Why do Some Beetles Develop in Fallen Twigs?

Many beetle species emerge in twigs pruned from the host tree by larvae inside the twig or externally girdled by adult females. Benefits of developing in fallen twigs have been afforded little experimental attention. If predation or parasitism in the canopy drive pruning and girdling behaviors, emergence is expected to be greatest in twigs on the ground, where predation and parasitism are expected to be minimized. Here, 220 twigs pruned from oak trees by larval Anelaphus parallelus (Newman) (Coleoptera: Cerambycidae) were randomly placed into one of four treatments in a forested hedge: 1) on the ground; 2) on the ground within a mesh bag; 3) tied into the hedge 2.5 m above the ground; 4) secured in a mesh bag and tied into the hedge; or 5) stored in a cool basement. The percentage of beetle emergence was greatest in twigs stored in the basement (87%) and bagged twigs on the ground (74%) and was least from unbagged twigs placed in the hedge (40%). Reduced rates of predation on the ground are, therefore, implicated as a potential selective force for pruning behaviors. The percentage of parasitized twigs was least from twigs stored in a basement (2%) but, unexpectedly, greatest from twigs placed in mesh bags in the hedge (24%). Support for parasitism as a determinant of pruning behavior was, therefore, ambiguous but may be confounded if mesh bags did not deter parasitoids. Considering results of other studies, selective forces for pruning and girdling behaviors may not generalize among species.