Sex-Biased Predation by Web-Spinning Spiders (Araneae) on Spruce Budworm Moths
- Jennings, Daniel T.
University of Maine
- Houseweart, Mark W.
University of Maine
Web-spinning spiders of 6 families, 12 general, and at least 15 species preyed on spruce budworm, Choristoneura fumiferana (Clem.), moths in Maine. Significantly more (G-tests, P≤0.05) male than female moths were captured by 9 web-spinner species, and the overall capture by spiders was significantly biased (P≤0.001) toward male moths. Most of the budworm-moth prey were found in webs of Frontinella pyramitela (Walck.) (30.6%) and Theridion pictum (Walck.) (45.0%). Multiple observations of the same web (temporal replication) indicated that T. pictum captured significantly more (Z=-4.36, P=0.000) budworm moths/web than F. pyramitela. However, such differences in prey-capture rates were not detected (Z=-1.49, P=0.14) over several locations (spatial replication). Web surveys during the spruce budworm's moth-flight period indicated that percentages of webs with budworm prey were about equal; F. pyramitela (x̄ = 19.5 ± 5.7), T. pictum (x̄= 18.9 ± 3.6), all species (x̄ = 16.0 ± 2.7).
Trees occupied by T. pictum were significantly taller (P ≤ 0.001) and webs significantly higher (P ≤ 0.001) than trees and webs of F. pyramitela. For both spider species, mean relative web heights was > 60% of tree height, possibly indicating nonrandom choices of foraging patch. However, tree height and web height were not significantly (P ≥ 0.05) related to prey-capture for F. pyramitela webs; tree height was significantly taller (P = 0.009) for T. pictum webs with budworm moths. Two species of keptoparasites, Argyrodes trigonum (Hentz) and A. fictilum (Hentz), were associated with host-spider webs that captured spruce budworm moths.
Possible explanations for the observed sex-biased predation include: 1) sex-pheromone mimicry, 2) uneven prey densities, 3) accidental capture, 4) moth behavior, and 5) moth-flight activity. Because of the potential impacts on spruce budworm reproduction, predation by spiders gains increased importance as a source of moth mortality.
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