Mortality risk promotes cooperation of wasps when paralysing hosts.

The role of risk in behavioural choices leading to cooperation in attacking a prey or a host has proven to be difficult to assess in most systems because risk is difficult to quantify. For parasitoid wasps, paralysing a host is a key step for successful parasitism and reproduction, but wasps risk death or injury because of host defences and may require group action, which provides a model system to decipher the mechanism driving cooperation. We explored scenarios where wasps behave as either cooperators or free-riding defectors by using the parasitoid Sclerodermus guani and its host Monochamus alternatus. First, when one or two wasps paralysed a single host, the mortality of both increased with increasing host size but was significantly lower when there were two wasps. Second, fitness analysis showed that whether a wasp was involved in paralysing a host did not affect the number of eggs laid. Finally, when two wasps paralysed single large hosts, higher mortality risks led to more cooperation, whereas lower risk led to more frequent free riding. Our study implies that risk is involved in mediating the level of cooperation and provides an experimental demonstration of risk-driven cooperation of parasitoids. • The wasp S. guani was tested to see how risk affects cooperation in paralysing hosts. • The risk level that wasps faced in paralysing hosts was proportional to host size. • The larger the host, the higher the observed cooperation level in paralysing hosts. • Results show direct empirical evidence for risk-driven cooperation in social insects. [ABSTRACT FROM AUTHOR]