Interactions between butterflies and caterpillars in the genus Pieris and plants in the family Brassicaceae are among the best explored in the field of insect-plant biology. However, we report here for the first time that Pieris brassicae, commonly assumed to be a typical folivore, actually prefers to feed on flowers of three Brassica nigra genotypes rather than on their leaves. First-and second-instar caterpillars were observed to feed primarily on leaves, whereas late second and early third instars migrated via the small leaves of the flower branches to the flower buds and flowers. Once flower feeding began, no further leaf feeding was observed. We investigated growth rates of caterpillars having access exclusively to either leaves of flowering plants or flowers. In addition, we analyzed glucosinolate concentrations in leaves and flowers. Late-second-and early-third-instar P. brassicae caterpillars moved upward into the inflorescences of B. nigra and fed on buds and flowers until the end of the final (fifth) instar, after which they entered into the wandering stage, leaving the plant in search of a pupation site. Flower feeding sustained a significantly higher growth rate than leaf feeding. Flowers contained levels of glucosinolates up to five times higher than those of leaves. Five glucosinolates were identified: the aliphatic sinigrin, the aromatic phenyethylglucosinolate, and three indole glucosinolates: glucobrassicin, 4-methoxyglucobrassicin, and 4-hydroxyglucobrassicin. Tissue type and genotype were the most important factors affecting levels of identified glucosinolates. Sinigrin was by far the most abundant compound in all three genotypes. Sinigrin, 4-hydroxyglucobrassicin, and phenylethylglucosinolate were present at significantly higher levels in flowers than in leaves. In response to caterpillar feeding, sinigrin levels in both leaves and flowers were significantly higher than in undamaged plants, whereas 4-hydroxyglucobrassicin leaf levels were lower. Our results show that feeding on flower tissues, containing higher concentrations of glucosinolates, provides P. brassicae with a nutritional benefit in terms of higher growth rate. This preference appears to be in contrast to published negative effects of volatile glucosinolate breakdown products on the closely related Pieris rapae.

Plants attacked by herbivorous insects emit volatile compounds that attract predators or parasitoids of the herbivores. Plant fitness increases when these herbivorous insects are parasitized by solitary parasitoids, but whether gregarious koinobiont parasitoids also confer a benefit to plant fitness has been disputed. We investigated the relationship between parasitoid load of the gregarious Cotesia glomerata (L.) (Hymenoptera: Braconidae), food consumption by larvae of their host Pieris brassicae L. (Lepidoptera: Pieridae), and seed production in a host plant, Brassica nigra L. (Brassicaceae), in a greenhouse experiment. Plants damaged by caterpillars containing single parasitoid broods produced a similar amount of seeds as undamaged control plants and produced significantly more seeds than plants with unparasitized caterpillars feeding on them. Increasing the parasitoid load to levels likely resulting from superparasitization, feeding by parasitized caterpillars was significantly negatively correlated with plant seed production. Higher parasitoid brood sizes were negatively correlated with pupal weight of Cotesia glomerata , revealing scramble competition leading to a fitness trade-off for the parasitoid. Our results suggest that in this tritrophic system plant fitness is higher when the gregarious parasitoid deposits a single brood into its herbivorous host. A prediction following from these results is that plants benefit from recruiting parasitoids when superparasitization is prevented. This is supported by our previous results on down-regulation of synomone production when Brassica oleracea was fed on by parasitized caterpillars of P. brassicae . We conclude that variable parasitoid loads in gregarious koinobiont parasitoids largely explain existing controversies about the putative benefit of recruiting these parasitoids for plant reproduction.

The western conifer seed bug, Leptoglossus occidentalis Heidemann, and the southern pine seed bug, L. corculus (Say), feed on the developing seeds of conifers causing great losses to seed crops (Koerber 1963; Connelly and Schowalter 1991; Schowalter 1996). When high populations threaten the yieldof genetically superior, high quality seed in seed orchards, chemical controls may be used (DeBarr et al. 1982). However, because even minor uses of chemical pesticides are increasingly restricted, particularly in British Columbia where seed orchards commonly interface with urban areas, damaging populations of L. occidentalis may not be controlled at all.