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Birds: Great Regulators of Forest Insect Pests

By Carrie Swadener

Many organisms have been introduced from their native areas to foreign lands, some with disastrous results. In the absence of naturel en enemies, populations of organisms such as the gypsy moth and the fire ant reach uncontrollable levels. Similar results are possible when naturel enemies of native organisms are eliminated.

Torolf Torgersen is an entomologist with the U.S. Forest Service's Pacific Northwest Forest and Range Experiment Station in La Grande, Oregon. He has been studying one example in Pacific Northwest forests, the effects of birds and other naturel enemies on two forest insect pests, the Douglas-fir tussock moth and the western spruce budworm. He is using the findings of these studies to emphasize to forest managers the need to provide for the wellbeing of these naturel enemies and thus prevent pest outbreaks.

Predators of the Tussock Moth

The first step in studying the effects of naturel enemies is to identify which organisms are the predators. In one of Torgersen's studies, a branch stocked with a known quantity of Douglas-fir tussock moth larvae or pupae was observed from a blind some distance away. In about 700 hours of observation, 21 species of birds were identified as suspected and/or observed predators of tussock moth larvae or pupae. In this same study the larval loss rates differed between study sites, and 78% of this variation could be accounted for by differences in bird density. Larval loss rates increased with bird density, indicating that birds were indeed important predators of the moth.

Other researchers, by analyzing the stomach contents of various bird specimens, had already identified about two dozen species of birds as predators of the western spruce budworm, half of which were also tussock moth predators.2

Together, these studies established that birds were important predatory forces on populations of the tussock moth and the spruce budworm.

How Effective are Bird Predators in Controlling insect Populations?

Further studies were now necessary to establish the effect bird predation has on populations of harmful forest insects. Torgersen assisted in devising an experiment to measure both bird and ant predation on the western spruce budworm. In this experiment, birds were excluded from some trees, ants from others, both birds and ants from others; and some trees were left exposed to both predator groups.3

He discovered that "either birds alone or ants alone were able to compensate [in reducing the survival rate of the spruce budworm], to a large extent, on trees from which the other group was excluded."3 In the absence of both birds and ants, however, any i "compensation [by any other predators 1 that occurred was trivial."4 At low densities, survival of the budworm on trees protected from both groups was ten to fifteen times higher than on trees' exposed to both predator groups. At higher densities survival on - protected trees was still two times higher.3 This study indicated that birds and ants were dominant controlling factors of budworm populations.

Are birds more effective predators than ants? Both groups have been frequently observed preying on all life stages of the budworm with the exception of egg masses, but ants seem to concentrate on the larger larvae and pupae. Since 2, birds can fly, they are able to catch "ballooning" larvae (larvae floating on strands of silk) and adult moths more efficiently than ground-dwelling ants.4 Birds are also more effective in foraging in the higher tree branches than ants.5 Some life of stages of the budworm occur during cola weather, and ants, being more affected by cold temperatures than birds, are less effective than birds during these times.4 However, it is very difficult to separate the effects of insectivorous birds from those of predacious ants, since both are present in the naturel forest environment. Birds and ants together play an important role in the population control of the western spruce budworm.

Effects of Insect Density on Bird Predation

In Torgersen's studies of both the Douglas-fir tussock moth and the western spruce budworm, an interesting discovery was made concerning birds' feeding habits. He found that as pest density increased, bird predation rate decreased.3 (See Figure 1.) This finding was instrumental in determining the role of bird predation on populations of these two forest pests. It seems unlikely that birds could be effective in controlling pest populations once they reach outbreak proportions. Instead, birds keep populations at low levers. Birds "help dampen or regulate insect numbers before an outbreak occurs."7

In any stand where the trees are susceptible to the spruce budworm or the tussock moth, these pests are present, usually at low levels.7 Under natural conditions, any such stand should have an adequate bird density for birds to play a significant role, together with other regulatory forces, in keeping pest populations at innocuous levels.4 The key phrase here, however, is "under natural conditions."

What happens when conditions are not natural? If something occurs to upset regulating forces, conditions become unstable and pest density increases. If left unchecked, pest density will reach outbreak proportions. As pest density increases, the regulatory role of birds and other predators fades in effectiveness. Under outbreak conditions, other factors become dominant in pest population control. To break the population explosion of the forest pest, a "synchrony of a lot of different factors,"4 such as disease or starvation, is necessary. Only when these other factors play their role in lowering the pest population from, outbreak levels will birds reclaim their role in pest population regulation.4 The influence of predators "in effect, is to lengthen the periods of forest stability between pest outbreaks."7 Birds play their most dominant role in pest outbreak prevention.

Managing Forests to Maximize Natural Controls

It seems that since predation is such an important factor in prevention of pest outbreaks, people would be eager to maximize the survival of natural enemies. Unfortunately, it is a human tendency to pay the most attention to outbreaks, and when the outbreak ceases, to ignore the organism and neglect preventive measures.4 This selective attention must be overcome; providing for maximal bird and other predation is fundamental to prevention of pest outbreaks. "The need to reduce damage to forests from insect pests suggests that managers view... natural enemies as a resource to be conserved and enhanced."2

The key to the maximization of predation is to provide adequate habitat for the predator, that's, to provide for food, nesting sites, cover and any other requirements necessary for survival of natural enemies. "Birds and ants can only perform as well as their habitat allows," says Torgersen.7

He makes several suggestions to forest managers in order for them to provide for the needs of natural enemies.7 The most important suggestion is to diversify the managed forest by utilizing irregularly shaped cutting techniques (as opposed to clear-cutting in square or circular patterns) or planting a variety of tree species rather than planting only one or two species. Conserving streamside or pond habitat can improve bird nesting success as well as improving the habitat of other animals. Torgersen also advocates minimizing application of herbicides to avoid the elimination of shrubs, grasses and other plants that results in reduction of the benefits these plants provide for many animals. In addition, insecticide use should be minimized to avoid unintentional yet harmful effects on nontarget organisms that could undermine the very survival of the targeted organism's natural enemies.

Current Research

Torgersen is presently involved in studies on the importance of leaving both fallen and standing dead wood in the forest environment. Dead wood is important as a source of food for insects and also for birds that prey on these insects. Since most birds feed on a wide variety of insects during different times of the year this also increases predation on pests of living trees. Dead wood provides nesting sites for both predacious ants and birds. Nutrients from dead wood are returned to the soil by many organisms, and these nutrients are then absorbed by the living trees. The more nutrients that are available in the soil, the healthier the living forest will be, and the less susceptible it will be to fatal damage as a result of insect attack."

Often, most of the dead wood from a forest area that has been cut over is removed, but through studies of Torgersen and others, forest managers are gradually becoming aware that leaving dead wood is critical to forest health and is an important factor in the enhancement of the habitats of man: y forest pests' natural enemies.4 7 addition to Torgersen's current involvement in studies about effects of dead wood on the for est environment, he is also working on an analysis of the effects of a recent application of carbaryl, a carbamate insecticide, over an area in eastern Oregon.

Torgersen has been an influence in the documentation of birds as predators of two common and damaging forest pests present in the Pacific Northwest, the Douglas-fir tussock moth and the western spruce budworm. He has also helped elucidate the role of birds and other naturel enemies in preventing outbreaks of these forest pests. Now he is attempting to communicate to forest managers the methods that will promote the survival of the natural enemies, since predation is such an important force in keeping pest populations under control.


1. Torgersen, T.R, et al. 1984. Avian predators of Douglas-fir tussock moth, Orygia pseudotsugata (McDunnough), (Lepidoptera: Lymantrlidae) In southwestern Oregon. Environmental Entomology 13(4):1018-1022.

2. Torgersen, T.R., R.R. Mason, and RW. Campbell, 1990. Predation by birds and ants on two forest Insect pests In the Pacific Northwest. Studies in Avian Biology 13:14-19.

3. Campbell, R.W., T.R. Torgersen, and N. Srivastava. 1983, A suggested role for predaceous birds and ants In the population dynamics of the western spruce budworm. Forest Science 29(4):779-790

4. Torgersen, T.R, entomologist, Pacific Northwest Forest Experiment Station. Personal communication. November 7,1991

5. Campbell, RW. and T.R Torgersen. 1983. Effect of branch height on predation of western spruce budworm (Lepidoptera: Tortricidae) pupae by birds and ants. Environmental Entomology 12(3):697-699.

6. Torgersen, T.R., RR Mason, and H.C. Paul. 1983. Predation on pupae of Douglas-fir tussock moth, Orygia pseudotsugata (McDunnough) Lepidoptera: Lymantriidae. Environmental Entomology 12(6):1678-1682.

7. Torgersen, T.R 1988-1989. Natural enemies In forest Insect regulation. Newsletter of the Northeast Oregon Natural History Society. Three-part series.

Citation for this article: Swadener, Carrie. 1991, " Birds : Great regulators of forest insect pests.", Vol. 11, No. 4, Winter 1991, pp. 13 - 14.

Copyright 1991 Northwest Coalition for Alternatives to Pesticides.

Reprinted with permission.

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