Contributions of female oviposition patterns and larval behavior to group defense in conifer sawflies (hymenoptera: diprionidae)

Abstract

We studied the effects of adult oviposition and larval interactions on the defensive potential of gregarious behavior in conifer sawflies. Aggregation size and distribution initially reflected adult host plant selection and oviposition behavior. The contagious distribution of egg clusters resulted in part from the utilization of individual trees by multiple females, and of single host shoots by several females. Trees with the greates degree of prior defoliation received the most eggs, even though the potential for larval crowding made resource depletion possible. Foliar monoterpene and nitrogen contents explained only a small proportion of variability in female host utilization. Conifer needle architecture restricted the size of larval subgroups within aggregations, and limited the degree of defensive cohesiveness between subgroups. Subgroup turnover was frequent and independent of local food depletion. Risk of predation from wood ants varied with larval aggregation size and predator foraging level. When ant activity was high, large aggregations suffered greater numerical losses, but showed lower per capita predatory risk, than small groups. Results suggest that female oviposition patterns are influenced in part by the defensive benefits gained by offspring in large aggregations. Against ants, dilution effects and defensive synchrony due to gregariousness contribute to the overall antipredator strategy of sawfly larvae.

Experimental evidence for associational resistance against the European pine sawfly in mixed tree stands

“…A female lays in August-September approximately 50 eggs (Juutinen 1967) by placing them in rows inside the current-year needles, and the eggs hatch in May-July in the next year. Gregarious N. sertifer larvae defend themselves as a group against enemies (Codella and Raffa 1995) by hooking their bodies and extracting resin droplets from their mouth. In late June-early July the larvae stop feeding and pupate in the litter layer of soil.…”

Two types of refuge have opposite effects on the size of larval aggregations in a tropical defoliator

“…As over 50% of all parasitism in the study was attributable to tachinids, this observation suggests that late instars located at the group perimeter are most susceptible to parasitoids. Thus, in this system, reduced rates of parasitism in larger (versus smaller) exposed aggregations may be due to both an increased efficiency of group versus individual defence [e.g., head-flicking is quite effective for groups of sawflies versus for individuals (Codella & Raffa, 1995)] and to an indirect effect of dilution associated with the position occupied in large groups. In larger aggregations a smaller proportion of larvae will be located on the group perimeter, where parasitism risk may be highest.…”

Comparison of the fat allocation patterns in female pine sawflies (Hymenoptera: Diprionidae)

“…D. pini and N. sertifer are gregarious and lay their eggs in one large cluster (Pschorn-Walcher, 1982;Blumke & Anderbrant, 1997). Sur vival of larvae increases with colony size (Lyons, 1962), probably because the feeding of the early instars is enhanced or large colonies are better able to defend themselves so reducing the mortality risk per capita (Heitland & Pschorn-Walcher, 1993;Codella & Raffa, 1995;Hunter, 2000). The data presented indicate that these sawfly species invested most of their fat in their egg load, producing more or larger eggs.…”