Abstract
Virtually all studies of plant-herbivore-natural enemy interactions focus on plant quality as the major constraint on development and survival. However, for many gregarious feeding insect herbivores that feed on small or ephemeral plants, the quantity of resources is much more limiting, yet this area has received virtually no attention. Here, in both lab and semi-field experiments using tents containing variably sized clusters of food plants, we studied the effects of periodic food deprivation in a tri-trophic system where quantitative constraints are profoundly important on insect performance. The large cabbage white Pieris brassicae, is a specialist herbivore of relatively small wild brassicaceous plants that grow in variable densities, with black mustard (Brassica nigra) being one of the most important. Larvae of P. brassicae are in turn attacked by a specialist endoparasitoid wasp, Cotesia glomerata. Increasing the length of food deprivation of newly molted final instar caterpillars significantly decreased herbivore and parasitoid survival and biomass, but shortened their development time. Moreover, the ability of caterpillars to recover when provided with food again was correlated with the length of the food deprivation period. In outdoor tents with natural vegetation, we created conditions similar to those faced by P. brassicae in nature by manipulating plant density. Low densities of B. nigra lead to potential starvation of P. brassicae broods and their parasitoids, replicating nutritional conditions of the lab experiments. The ability of both unparasitized and parasitized caterpillars to find corner plants was similar but decreased with central plant density. Survival of both the herbivore and parasitoid increased with plant density and was higher for unparasitized than for parasitized caterpillars. Our results, in comparison with previous studies, reveal that quantitative constraints are far more important that qualitative constraints on the performance of gregarious insect herbivores and their gregarious parasitoids in nature.
Highlights
In nature, there is often considerable variation in the ways in which a species distributes itself within its habitat relative to conspecifics
Insect herbivores obtain 80% or more of biomass in the final instar, we examined temporal food deprivation in L5 P. brassicae on the performance of herbivores and parasitoids
Effect food deprivation duration of L5 Pieris brassicae on the performance of herbivores and parasitoids
Summary
There is often considerable variation in the ways in which a species distributes itself within its habitat relative to conspecifics. For group living to be favored, the overall benefits must outweigh the costs, and the resulting fitness pay-off to each group member must be greater than for a solitary individual [1]. Similar patterns may be observed within invertebrate groups, e.g. insects, where gregarious living is often based on the genetic relatedness of the individuals in the group [3]. This is certainly true in the social Hymenoptera, such as ants, bees and wasps, which reproduce via haplodiploidy and where distinct castes perform specific functions that optimize colony fitness [4]. In the Lepidoptera (butterflies and moths), for example, large clutch size and aggregative feeding by the larvae has evolved independently in up to 23 families [8]
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