Habitat preferences and reproductive success forthe threatened longhorn beetle Plagionotusdetritus

Abstract: Plagionotus detritus is a threatend longhorn beetle that only exists at one site in Sweden. It is saproxylic and depending on recently dead coarse oak wood for its larval development. Trees at Djurgården, Stockholm that have been colonized by Plagionotus detritus has been studied to find out the habitat preferences of the species and to see what affects the density of beetles in a tree. The bark of some trees and wood from the breeding project at Nordens Ark has also been studied to find out what affects the species reproductive success. The reproductive success was measured in two ways, the larval mortality and the size of the hatching holes. My hypothesis was that sun exposed, coarse trees with thick bark would produce more beetles. I found that the south side of the trunks had more hatching holes per m2 compared to trunks that faced the north side. No relationship was found between the production of beetles and bark thickness and tree diameter. My hypothesis was to find a decreased size of the hatching holes and increased mortality when the density of larvae increases and the bark gets thinner. I also expected an increased reproductive success when the larvae consumed more bark. When the mortality was studied did all my result supported these hypotheses and mortality seems to be a good way to measure fitness. The size of the hatching holes did not show any relationship with the factors and was probably not a good way to measure fitness. The results can be used in the future conservation work with Plagionotus detritus. Sun exposure is showed to be an important factor that will increase the production of beetles in a tree. It is important to cut around the potential host trees to make them as sun exposed as possible. Enough breeding material should be supplied to avoid a to high density of larvae and allow the larvae to consume as much bark as possible. An estimation from my data is that one female that deposits 50-80 eggs needs 2,5-400 m2 of bark to make all the eggs hatch. The breeding material should preferably have thick bark and be place in an sun exposed area to minimize the risk for mortality and there by increase the fitness.