Patterns of spider (Araneae) abundance during an outbreak of western spruce budworm (Lepidoptera: Tortricidae)

Spiders are acknowledged predators on all life stages of western spruce budworm, Choristoneura occidentalis Freeman, yet little is known about their communities in budworm infested forests. Systematic samples of arboreal spiders were taken from the mid-crowns of Douglas-fir, Pseudotsuga menziesii variety glauca (Beissner) France, and grand fir, Abies grandis (Douglas ex D. Don) Lindley, for 3 consecutive years during a budworm outbreak. Densities over the 3-yr period averaged 5.6 spiders and 288.0 fourth-instar budworms per square meter of branch area. Spider densities were significantly higher on Douglas-fir (7.8/m(2)) than on grand fir (4.6/m(2)); however, species composition of spiders was similar on both tree species. Overall densities of spiders did not differ significantly over the 3 yr. All sampled spiders were classified by family, but because of large numbers of juveniles only 20% could be identified to species. Nonetheless, 13 families and at least 26 species were represented in die samples. Relative abundance of both families and species followed a typical logarithmic-series distribution in which a relatively few taxa accounted for most of the individuals sampled. Total number of spiders sampled was about evenly divided between web-spinning (47.3%) and hunting (52.7%) guilds; however, there were almost twice as many web-spinning species as hunting species. Species-diversity indices also were consistently higher each year for web-spinners than for hunters. The salticid species Pelegrina aeneola (= Metaphidippus aeneolus) (Curtis), a well-known predator of defoliating insects, was clearly dominant each year and accounted for almost half of all hunting spiders. The results offer a convenient marker for comparison with the relative structure of arboreal spider communities in other forest ecosystems.