Gypsy Moth (Lymantria dispar L.) is the most serious insect pest of hardwood trees in United states. In the 80 and 90, the gypsy moth defoliated more than a million acres each year in the eastern. Infestations alternate between years of little defoliation followed by periods of 2 to 4 years of heavier defoliation when gypsy moth populations are heavier. Of late infestations have been considerably reduced.
Gypsy moth caterpillars feed on most hardwood trees, except ash. They prefer oak, poplar, gray birch and fruit trees. When half grown or larger, the larvae are also likely to feed on evergreens. As the number of gypsy moth larvae in an area increases, their favorite food sources become depleted. The larvae are then likely to feed on other ornamental trees and shrubs.
Healthy hardwoods can usually withstand two to three years of defoliation; however, secondary attack by insects or diseases can shorten or affect the life of weakened trees. Weakened, diseased, insect-damaged or shaded trees, particularly those already struggling with poor soil or moisture conditions, are especially vulnerable. Severely defoliated evergreens are less likely to survive than other trees.
The Gypsy Moth has special methods of dispersal. The young larvae have hairs with small air pockets that create buoyancy, allowing them to travel great distances when the wind is strong. They have been found as high as 2,000 feet in the air, and are known to travel five miles a day by this method. Adult females commonly pupate and deposit egg masses on motor vehicles, especially trucks and recreational vehicles that are parked near or under trees.
Life Cycle: The gypsy moth passes through 4 stages: egg, larva, pupa, and adult moth stage. Only the larval stage damages trees and shrubs. The female adult gypsy moth lays her eggs in buff-colored egg masses during the summer. They remain on host trees over fall and winter months. Hatching of eggs coincides with budding of trees in early spring, usually by mid-May.
The small, first instar larvae are easily dispersed by “ballooning.” The wind carries the tiny caterpillar sometimes great distances as it hangs by a silken thread. Shortly after emerging from the egg in early May, the caterpillar is mostly black and 1/8 to 3/16 inch long. Its long hairs are somewhat bristly, not soft and silky like the hairs of the tent caterpillarThe . These hairs grow out of knob-like tubercles along the back. The tubercles caterpillar just behind the head are larger than the others, which helps distinguish the gypsy moth from other caterpillars.
After a few days, each larva sheds or molts its skin. After the first molt, the caterpillar increases in length to about 3/8 inch and has a tan stripe down the back. When it molts the second time, it is larger, about 5/8 inch long, and has several bright-orange marks down its back. By the time it is fully grown, to about one inch long, it will have five pairs of blue spots down its back, followed by six pairs of red spots. The first 3 larval instars stay in the branches of the crown of host trees. Larger fourth instar caterpillars move down the tree to rest during daylight hours, returning up the tree trunk to feed at night. Larvae reach full size by mid-June to early July.
The pupa stage is spent in locations where the larvae hide, such as in bark crevices or on the ground. Males emerge before females. Males fly in a characteristic zigzag pattern. Females do not fly; they attract the males to their location by emitting a pheromone. After mating, females lay eggs in July or August to complete the life cycle.
Damage: As a defoliator, the gypsy moth can totally strip the foliage from a wide variety of trees. While oak, poplar, birch and willow are a few of the preferred hosts, the gypsy moth will feed on most deciduous trees and even on coniferous trees when hardwood foliage is not available and the caterpillars are in their final development stages. There are, however, plants that are usually avoided by the gypsy moth. In anticipation of increasing problems with gypsy moth, non-preferred tree and shrub species such as these should be selected for landscape plantings.
In urban areas, the decline and death of unhealthy hardwood trees will be accelerated when they are defoliated for two or more consecutive years. Growth loss in healthy trees will be roughly proportional to the amount of defoliation above 50%. Healthy deciduous trees can enter a period of decline following consecutive years of defoliation. Most coniferous trees will not survive one complete defoliation in a single season, though complete conifer defoliation by gypsy moth caterpillars is rare.
Homeowners and recreationists in infested areas will also have to deal with the considerable annoyance of gypsy moths. In addition to the unappealing appearance of defoliated trees, masses of caterpillars will drop or crawl from heavily infested trees onto sidewalks, trails, picnic areas, cars, homes, cabins and other objects. Fecal droppings also abound in outbreak areas and the larval hairs can cause welts and rashes when susceptible individuals come in contact with the larvae. While not dangerous, these allergic reactions add to the annoyance of gypsy moth for some people.
Control: Eggs can be crushed, but it is difficult to crush all the eggs in an egg mass on bark or other rough surfaces. Eggs can also be scraped or otherwise removed and destroyed. But they must not be allowed to fall to the ground, where snow cover will increase the chance of hatching.
Burlap can be wrapped around the trunks of trees to collect caterpillars and prevent them from climbing into branches to feed. Collected caterpillars can then be killed. Crushing any female moth, caterpillar, pupae or eggs helps protect ornamental trees, but only if a high percentage of the population is killed.