Early instar mortality of a forest pest caterpillar: Which mortality sources increase during an outbreak crash?

Background/Question/Methods

Collapses of insect pest outbreaks are often attributed to delayed density dependence of predation. The forest tent caterpillar, Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), is an outbreaking pest species defoliating mixed wood boreal forests in eastern Canada. This species presents periodic population dynamics with peaks every 10 years and outbreaks lasting 1–3 years impacting the health of its host tree. However, the mechanisms behind this pattern are still not well understood. In this study, we examined the relative importance of various sources of early-instar mortality during and after the crash of an outbreak, testing for density dependence. We used a triad set-up of complete predator exclusion, partial exclusion, and free colonies to distinguish between intrinsic mortality (caused by, e.g., pathogens) and mortality caused by flighted and walking natural enemies. This way, we can test and distinguish patterns specific to different functional groups representing differ sources of mortality. We then compared sites that were defoliated during the last outbreak with control sites that were not afflicted, in order to examine if the pressures of outbreaks drove different population dynamics of natural enemies than a site that did not experience those pressures.

Results/Conclusions

Overall, survival was lowest in the free colonies and increased with partial and complete predator exclusion. Survival was also higher in control than in outbreak sites and higher in the final year of the outbreak than in the following year. We observed no changes in mortality from walking enemies, but an increase in intrinsic mortality and mortality from flighted enemies. In the year following the outbreak. This increase is consistent with density dependence of these mortality sources, but its occurrence in the control sites as well was unexpected. These findings show that mortality from flighted natural enemies and intrinsic source increases after the outbreak, but do not contribute to the population crash. However, they help maintain low endemic levels between outbreaks. These findings are important for integrated pest management in forests, making sure that the ecosystem is suitable for flighted natural enemies as well as pathogens that are relatively specific to pests.