Deadwood in commercial forest: a source of danger for the outbreak of pestspecies?

In a commercial mixed oak-beech forest situated in the Westerwald (325 m ab ove sea level) the saproxylic insect fauna emerging from 1.7 m(3) deadwood was investigated. Using eclectors, insects were collected from deadwood ori ginating from oak and beech trees which prior to sampling had been freshly cut and exposed either to sunny or shaded areas for the length of one year. Roughly, 36000 insects were collected of which more than 10000 individuals belonged to 122 different species of Coleoptera. Diptera were represented by about 7500 individuals whereas 2500 individuals belonged to the parasito ids of Hymenoptera. The actual numbers of the different beetle species were always higher in areas exposed to more sunlight compared to shaded areas. However, the abundance of individuals in the sunlight areas exposed to outn umbered those of the shaded areas only when their development had taken pla ce in branches with diameters of 15-17 cm (p < 0.001). The emergence of ind ividuals from the twigs (circle divide = 5-7 cm) was not influenced by expo sure to sunlight (p > 0.05). Remarkably high emergences were found for the bark beetle Taphrorychus bico lor from the branches of beech trees which had been exposed to strong sunli ght (maximum emergence per surface area of bark: 850 ind./m(2)). With incre asing numbers of T. bicolor the number of possible predators also increased . Apparently, predators were not able to reduce the population density of T . bicolor significantly. Based on results obtained from our studies, if there is an increase of dead wood in a commercial forest characterized by diameters of 15 cm or more, we would strongly recommend to exercise control of at least the sunny points. On the other hand, a more natural management and the cultivation of graded edges of forests would possibly reduce outbreaks of pest species.