Gypsy Moth

Pest: Gypsy Moth (Lymantria dispar)

Order: Lepidoptera

Family: Lymantriidae

Host Plants:

White oak is the preferred host but most other oak species (in the Northeast) are also highly susceptible as well as many other deciduous species. Conifers may also be attacked when the preferred host plants are in short supply.

Description:

The Gypsy Moth was accidentally introduced into the United States in Massachusetts in the 1860’s. Since that time, it has spread throughout the Northeast and well beyond. It is a voracious feeder on oaks but can also attack many different deciduous hosts. In addition, it will also feed on many different conifers when stressed for food. It can be a serious pest of trees and a nuisance due to the irritating hairs on its body and the copious amount of excrement (frass) that it produces in high population years.

The Pest:

The gypsy moth over-winters as an egg in a cluster of 500 or more eggs. Eggs typically hatch in the spring during the first week in May, in Massachusetts, but variations in climate and spring weather can either accelerate or delay egg hatching. Once hatched, the tiny, hairy caterpillars migrate upwards to the tree tops and then each one spins down on a long silken thread. They hang in the air waiting for a strong wind to break the thread and carry them to a new location. This process of dispersal is known as "ballooning" and is somewhat common in caterpillar species where the adult females do not fly. It is the only silk that this species produces. Gypsy moth caterpillars do not make silken webs or tents.

Once the caterpillars settle on a new host, they begin feeding on the foliage. Small to moderate sized populations will often feed at night and come down out of the trees during daylight hours to avoid predators and parasites. Caterpillars in high populations usually stay in the trees around the clock due to intense competition for foliage.

The caterpillar stage lasts until about the third week in June (Massachusetts) whereupon they pupate; adults start to appear by late June/early July. Neither the male nor female moths feed.

Gypsy moth caterpillars start out being about a 1/16 of an inch in length and may exceed 3 inches by the time they pupate, six or more weeks later. The caterpillars have hairy bodies; along the length of their backs they have five pairs of blue dots followed by six pairs of red dots. Adult male gypsy moths are brown with black markings and have highly feathered antennae. Female moths are white with black markings and have straight, threadlike black antennae; female gypsy moths, of this species, do not fly.

Population sizes of this pest can change dramatically from one year to the next but there is no set cycle for these fluctuations. Gypsy moth caterpillars have numerous hairs on their bodies, as do the adults. Many people experience allergy-type reactions to these hairs. Symptoms range from itchy skin irritation to sinus allergies with itchy eyes and a runny nose.

Management Strategies:

Once the caterpillars have settled to feed, they can be successfully treated with a compound known as Bacillus thuringiensis (kurstaki) or commonly known as B.t. This is a naturally occurring bacterium that is specific to caterpillars that become moths or butterflies (Lepidoptera). It is relatively safe for beneficial organisms and other insects. However, once the caterpillars are older, B.t. is much less effective. To establish whether B.t will work or not, inspect the caterpillars on the host plant; younger gypsy moth caterpillars have a head capsule that is all black while the older ones have obvious yellow markings on the head and these larvae are less susceptible to B.t. In this case, other compounds, such as a chemical pesticide, may be necessary. Even though numerous chemical pesticides are available, it is best to manage problematic populations of this pest early and rely on B.t.

A commercial applicator will need to be consulted for larger trees due to the necessary application equipment required.

Traps for the adults offer no benefits whatsoever in managing this pest.

During wet springs, an entomopathogenic (insect killing) fungus (now "naturally" common in Massachusetts), known as Entomophaga maimaiga, works extremely well in keeping this pest in low numbers. Observing dead gypsy moth caterpillars that are hanging head-down on the trunks of host trees is a good indicator that this fungus has been effective.

Written by: Robert Childs
Revised: 10/2011