The Spruce Budworm

R. C. Brown.

The spruce budworm has destroyed thousands of square miles of coniferous forests in the United States and Canada.

An article in Trees, the Yearbook of Agriculture for 1949, discussed an outbreak in Canada and emphasized the importance of proper forest management as a way to alleviate the damage that the budworm inflicts on forests.

Since then the infestation has spread eastward over a wider area in Quebec, and tree mortality has greatly increased there and in Ontario. Noticeable defoliation of balsam fir and spruce is now evident over a quarter million acres in northern Maine. Heavily defoliated areas covering about 1.5 million acres have appeared in New Brunswick. Nearly 3 million acres of Douglas-fir have been severely defoliated in eastern Oregon since 1945, and severe outbreaks have occurred south of Mount Hood and near Eugene. Heavy outbreaks have started in Montana. In 1949, 1950, and 1951, nearly 2.5 million acres of Douglas-fir in Oregon were sprayed with DDT from airplanes in an attempt to save the forests from destruction.

The menace of the spruce budworm to the pulp, paper, and the lumber industries, the devastation already done in this country and Canada, and the threat of much greater damage has stimulated the Department of Agriculture to use every effort to combat it. Surveys are being conducted by the Department and State and private agencies to detect and appraise outbreaks. Where defoliation is sufficiently severe to cause trees to die, steps are taken to apply DDT to kill the feeding budworms. Foresters and entomologists are urging timberland owners to carry out cutting operations designed to lessen the impact of spruce budworm defoliation in threatened areas. Investigations are conducted on nearly every phase of the problem.

THE SPRUCE BUDWORM has worried timberland owners, foresters, and entomologists for 4o years. An investigation was conducted by J. M. Swaine and F. C. Craighead in eastern Canada in the early 1920's. It was primarily a study of post-outbreak conditions, for the heaviest defoliation had occurred in the preceding decade. But that did not detract from the value of the studies, for they permitted an appraisal of the damage under a wide variety of forest conditions and revealed the stands that best withstood the onslaught. They furnished information about ways to manage forests so as to minimize budworm damage. Their findings have been supported by later investigations in Canada. Observations in Ontario and Quebec have shown, for instance, that severe damage has occurred where balsam fir was the predominant species in the forest and that pure stands of black spruce have suffered practically no damage, while white spruce has been seriously injured. White spruce, however, is relatively unimportant as a component of the spruce-fir forests of the Northeastern States.

The studies are a basis for recommending three general methods for managing spruce-fir stands: To clear-cut mature and overmature balsam stands; to operate balsam stands on a short rotation ; to try to increase the proportion of red and black spruce in the stands.

Studies conducted by S. A. Graham and L. W. Orr in Minnesota in the late 1930's on the silvicultural aspects of the problem substantiated the findings of Swaine and Craighead and gave further support to the forest-management methods I have described. The investigations added another important suggestion : "To limit the size of individual tracts of susceptible types to the minimum size that can be economically handled as a unit, and either by cultural operation or by logging, to separate them from one another by non-susceptible types or by stands that will not become susceptible until a later date."

The current outbreak in Canada, which reached epidemic proportions in Ontario in 1935, caused the Dominion Department of Agriculture to initiate a research program. With the backing of the pulp and paper industry, modern laboratories have been constructed where fundamental research work on the spruce budworm is being done. Field studies are conducted over large areas in the Canadian forests.

By 1944 the budworm infestation in Ontario and Quebec covered about 125 million acres. An enormous amount of timber had been killed. That and the memory of an earlier outbreak in Maine prompted the timberland owners of the Northeastern States to ask Congress for funds to study methods for combating the spruce budworm. In 1944 the funds were made available to the Bureau of Entomology and Plant Quarantine and the Forest Service. Only a few specimens of the insect could be found then in New England, and plans were made to initiate studies in 1945 in Quebec in cooperation with Dominion entomologists. In that year an infestation was discovered in the Adirondacks of New York. Field studies were carried on there until 1948, when the infestation subsided. Meanwhile an increase in budworm population occurred in northern Maine, and field investigations were transferred there.

The research program in the Northeast has involved the following projects : The development of survey techniques; biological and natural control investigations; the application of biological information to silvicultural practices; control of the insect through the airplane application of insecticides; and the effect of the application of DDT on fish and wildlife.

One of the first problems was to develop a method for measuring the numbers of the insect in order to evaluate the effect of natural control factors and to measure the degree of control obtained from insecticide applications. Early investigations proved the futility of attempting to obtain records of the population of the insect on the entire tree because of its tiny size in its early stages, its concealed habit of feeding, and the labor and time involved in examining large volumes of foliage. Because only a small portion of the foliage from an individual tree could be effectively sampled, a 15-inch twig was adopted as the sampling unit. This sample could be used to obtain population records for larvae in all stages of development, for pupae, and also for egg masses of the insect. Although it did not provide a value for the total number of budworms on a tree or in an area, it served a useful purpose in comparing population densities from place to place or year to year. Reductions in population from stage to stage during a single generation could also be measured.

The 15-inch twigs taken with a pole pruner at a height of about 20 feet from the ground gave a fair measure of budworm populations when a number of representative trees were sampled. Thus, in conducting surveys to determine population trends, five 15-inch twigs taken from each of five trees at a sampling station was adopted as the standard procedure for measuring larval, pupal, and egg-mass populations. Estimates of percentages of defoliation are also recorded.

Much of the spruce-fir region of Maine cannot be reached by roads or navigable streams, and aerial surveys have been valuable in detecting defoliation. The Cessna 195 seaplane, a five-seated high-wing monoplane, with no struts to obstruct the view, is good for the survey work. By flying at an average speed of 95 miles an hour and at a constant elevation of 200 feet above the ground, an observer can detect defoliation in excess of 20 percent.

By following flight lines on a map and using an operation recorder, one can obtain a permanent record of defoliation. The records from the individual flight lines may then be transferred to a map in much the same manner that a type map is made up for a timber cruise. Such a system makes it possible to conduct aerial surveys annually along the same flight lines and thus obtain accurate information on the progress of the infestation. In areas where surveys had previously indicated the presence of an infestation of the budworm, flight lines were spaced 3 miles apart. When the observers could no longer detect defoliation along a line, the distance between flight lines was increased to 6 miles. If no defoliation showed up on those lines, the distance was increased to 1 miles. Ground checks made at several points revealed that the defoliation records obtained from the aerial survey were surprisingly accurate.

Emphasis has been given to studies of the biology and natural control of the spruce budworm. The studies have provided information on the seasonal development of the insect and its food-plant preferences. They indicated that the larvae prefer the foliage of balsam fir and develop readily on both old and current growth. When larvae emerge from hibernation they first mine the old needles. Then they enter the opening buds as soon as growth starts. The larvae develop readily on the new foliage of red and black spruce, but survival is poor on the old foliage of those species. The buds on red and black spruce open later than those on balsam; red and black spruce therefore are less favorable food plants than balsam fir. Such fundamental information on the biology and feeding habits of the budworm is a basis for formulating methods for its silvicultural control.

Also emphasized is the effect of natural factors of control particularly measures of the controlling effect of egg, larval, and pupal parasites, diseases of larvae and pupae, predation by birds, and overwintering mortality.

Eleven quarter-acre plots representing different forest types were established in the Adirondacks in 1946 to get information on the degree of parasitization of the spruce budworm. Collections of eggs, hibernating larvae, small larvae in the buds, full-grown larvae, and pupae were made at each point and either dissected or reared to determine the percentage of parasitization. Aggregate percentages for all plots increased from 62 in 1946 to 72 in 1947 and to 75 in 1948.

Sixty-one species of parasites of the spruce budworm are known, but only about 12 of them were important in the Adirondack region. Disease apparently played an insignificant part in controlling the budworm. Mortality that occurred between the time the eggs hatched in late summer and the time the tiny larvae started mining needles in the spring accounted for a loss of 76 percent in 1946-1947 and 80 percent in 1947-1948. No quantitative measure of the effect of birds in controlling the budworm was obtained, but entomologists believe that the tremendous reduction in population during the late larval and pupal period in 1947 could be largely attributed to birds. That was possible because there were enough birds in the region almost to eliminate the budworms left after other natural factors had taken their toll.

The infestation that appeared in 1945 and threatened to attain outbreak proportions in 1946 was brought under control by 1948. Natural factors were responsible the composition and characteristics of the forest in the Adirondack region were particularly favorable for the natural enemies of the spruce budworm. The spruce-fir stands in the area are relatively small and are interspersed with hardwoods, in contrast with regions farther north where large continuous areas of spruce and fir exist and where devastating outbreaks have occurred.