Top-down pressure by generalist and specialist natural enemies in relation to habitat heterogeneity and resource availability

Habitat heterogeneity might promote the abundance and richness of natural enemies potentially leading to higher top-down pressure on herbivorous insects. Heterogeneous habitats could provide natural enemies with more abundant and alternative resources and a greater variety of micro-habitats. Natural enemies with different searching behaviours, e.g. generalists and specialists, could be affected in different ways by habitat heterogeneity, thus affecting their pressure on herbivorous insects.To understand how top-down pressure on herbivorous insects is promoted by habitat heterogeneity, it is crucial to investigate which parameters contributing to habitat heterogeneity affect not only the abundance and richness but also the searching behaviour of different natural enemies. We investigated the relationship between heterogeneity in forest habitats and the top-down pressure exerted by generalist predators and specialist parasitoids on larvae of the European pine sawfly (Neodiprion sertifer).We used forest stands with endemic or epidemic densities of resident sawfly populations. Within each stand we selected experimental trees to create variation in tree species diversity and density in their surrounding area, i.e. habitat heterogeneity. We found that a higher tree density increased the predation by generalists on sawfly larvae in stands with endemic sawfly densities. Parasitoids were less successful in stands with endemic sawfly densities. Total mortality depended on stand character and the proportion of pine around experimental trees.The explained variation in the response variables by the models is relatively low, indicating that other measures of heterogeneity, like understory vegetation and presence of dead wood could contribute to the observed variation. Also, interference between generalist and specialist enemies could affect the realized mortality pressure. Thus, the effect of tree species diversity in combination with these other measures of heterogeneity needs to be recognized to promote the presence and the activity of natural enemies in managed habitats.