actively feeding. BtK must be ingested by the caterpillars to be effective. Once the caterpillars attain a larger size, when they are approximately ¾ inch in length or larger and when they develop a yellow color to their head, they will not be as susceptible to BtK. (Organic growers should check regulations, of course, before they apply any sprays, even BtK.)

What should you do if your caterpillars do become large and hairy? I asked Petersham’s Tree Warden and Certified Arborist at Shelter Tree Melissa LeVangie. First of all, she said that lures and traps mostly do not work, nor does spreading Vaseline. Placing some sorts of tape around tree trunks may work if population densities are low.

I also asked Felicia Andre, DCR Forester, who spoke about gypsy moths at the Petersham Grange’s Horticultural Fair last August to explain. “When population densities that are low,” said Andre, “the GM caterpillar does move up and down the tree to avoid predators. However if the population density is high (outbreak) then the caterpillars stop moving up and down as much. Some caterpillars also balloon in from other trees and never even touch the trunk of the tree they are in. So folks using barriers need to understand they may not stop defoliation in out break populations.”

She goes on to describe how to look at a dead caterpillar in order to tell what killed it. A “V-shape is characteristic of the NPV virus which helps control the population in high densities. When Entomophaga kills a caterpillar it ends up on the trunk of the tree in a straight up and down position and if you look close you should be able to see the fungal spores.”

Melissa LeVangie’s best advice for your valuable orchard trees is to work with nature to encourage the fungus: “First, clear grass and weeds from under each tree, then lay down a layer of shredded leaves or mulch. Keep the mulch wet and watch for the dead caterpillars that will be found swaying in the wind. Knock them down into the wet mulch where they will enhance the production of fungus. If the healthy caterpillars come down to the ground at night (and many of them do) they will not rise again! In addition you will be helping to spread the fungus, especially during dry times.”

For more information, the University of Massachusetts Extension Service will be sending out reports about gypsy moth such as this article under “News for Gardeners” which may be found in their Garden Clippings publication here.

Gypsy Moth Egg Mass (Courtesy Tawny Simisky)You should begin to identify the locations in your yard or orchard where there are egg masses and begin to watch them during the last week in April, especially if it is warm and dry at that time.

If you would like to see the egg masses as they exist this spring, the Petersham Grange has arranged to have well-known holistic orchardist, Michael Phillips, walk through the Town of Petersham on Saturday, April 21st at 10 AM, meeting near the Petersham Country Store on the corner of Main and East Streets. A brown bag lunch and a formal speech will follow. Watch the Petersham Grange’s Facebook Page for details.

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