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.

事实证明，在大多数系统中，风险在行为选择中的作用在导致合作伙伴攻击猎物或宿主方面很难发挥作用，因为风险难以量化。对于寄生类黄蜂，使宿主瘫痪是成功寄生和繁殖的关键步骤，但由于宿主的防御，黄蜂冒着死亡或受伤的危险，可能需要集体行动，这为破译推动合作的机制提供了模型系统。我们研究了通过使用寄生性类固盘菌（Sclerodermus guani）及其寄主莫尼莫斯天牛（Monochamus alternatus），黄蜂扮演合作者或搭便车者的行为。。首先，当一个或两个黄蜂使一个寄主瘫痪时，两者的死亡率都随着寄主大小的增加而增加，但是当有两个黄蜂时，两者的死亡率显着降低。其次，适应性分析表明，黄蜂是否参与了宿主的瘫痪，并不影响产卵数。最后，当两个黄蜂瘫痪一个大型寄主时，较高的死亡风险导致更多的合作，而较低的风险导致更为频繁的搭便车。我们的研究表明，风险参与了合作水平的中介，并提供了由风险驱动的寄生虫合作的实验性证明。