Kinship effects in quasi-social parasitoids II : co-foundress relatedness and host dangerousness interactively affect host exploitation

Sclerodermus brevicornis is a parasitoid that exhibits cooperative multi-foundress brood production. Prior work showed that the time lag to paralysis of small-sized hosts is shorter when co-foundress relatedness is higher and predicted that the greater risks and greater benefits of attacking larger hosts would combine with co-foundress relatedness to determine the limits to the size of a host that a female is selected to attack as a public good. It was also predicted that the time to host attack would be affected by an interaction between host size and relatedness. Here, we show empirically that both host size and kinship affect S. brevicornis reproduction and that they interact to influence the timing of host attack. We also find effects of co-foundress relatedness after hosts have been suppressed successfully. A public goods model using parameters estimated for S. brevicornis again suggests that selection for individual foundresses to attack and, if successful, to share hosts will be dependent on both the size of the host and the relatedness of the foundresses to any co-foundresses present. Females will not be selected to bear the individual cost of a public good when hosts are large and dangerous or when their relatedness to the co-foundress is low. We conclude that although reproductive behaviours exhibited by Sclerodermus females can be cooperative, they are unlikely to be exhibited without reference to kinship or to the risks involved in attempting to suppress and share large and dangerous hosts.