Long-term effects of forest management on parasitoid wasps

Abstract

Forest ecosystems are important for maintaining multiple ecosystem services, providing timber and habitat for pest controlling species. Parasitoid wasps are important natural pest controls, but studies on how parasitoid wasps are influenced by forestry are scarce. Clear cutting is an intensive forest management practice used in Norway, and the forests that were clear-cut in the 1940s-1960s have recently reached a state that can be compared with mature forests that have been exposed to selective cutting in the past (termed near-natural). Studying the abundance of parasitoid wasps between the two forest management types, could yield valuable insights into the most effective forest management practices for maintaining a robust population of these beneficial insects over the long-term. To compare the two forest management types on parasitoid wasps, I collected and sorted specimens of parasitoid wasps (Hymenoptera, Ichneumonoidea) from 12 locations. Each location had two field areas – one in a former clear-cut forest and one in a near-natural forest – in southeastern Norway. The collection was done using Malaise traps. I found that the abundance of parasitoid wasps was similar in mature clear-cut forests and near-natural forests at the family level. However, at the subfamily level, there were greater variation with some subfamilies being more abundant in near-natural forests. Interestingly, one family, one subfamily and generalist species showed increased abundance with the increase of total volume of dead wood, but this pattern did not hold true for saproxylic Ichneumonoidea. Investigating the impacts of forest management at the family level may be overly broad due to the presence of numerous subfamilies with different life history strategies. To enhance clarity, it is essential to study these parasitoid wasps at the lowest possible taxonomic level. The complexity in the observed variation in parasitoid wasp abundance between the forest management types makes it difficult to conclude which forest management practice yields the most abundant parasitoid community.