The world is astoundingly variable, and organisms – from individuals to whole communities – must respond to variability to survive. One example of nature’s variability is the fluctuations in populations of spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), which cycle every 35 years. In this study, we examined how a parasitoid community altered its parasitism of budworm and other caterpillar species in response to these fluctuations. Budworm and other caterpillar species were sampled from balsam fir (Pinaceae) in three plots for 14 years in Atlantic Canada, then were reared to identify any emerging parasitoids. We found that the parasitoid community generally showed an indiscriminate response (i.e., no preference, where frequencies dictated parasitism rates) to changes in budworm frequencies relative to other caterpillar species on balsam fir. We also observed changes in topology and distributions of interaction strengths between the parasitoids, budworm, and other caterpillar species as budworm frequencies fluctuated. Our study contributes to the hypothesis that hardwood trees are a critical part of the budworm–parasitoid food web, where parasitoids attack other caterpillar species on hardwood trees when budworm populations are low. Taken together, our results show that a parasitoid community collectively alters species interactions in response to variable budworm frequencies, thereby fundamentally shifting food-web pathways.

世界千变万化，生物体——从个人到整个社区——必须对变化做出反应才能生存。大自然可变性的一个例子是云杉芽虫，Choristoneura fumiferana Clemens（鳞翅目：Tortricidae）种群的波动，它每 35 年循环一次。在这项研究中，我们研究了寄生蜂群落如何响应这些波动而改变其寄生于 budworm 和其他毛虫物种的情况。14 年来，在加拿大大西洋沿岸的三个地块中，从香脂冷杉（松科）中采集了芽虫和其他毛虫物种，然后进行饲养以识别任何新出现的寄生蜂。我们发现寄生蜂群落普遍表现出不分青红皂白的反应（即，没有偏好，其中频率决定了寄生率）相对于香脂冷杉上其他毛虫物种的芽虫频率变化。我们还观察到随着 budworm 频率波动，寄生蜂、budworm 和其他毛虫物种之间的拓扑结构和相互作用强度分布的变化。我们的研究有助于假设硬木树是 budworm-parasitoid 食物网的重要组成部分，当 budworm 种群低时，寄生虫会攻击硬木树上的其他毛虫物种。综上所述，我们的研究结果表明，寄生蜂群落共同改变了物种相互作用以响应可变的芽虫频率，从而从根本上改变食物网途径。