Information about Gypsy Moth Outbreaks and Management Options for Woodland and Residential Properties

Over the past 2 years, many counties across NYS have experienced gypsy moth outbreaks and based on the density of egg masses recorded in some areas of the state, it might very well be another high population year. Calls and emails have already started coming in from concerned individuals who experienced tree defoliation last year or have noticed egg masses lining the branches of their trees. Over the weekend, I was weeding along the side of my house and thought I saw ants crawling along the foundation. Upon closer inspection, much to my disappointment, they were tiny little gypsy moth caterpillars emerging from an egg mass hidden just under the lip of the siding. I ran into the house and filled a container with soapy water and knocked the caterpillars into the water -- I took care of them! But, what about all the egg masses plastered on the branches of my neighbors trees and so many of the trees in my neighborhood? Unfortunately, I think I will be seeing more caterpillars soon. So, what can I do? What can you do if you are facing this scenario as well or have the egg masses on your tree branches? Or maybe you have a woodlot or sugarbush that you are concerned about. I can't offer any magical solutions, but here is some information about gypsy moth outbreaks, their impact on trees, and some management options.

Gypsy moths are known as an outbreak pest, as populations can quickly increase every 5 to 10 years (possibly due to weather patterns) after an extended period of low, nearly undetectable levels. Multiple factors affect the size of the gypsy moth population each year, such as available food sources, parasites, predators, and disease. At low gypsy moth population densities, small mammal predators are the primary source of mortality, but they do not actually serve as a means of population control, especially during an outbreak. At higher population densities or during an outbreak, disease (a fungus and a virus) tends to be the greatest source of gypsy moth mortality. Both the fungus and virus require moisture, so control of gypsy moth populations by these diseases is better during wet springs.

Fortunately, healthy deciduous trees can generally regrow leaves after defoliation from gypsy moth caterpillars and can usually withstand 2-3 years of successive defoliation. During outbreaks, when populations are high and food becomes scare, gypsy moth caterpillars will feed on almost any vegetation including pines and spruce. Conifers do not regrow needles as easily as deciduous trees regrow leaves, and are more likely to die as a result of defoliation. Unhealthy trees are also less likely to withstand defoliation and might die after one defoliation episode. In many cases, the actual cause of death in trees attacked by gypsy moths is not the defoliation itself, but instead a secondary organism that invades the weakened tree, such as a fungus or borer. Even though gypsy moths can cause significant stress to a tree or forest during an outbreak, overall they do not pose a major threat to NY’s forests.

Management Strategies:

  • When populations are low, caterpillars and adults can be removed by hand and killed. Scraping egg masses off and dropping them on the ground will not kill them, so make sure to place them in soapy water or thoroughly crush them. Some people are allergic to gypsy moth hairs, which are present during all life stages, so wear gloves, protective clothing and a dusk mask.
  • Wrapping burlap around the trunk of the tree, tying a string around the center, and folding the burlap in half to trap caterpillars as they move up and down the tree and then crushing or knocking them into soapy water.
  • There are also some cultural practices that can help reduce the damage caused by gypsy moths. These include maintaining tree and/or forest health, and watering landscape trees during droughts and placing mulch around the base of the tree to retain moisture.
  • During outbreaks, manual removal may become tedious and does little to reduce the overall population of gypsy moths, so spraying may be a practical and effective option. Various insecticides are available for gypsy moths, including microbial/biological and chemical sprays. The most commonly used microbial/biological insecticide used for gypsy moths is Bacillus thuringiensis (kurstaki), commonly known as Bt or Btk, which is naturally occurring on plants and in soil. It is harmless to people, animals and plants but does affect other young moth and butterfly larvae. Btk is most effective on young caterpillars. Younger gypsy moth caterpillars have an all-black head capsule, while older caterpillars will develop yellow markings on the head – these caterpillars are less susceptible to Btk. In certain cases, such as in a sugarbush, aerial spraying of Btk may be economically feasible since severe defoliation can reduce maple syrup production. Several chemical insecticides are labeled for use on gypsy moths but keep in mind that these can have an impact on a wide variety of beneficial, native insects, birds, and other animals, so they should be used wisely. Spraying of Btk or chemicals is not at all effective during the pupae or egg mass stages.

These management options all have their pros and cons and vary in degree of effectiveness, especially in high population years. I encourage you to implement the management practices that you are able to do, but it is also really important to focus on overall tree and forest health so that your trees can better withstand future gypsy moth outbreaks.  

Some more educational information on gypsy moths is available at these links [compiled by Peter Smallidge]:

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