The Gypsy Moth

John M. Corliss.

The gypsy moth is a leaf-eating insect, native to Europe and Japan, that was accidentally introduced into this country in 1869. It was brought from France to Medford, Mass., by a French scientist for experimental purposes but escaped and became established there. It gradually spread through most of New England.

It feeds on many fruit, shade, forest, and ornamental trees. In epidemic outbreaks few species of trees are untouched. Evergreen trees, on which only the larger caterpillars feed, are particularly susceptible to attack and damage when grown near oak and other hardwood trees that the insect favors. Defoliation caused by the gypsy moth has retarded growth and killed trees over extensive areas.

The gypsy moth has four stages in its life history: The egg, larva or caterpillar, pupa or resting stage, and adult or moth. During late July and August the female moth deposits about 400 eggs in a cluster, and the insect over-winters in this stage. The eggs hatch in May and the caterpillars feed on foliage for about 6 weeks. This is the stage when the insect may be most effectively controlled. When full- grown, the caterpillars change into pupae, and the adult moths emerge from the middle of July until the middle of August. A new generation is then started. The male gypsy moth is brown with black wing markings and is a strong flier. The female is nearly white, with black wing markings, but cannot fly because of her heavy body.

In Europe and Japan the gypsy moth has many natural enemies, which contribute toward its control, but when it was introduced into this country none of these was present. In 1905 the State of Massachusetts and the Federal Government began to introduce parasites and other natural enemies of the gypsy moth. Much parasitized material has been received since then and several natural enemies have become established in this country.

But control of the gypsy moth by its natural enemies was less effective than people expected because some of the most important parasites from Europe could not be established alternate host insects were not available here for the second or succeeding generations of the parasite. The wilt disease, caused by a polyhedral virus, attacks the gypsy moth in the larval stage and often kills large concentrations of caterpillars.

Congress first provided funds for Federal work on the gypsy moth in 1906 after the insect had spread through eastern Massachusetts and southern New Hampshire. The aim was to control and prevent the spread of the insect and eradicate outlying infestations. In 1923 a barrier zone was established where clean-up operations to prevent the further westward spread would be centered. The zone extended along the New England-New York State line from Canada to Long Island. The zone was replaced by a suppressive area in 1945 because the infestation had spread beyond the zone in New York after the hurricane of 1938.

Isolated infestations were found from time to time in New York, New Jersey, Pennsylvania, and Ohio. The smaller infestations were rapidly cleaned up.

In 1920 a well-established infestation was found in New Jersey near Somerville. It eventually covered some 1,450 square miles. Investigations traced the outbreak to blue spruce seedlings, which were imported from Europe and contained gypsy moth egg clusters. By 1935 complete eradication was effected by New Jersey and Federal cooperation, funds, and men. Spraying was done with hydraulic sprayers. Arsenate of lead was the insecticide.

In 1932 an infestation over 1,000 square miles was located near Pittston, Pa. In 1948 an infestation over 250 square miles was discovered at Quakertown, Pa. Both were sprayed with DDT by airplane. In 1951 only one isolation was known to exist in Pennsylvania. It was discovered near Pittston in August 1951, following the capture of male gypsy moths in two nearby traps.

The infested area in New England and New York is regulated by Federal Quarantine No. 45. The purpose is to prevent spread of the gypsy moth to uninfested sections of the country. Shipments of regulated plant material to points outside the infested area must be accompanied by a certificate of inspection.

DEFOLIATION by the gypsy moth, causes economic damage by ultimately killing the trees, retarding their growth, or creating other conditions that impair land value. The amount of defoliation varies from year to year.

The death of trees may be caused by single or repeated defoliations and is more extensive when partially developed deciduous foliage from primary buds has been killed by heavy frost earlier in the same season and when defoliation occurs during long periods of drought. White pine trees and some other conifers die almost always after a single stripping of the foliage.

Estimates based on a 20-year study of some 200 representative observation points in eastern New England put the value of all hardwood trees killed in those areas at 16 million dollars. Further estimates for the remainder of the infested area during that period placed the loss at 10 million dollars. No evaluation was placed on the accompanying mortality of young white pine trees, which has continued since the studies were made and is hard to estimate because the trees are usually killed before they have obtained sufficient growth to have any more than potential value.

A significant effect of defoliation is the loss in increment of tree growth. Apparently the loss of growth in trees varies proportionately with the amount of defoliation a tree defoliated 75 percent generally will put on only 25 percent of its annual normal growth. Estimates place the monetary loss of growth of trees from defoliation and consequent loss of lumber in the infested area at an average of 1.5 million dollars yearly.

The death of trees in woodland creates several problems. One is the opening of the forest stand, which affects the remaining growth so that the subsequently developing trees may be unsuitable or of inferior quality for timber. It also eliminates the good forest cover that helps to regulate stream flow and minimize floods. The loss of trees in recreation areas lowers land values and the production of wildlife. Heavy infestations of the caterpillars can make a place unpleasant to visitors and increase the hazard of spread of the pest by the traveling public.

EFFECTIVELY PLANNED spray operations must be preceded by surveys to delimit areas of infestation. Before establishing priorities for spraying one must know whether the growth is conducive to rapid build-up of infestation and determine whether the physical features of the forest are such as to increase the hazard of spread.

Special traps are an effective and economical way to survey extensive areas to find out if gypsy moth infestation exists and to check on the effectiveness of spraying operations. The traps are baited with a substance that attracts adult male gypsy moths in a radius of one-half mile or more. The substance is obtained by clipping the last two abdominal segments or tip of the virgin female adult and putting them in benzol, which extracts the material from the sex glands. The tips are then processed at Beltsville, Md., in order to stabilize and increase the potency of the attractant. They are then used at the rate of 15 tips to a trap. The material has been obtained by collecting large numbers of female pupae in heavily infested areas, but the spraying of extensive areas has reduced the numbers of pupae available in the United States.

The most satisfactory traps are made of salvaged cans, about 7 inches long and 4 inches in diameter, and are known as the Graham trap. The sex attractant is placed inside in a cardboard cartridge. The inside of the trap is lined with a sticky material that catches the moths. The ends of the can have cone-shaped screens with holes at the center to allow the moths to enter.

The traps are hung by wires to trees and spaced uniformly throughout the zone to be surveyed. They are placed in the field not later than early July, before the beginning of the moth flight, and removed the latter part of August or first of September. The traps are inspected every 7 to 10 days to remove moths in them and to renew the sticky material.

During the summer of 1950 trapping was done in New York, Pennsylvania, New Jersey, Vermont, Massachusetts, Rhode Island, and Connecticut. A total of 19,608 traps were placed in areas aggregating nearly 7,194,000 acres. No moths were caught in Pennsylvania and New Jersey.

In the fall and winter, scouting surveys are conducted in the vicinity of traps where moths were caught during the summer and in areas not trapped where infestation is known to occur in order to determine extent and intensity of infestation. Scouting is also carried on at points where infestation could be started by the importation of egg masses or living larvae by vehicles of all types. The latter work is largely confined to through highways, population centers, recreation areas, carrier terminals, State parks, and other locations of similar character. During the fall and winter of 1949-1950, more than 1,837,.000 acres were scouted and 888 infestations were located.

SPRAYING IS DONE in spring. Priorities are given to areas where surveys have indicated the hazard of spread of the pest is greatest.