Extension Corner: Gypsy Moth Update

Laura Bailey, Cornell Cooperative Extension of Yates County Natural Resources Educator & Northwest Regional Director of the Master Forest Owner (MFO) Program

FINGER LAKES — Currently, in the Finger Lakes region, gypsy moths are still in the caterpillar life stage and will continue feeding on leaves for a few more weeks before going into cocoons. They will be in cocoons for 1-2 weeks before emerging as moths. Moths do not feed. 

No surprise that gypsy moth caterpillars like this one were seen June 11, 2021 during a walk and workshop on invasive species.

How to manage gypsy moths

If you are looking for ways to manage gypsy moths, there are some practices you can employ. Overall, these practices will not have a large impact on population control on a regional or statewide level, but they may help at an individual tree or landscape scale. Gypsy moth caterpillars can move by sending out a silken thread and catching the wind, so you must be diligent with these practices as more caterpillars may move into your trees. Some people are allergic to gypsy moth hairs, which are present during all life stages, so wear gloves, protective clothing, and a dust mask.

1) For this year, it is too late to spray tree leaves with BTK or BT as it is only effective on small, young caterpillars, but if you have young caterpillars next spring, this is an option.

2) Wrap a piece of burlap around your tree trunk. Tie a piece of string around the center of it and fold it over so you have a 2-ply layer of burlap. As the caterpillars move up and down the tree, some will get caught between the layers of burlap. Squish the ones that get caught.

3) Caterpillars will be going into cocoons in the next few weeks and will be in their cocoons for 1-2 weeks before emerging as moths. Look for cocoons on the trees and crush or burn them.

4) When moths emerge from cocoons, swat the females. Females are cream-whitish colored and cannot fly. This will reduce the number of females laying eggs on your tree.

5) This fall and winter look for egg masses that are within reach. They are tannish colored and velvety. Use a plastic putty knife or butter knife to scrape egg masses into a container. Fill the container with soapy water and let it sit for 2 days before discarding. Just scraping egg masses on the ground will not prevent them from hatching in the spring.

6) 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.

The gypsy moth is an invasive, tree-defoliating insect, whose caterpillars are capable of feeding on approximately 500 different species of trees and shrubs. White oak is the preferred tree species, but most other oaks species in the Northeast are highly susceptible, as well as several other deciduous trees (lose their leaves each fall) including maple, apple, crabapple, aspen, willow, birch, mountain ash, hawthorn and sometimes even conifers (have needles or scales and are generally evergreen), such as pine and spruce.

Damage caused by gypsy moths 

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.

Identification of gypsy moth life stages 

Eggs of the gypsy moth hatch in the spring, from clusters of 500 or more eggs, which were laid the previous summer and overwintered. Egg masses are approximately 1.5 inches long by 0.75 inches wide and are typically laid on tree trunks, but can also be found on stones, walls, logs and other outdoor objects. They are covered with buff or yellowish “hairs,” giving them a fuzzy or velvety appearance. In NY, eggs usually begin hatching around April or May.

Once hatched, the larvae remain in the lower forest canopy, or if it is an outbreak year and populations are high, they will move up into the canopy or disperse by “ballooning,” spinning down on a long silken thread and waiting for a strong wind to break the thread and carry them to a new location. At first, the larvae are very tiny, hairy and grayish but become easier to identify as they are about half grown and develop several pairs of red and blue dots on their back. Mature caterpillars are 1.5 to 3 inches long.

Only the caterpillar stage of the gypsy moth feeds. Gypsy moth caterpillars do not make silken webs or tents. Caterpillars stop feeding when they enter the pupal or cocoon stage around mid-late June. Adult moths emerge from dark brown pupal cases about 2 weeks later, in late June-early July. Adult male gypsy moths have light tan to brown wings with dark, wavy bands and feathered antennae and have a wingspread of 1.5 inches. Female moths are white with dark, wavy bands and straight, threadlike antennae. Females are larger than males, with a wingspread of 2.5 inches, but they cannot fly. Neither the male or female moths feed, adults mate and lay eggs only.

Population cycles of gypsy moths 

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.

Other management strategies

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. 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 on larger caterpillars or during the pupae or egg mass stages. Spraying is not commonly done by state or local agencies unless there have been subsequent defoliation events and/or defoliation at an ecologically or culturally unique area.

For questions or additional information, contact CCE Yates Natural Resources Educator, Laura Bailey at lb698@cornell.edu or 315-536-5123.