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.

Host diet affects the defensive efficacy of Neodiprion sawflies. In laboratory assays with wood ants (Formica obscuripes), secretions from larvae reared on Pinus banksiana were the most repellent, while those from P. resinosa feeders were the least so. This was explained diterpene resin acid content, but not total monoterpene content. The terpene content of regurgitant generally reflected dietary concentrations. Compounds were sequestered nonselectively by larvae. Host-based differences in defense persisted at the behavioral level. P. banksiana feeders regurgitated greater volumes of fluid and were less likely to be disabled or killed by ants in one-on-one interactions than were larvae fed on P. resinosa. The defensive advantages of host diet conflicted with developmental requirements. N. sertifer reared on P. banksiana (the best diet for defense in all cases) had lower cocoon weights (a correlate of fecundity) than those reared on other diets, and had prolonged larval development compared to insects fed P. sylvestris. No such tradeoff was detected in N. lecontei. Larvae of both species strongly preferred P. banksiana over P. resinosa in feeding choice assays.

The black swallowtail butterfly (Papilio polyxenes asterius Stoll), is commonly assumed to exhibit female-limited Batesian mimicry ofthe aposematic pipevine swallowtail (Rattus philenor [L.]), since the dorsal wing surfaces of P. polyxenes females, but not males, resemble those of the model. However, the ventral wing surface is monomorphic and closely resembles that ofthe model in both sexes. Thus both sexes of P. polyxenes should benefit from mimicry during periods of ventral surface exposure, such as during overnight roosting and other times ofhigh predatory risk.Eight blue jays (Cyanocitta cristata L.) were offered ventrally and dorsally exposed butterfly prey items in an outdoor aviary. Model-conditioned birds refused male and female P. polyxenesequally when the butterflies were presented ventrally. However, significantly more males than females were attacked when the dimorphic dorsum was visible. Both sexes are thus similarly protected when the ventral wing surface is displayed during roosting. The high degree of bird-to-bird variability in response to P. polyxenes mimics suggests that there is a spectrum in ability or willingness of predators to discriminate among mimics of varying similarity to the model. Sexual dimorphism of the dorsal surface of P. polyxenes wings may reflect sexual selection favoring males that are recognizable as satisfactory mates or intrasexual competitors.

No abstract

No abstract