Determinants of the Southern Range Limit of an Infamous North American Forest Defoliator

Abstract

Aim

Predicting climate driven species range shifts requires knowledge of mechanisms limiting species fitness under various climatic conditions. The traditional approach of modelling ranges of herbivorous insects by fitting environmental niche models to occurrence records is generally incapable of differentiating direct effects of climate on insect populations versus indirect effects acting on the range of their host plants. Here, we delimit the southern extent of a major forest defoliator, the spruce budworm, Choristoneura fumiferana, in North America, and investigate whether its range is limited directly by climate or indirectly by the range of its host trees.

Location

Eastern North America.

Methods

We use pheromone traps to survey populations; these are sensitive tools for detecting low-density populations. We applied mechanistic models of the effect of temperature on C. fumiferana fitness to evaluate whether these patterns of occurrence can be explained by the direct effect of climate.

Results

We found that this insect species is widely present through most of the southern distribution of host spruce and fir species, both in the upper Mississippi River valley and the central and southern Appalachian Mountains. The species was present in ca. 50% of spruce-fir sites surveyed in the Appalachian Mountain region but was absent in the five most southerly sites where spruce and fir occur in eastern North America. Simulation using historical meteorological data indicated that all survey sites are climatically suitable for C. fumiferana.

Conclusions

Simulation of climate effects, along with the species' absence from the most southern distribution of hosts, indicate that while the southern range limit of C. fumiferana is limited by host availability it is not directly constrained by climate. Furthermore, populations at the species' southern extent may be in a dynamic state of extinction and recolonisation, thus explaining at least the temporary absence of C. fumiferana from certain locations.