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.
Four genera and 9 species of ants were collected by pitfall traps in a spruce budworm-infested forest of northern Maine. Myrmica detritinodis and Camponotus herculeanus were most abundant. In 1977, significantly more individuals and species were trapped in dense spruce-fir stands than in either uncut residual or clearcut strips. In 1978, clearcut-strip and dense-stand means were nonsignificant for individuals and species, but significantly more individuals and species were trapped in clearcut than in uncut residual strips. Ants were active during the spruce budworm’s developmental stages. Diversity of ant species was generally greater in dense stands than in strip-clearcut areas. Individuals were distributed unevenly among species but more evenly in dense stands than in strip-clearcut areas (residual + clearcut strips). Coefficients of community (CC) and percentage similarity (PS) values indicated ant species and individuals shared forest conditions in common; however, the most dissimilar neighboring habitats (uncut residual and clearcut strips) had few species in common. Neither age of strip clearcut (1–6 years) nor litter depth had much influence on mean catches and mean number of species of ants/trap/week.
The effects of three trap variables (age, saturation, and density) on catches of male spruce budworm moths were tested using Pherocon 1CP® traps baited with synthetic sex attractant in high-density budworm populations in Maine and low-density populations in Ontario. As trap age increased, the number of moths captured decreased significantly; traps aged 2, 10, and 16 days caught an average of 5.4, 28, and 27% fewer moths than unaged traps. Traps functionally saturate and trapping efficiency diminishes in both high and low density populations after ca. 50 moths are caught, although maximum observed catch was 193 moths/day. Density or spacing of traps also affects trap efficiency. Traps spaced 5 m apart may act as a single attraction source. Interference between traps was evident for traps spaced 10, 20, and 30 m apart, while traps spaced at 40 m had the least interference. However, interference at all spacings was noted during peak moth flight in 1978 when mean catch was ca. 140 moths/trap/day.Pherocon 1CP traps can be used for detecting and monitoring spruce budworm populations; however, influences of trap age, timing of trap placement, trap saturation, spacing of traps, and lure strength should be taken into account.
Mean daily progeny production by Trichogramma minutum (“Maine strain”) was 15.2 in Choristoneura fumiferana and 10.9 in Sitotroga cerealella eggs. Total progeny production was higher in S. cerealella, but not significantly different from that of C. fumiferana eggs. Significantly more eggs were deposited by T. minutum the first day than in subsequent days regardless of host. We found no significant relationship between progeny produced by females and the day of male death as previously reported. Ratio of females:males decreased significantly with increasing age and opposition activity of the mother. The oviposition period spanned 68% of the female's life span when S. cerealella eggs were available; whereas females spent significantly less time (60%) ovipositing in C. fumiferana eggs.
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