Many insects with the propensity to cause widespread forest damage do so in close association with fungi and other microorganisms. Often such interactions are mutually beneficial; fungi aid insects in overcoming host tree defenses and/or concentrate nutrients to feed developing larvae, while insects provide transport to new hosts. Assessing the nature, strength and ecological context of such feedbacks is essential to understanding insect pest and pathogen population dynamics and predicting resulting impacts on forest structure and function. Using a spatially replicated, 13-year time series (29 sites throughout the eastern deciduous forest from 1979-1992), we test the hypothesis that coupled dynamics between causal agents of beech bark disease (the felted beech scale – Cryptococcus fagisuga - and either one of two ascomycete fungi of the genus Neonectria) are broadly predictive of fluctuations in disease severity over time. Using a model selection approach, we assess the relative importance of endogenous regulation, population density of associated species, and selected potential exogenous effects (e.g. climate variables, stand characteristics, etc.) in driving per capita population growth rates of insects and fungi. We test specifically for evidence of positive feedbacks between disease agent populations which may result in system instability.
Results/Conclusions
Population growth rates of both scale insects and fungi show strong evidence of endogenous regulation. For each, the relative contribution of the population densities of the associated species on per capita population growth was small but consistently negative, suggesting that these two putative mutualists in fact functioned as weak antagonists during the 13 years assessed. These results are consistent with broad-scale spatial patterns indicating that in the center of the range of beech bark disease where disease agent populations have been established for >50 years, insects and Neonectria fluctuate more or less independently, but that overall, scale insect densities are highest along the edge of the range where fungal populations are low. Considerable among-site variation in estimates of the intrinsic population growth rate (r) and carrying capacity (K) was explained by exogenous variables including stand density, snow depth and low winter temperatures. In general, however, the best predictor of population dynamics was the duration of infection with beech bark disease, suggesting that forest history or disease ontogeny is an important driver. These results improve our understanding of the beech bark disease system and offer insights into the nature of forest disease where multiple players are important in determining outcomes.
