Search

In the parasitoid genus Sclerodermus, multiple foundresses produce and care for communal broods on large hosts, which can lead to greater reproductive success for group members than attempting to reproduce alone. We explore the consequences of foundress group size on the benefits of cooperative brooding and on brood sex ratios by providing groups of 10-55 foundresses with a single host and no alternative reproductive options. Within this range, increasing foundress group size leads to increasingly common failure in brood production and diminished per capita success. Group production of adult offspring declines once foundress number reaches around 25. Brood failure is usually at the early developmental stages, and current evidence suggests that there may be competition among foundresses for oviposition sites, possibly involving reproductive dominance and ovicide, which also delays initial brood production. Once broods become established, their rate of development is enhanced by large foundress numbers. The sex ratios of broods are very strongly female biased, irrespective of the foundress number. As this bias is not easily explained by standard models of local mate competition or by a recent model of local resource enhancement, we suggest an explanation based on control of sex allocation by a minority of dominant foundresses, which monopolise the production of adult males.

1. Plant resistance against herbivores can act directly (e.g. by producing toxins) and indirectly (e.g. by attracting natural enemies of herbivores). If plant secondary metabolites that cause direct resistance against herbivores, such as glucosinolates, negatively influence natural enemies, this may result in a conflict between direct and indirect plant resistance. 2. Our objectives were (i) to test herbivore-mediated effects of glucosinolates on the performance of two generalist predators, the marmalade hoverfly (Episyrphus balteatus) and the common green lacewing (Chrysoperla carnea) and (ii) to test whether intraspecific plant variation affects predator performance. 3. Predators were fed either Brevicoryne brassicae, a glucosinolate-sequestering specialist aphid that contains aphid-specific myrosinases, or Myzus persicae, a non-sequestering generalist aphid that excretes glucosinolates in the honeydew, reared on four different white cabbage cultivars. Predator performance and glucosinolate concentrations and profiles in B. brassicae and host-plant phloem were measured, a novel approach as previous studies often measured glucosinolate concentrations only in total leaf material. 4. Interestingly, the specialist aphid B. brassicae selectively sequestered glucosinolates from its host plant. The performance of predators fed this aphid species was lower than when fed M. persicae. When fed B. brassicae reared on different cultivars, differences in predator performance matched differences in glucosinolate profiles among the aphids. 5. We show that not only the prey species, but also the plant cultivar can have an effect on the performance of predators. Our results suggest that in the tritrophic system tested, there might be a conflict between direct and indirect plant resistance