Habitat heterogeneity affects predation of European pine sawfly cocoons

Abstract

Habitat heterogeneity is thought to affect top‐down control of herbivorous insects and contribute to population stability by providing a more attractive microhabitat for natural enemies, potentially leading to reduced population fluctuations. Identifying the parameters that contribute to habitat heterogeneity promoting top‐down control of herbivorous insects by natural enemies could facilitate appropriate management decisions, resulting in a decreased risk of pest insect outbreaks because of a higher level of predation. In our study, we measured the top‐down pressure exerted by small mammals on the cocoons of a notorious pest insect in pine forests, the European pine sawfly (Neodiprion sertifer), which is known to be regulated by small mammal predation. The forest stands used differed in heterogeneity measured in terms of differences in tree diversity and density, understory vegetation height, presence/absence, and density of dead wood. We found higher predation in more dense spots within forest stands. Further, the effect of dead wood on sawfly cocoon predation depended on the pine proportion in forest stands. The addition of dead wood in a manipulation experiment had a slight positive effect on cocoon predation, while dead wood removal caused a clear decrease in predation rate, and the decrease was more pronounced when the proportion of pine increased. Our results show that habitat heterogeneity affects predation by generalist predators on herbivorous insects. This knowledge could be applied to reduce the risk of insect outbreaks by applying management methods that increase heterogeneity in perennial systems such as forests and orchards, thus decreasing the levels of insect damage.

Mixed forests to mitigate risk of insect outbreaks

“…Throughout the life cycle of N. sertifer, mortality caused by parasitoids in the egg and larval stages, walking predators like spiders and ants in the early larval stages, and small mammals in the cocoon stage are found to be important predators. The predation rates of N. sertifer cocoons imposed by small mammals are known to be positively affected by forest structure (Hanski and Parviainen 1985;Kollberg et al 2014;Bellone et al 2017). Here we found that parasitoids seemed to be affected by tree size but not tree diversity.…”

The Influence of Tree Diversity on Natural Enemies—a Review of the “Enemies” Hypothesis in Forests

“…Most evidence comes from mixing conifers with broadleaved trees [33], where mixtures had higher parasitoid abundance [115] and overall predator richness [84••]. For the pine sawfly (Neodiprion sertifer) in Scandinavia, mortality rates were higher in mixed stands than in pine monocultures [116], which was mediated top-down by ant abundances ( [117], see also [9]). In the same observational system, parasitism rates of sawfly larvae increased with host tree size [11], suggesting size-dependent identity effects.…”

Effects of retained dead wood on predation pressure on herbivores in young pine forests

“…Besides acting as breeding substrate for many forest species [ 2 ] it is an important structural component, providing for shelter and altering micro-climate [ 3 ]. Dead wood may also affect top-down regulation of herbivores through effects on their natural enemies [ 4 ].…”

“…In addition, forest heterogeneity and complexity created by dead wood could alter ecological processes such as pest suppression [ 4 ]. Higher predation rates of European pine sawfly cocoons were found when dead wood was present in monocultures [ 4 ], potentially by providing shelter to small mammalian predators [ 17 ].…”