Ecologists have explained outbreaks of caterpillars in temperate forests using a variety of models to capture the temporal and spatial dynamics of pest populations. Dispersal patterns, natural enemy response, and disease are known to regulate these outbreaks at various scales. Yet a comprehensive understanding of how and where outbreaks begin still eludes researchers. Ecological stoichiometry, an emerging area of ecological theory, may aid in understanding outbreaks by examining the intersection between insect growth rates and elemental requirements, and plant nutrient content. Growth rates of herbivorous insects are known to be constrained by the nutrient levels in their food, specifically by the high carbon (C):nitrogen (N) or C:phosphorus (P) ratios in plants, but growth rates and elemental ratios both vary predictably between insect species. Under the growth rate hypothesis of ecological stoichiometry, P may be especially important for individual growth rate, yet it has not been as well studied as N in forest insect systems. We measured elemental ratios of Lepidoptera in eastern North American temperate forests to determine whether outbreak and non-outbreak species differed in constituitive elemental ratios. We also examined the correlation between abundance (as measured by larval surveys) and stoichiometric composition of the moths. Finally we documented the spatial pattern in foliar C:N:P in a riparian forest tree, sampling leaves during an outbreak of a forest pest (the fall cankerworm Alsophila pometaria).
The three sets of data suggest the inherent nutritional mismatch between caterpillars and their host plants is more dynamic than has previously been recognized. Caterpillar species clearly differed in their nutrient ratios, and while there was not a strong correlation between abundance and nutrient levels, some numerically dominant species were higher in P. Spatial distribution of plant quality in the form of resources for the herbivores was highly variable across multiple scales, and varied by site in strength of correlation to the density of the outbreaking caterpillar. The fundamental nature of stoichiometric limitations thus interacts with other important controls on caterpillar abundance. Considering the spatial variability of the availability of nutrients, especially P, may add to an understanding of where, and with which herbivores, outbreaks occur.