How much additive mortality is needed to suppress an outbreak?—A neglected question in forest insect pest management

Abstract

The concepts of compensation and additive mortality form the ecological basis for understanding animal population responses to exploitation by humans. In the context of pest management, compensation is a density-dependent response that allows populations to offset control-related mortality, often via increased survival or reinvasion. Additive mortality, in contrast, accrues when a population's compensatory capacity is insufficient to offset losses, resulting in a net reduction in population size or growth rate. These concepts are rarely considered in forest insect pest management, which tends to emphasise short-term plant protection over long-term population control. We used published life table data for a major native forest insect defoliator, the spruce budworm (Choristoneura fumiferana [Lepidoptera: Tortricidae]) to simulate the amount of additive mortality required to suppress an outbreak. Simulations also assessed how the failure to account for different compensatory responses could hinder successful control. Our results suggest that only relatively modest amounts of additive mortality (perhaps as low as approximately 8%–18%) may be needed to stop spruce budworm from outbreaking, with immigration being the strongest potential compensatory hindrance to outbreak suppression. Many of the compensatory responses that thwarted outbreak suppression in the past (e.g., low detection efficiency, immigration, indiscriminate killing of predators and parasitoids) have contemporary solutions that could increase additive mortality and thereby enhance the feasibility of population control strategies for native forest insect pests. Our results suggest that some native forest insect pests may require relatively little additive mortality to suppress outbreaks if compensation-limiting strategies are used. Incorporating theoretical and strategic frameworks used in vertebrate population management could advance the development of native insect population control programmes.