Predation by Birds on Spruce Budworm Choristoneura Fumiferana: Functional, Numerical, and Total Responses

Abstract

This study examined the hypothesis that predation by forest birds restricts expansion of low-density populations of the spruce bud worm Choristoneura fumiferana (Lepidoptera: Tortricidae). Bird populations were censused in spruce-fir stands of low to moderate spruce budworm density in northern New Hampshire and western Maine. Diets were determined from stomach contents of collected birds. Bud worm population densities were estimated by sampling fourth-instar larvae and successfully emerged pupae (counted after moth emergence).Populations of spruce bud worms ranged from 80 x 10 3 to > 22 x 10 6 larvae/ha and from 0 to 1.2 x 10 6 pupae/ha. Birds ate spruce budworms throughout the daylight hours.Blackburnian Warblers consumed the most Choristoneura (X = 28 420 budworms/ha), followed by Cape May Warblers (X = 26 220 budworms/ha), and grading down to no consumption by some species.In plots where spruce budworm densities ranged from low to transitional, the entire bird community as well as specific groups of birds, i.e., overstory warblers and Goldencrowned Kinglets; sparrows, vireos, and juncos; and nuthatches, thrushes, and Purple Finches, showed significant functional responses to increasing budworm density. When transitional bud worm densities were eliminated from the analysis, only the warbler-kinglet group exhibited a significant functional response. Only two species, Canada Warbler and Golden-crowned Kinglet, showed numerical responses to increasing budworm numbers. The percentage of the budworm population consumed by birds decreased as budworm numbers increased. Birds consumed 84% of the larvae and pupae where budworm populations were low, 22% where budworm populations were intermediate between low and high density. Once fourth-instar larvae exceeded 10 6 individuals/hectare, bird predation was ineffectual. The hypothesis that birds can effectively limit budworm increase is compatible with three recent theories to explain budworm outbreaks.The forest stands we studied were dense, even-aged, with little understory. Numbers of birds potentially can be increased by modifying these stands through silvicultural means . Birds are capable of dampening the seriousness of spruce budworm infestations when habitats are suitable for supporting adequate populations of these effective predators.

Nonlinear dynamics in ecosystem response to climatic change: Case studies and policy implications

“…Its preferred foods include balsam fir (A. balsamea) and several spruce species (Picea spp.). Spruce budworms are usually present at low densities (<100,000 larvae ha À1 ), but periodic outbreaks occur throughout the species range with densities reaching 22,000,000 larvae ha À1 (Crawford and Jennings, 1989). These outbreaks can extend over more than 70 million ha and last for up to 15 years, causing annual loss of more than 80% of foliage on fir and spruce trees.…”

“…These outbreaks can extend over more than 70 million ha and last for up to 15 years, causing annual loss of more than 80% of foliage on fir and spruce trees. This extreme foliage loss can kill many of the trees in mature stands of balsam fir (Crawford and Jennings, 1989;Fleming and Volney, 1995). Trees not killed by defoliation are often at risk from other phytophagous insects and pathogenic diseases and standing dead trees increase the risk of fire.…”

“…The control of some populations of spruce budworm may be aided by avian predators, especially some of the wood warblers (Crawford and Jennings, 1989). Several warbler species, including Cape May (Dendroica tigrina), bay-breasted (D. castanea), blackburnian (D. fusca), Tennessee (Vermivora peregrina), and Nashville (V. ruficapilla), are important predators of spruce budworms.…”

“…Several warbler species, including Cape May (Dendroica tigrina), bay-breasted (D. castanea), blackburnian (D. fusca), Tennessee (Vermivora peregrina), and Nashville (V. ruficapilla), are important predators of spruce budworms. Some of these species, especially bay-breasted warbler, show functional responses to increases in budworms with individual birds moving into the area and increasing their reproductive output in response to increases in the budworm's population (Crawford and Jennings, 1989). Birds consume up to 84% of budworm larvae and pupae when budworm populations are low (approximately 100,000 ha À1 ) and up to 22% when populations reach approximately 500,000 ha À1 .…”

“…Birds consume up to 84% of budworm larvae and pupae when budworm populations are low (approximately 100,000 ha À1 ) and up to 22% when populations reach approximately 500,000 ha À1 . However, bird predation cannot effectively neutralize budworm populations in concentrations exceeding 1,000,000 larvae ha À1 (Crawford and Jennings, 1989).…”