Chapter 10 - Herbivore-Natural Enemy Interactions in Fragmented and Continuous Forests

Abstract

This chapter discusses herbivore–natural enemy interactions in fragmented and continuous forests. Studies of forest insect population dynamics have generally taken two approaches, using either long-term life-table data from a single forest stand or data from several widely separated sites, where each site is treated as a replicate population and a composite picture of the dynamics is inferred from the multiple sties. Under each hypothesis, the effect of fragmentation is to uncouple the forest tent caterpillar population from the parasite/pathogen community associated with population collapse. At each sample point in the megagrid and mesogrid, one estimates forest tent caterpillar's abundance and several discrete mortality factors. The spatial pattern of defoliation provides a detailed record of caterpillar abundance, from which one can measure the spread of increasing populations and the contraction of declining populations. Cocoons from isolated patches were less likely to be parasitized than those from continuous forest and the effect was much stronger for parasitism by P. pachypyga than for S. aldrichi. Two of the parasitic flies that attack forest tent caterpillar, Sarcophaga aldrichi and Patelloa pachypyga, are affected in different ways by forest fragmentation. The large-scale pattern of prolonged outbreaks of tent caterpillar in fragmented boreal forests in Ontario could result from a variety of mechanisms, including the reduced efficiency of parasitoids and a reduction in parasitoid movement in response to herbivore abundance in fragmented forests. Ultimately, population dynamics studies will need to be placed within the landscape framework for dynamics to be fully understood.