ABSTRACT
Weakened and dead trees form part of every healthy forest. Trees may become weakened by natural disturbances such as wind, fire or disease, or by competing with other trees for water, nutrients and light. Many insects selectively colonize weakened but still living trees, and often then kill them off. Several bark beetle species infest weakened trees, in particular conifers, which usually do not survive the attack. Leaf-feeding insects such as the caterpillars of some moths can repeatedly defoliate their host trees, which may also result in the most weakened trees dying. These selective infestations remove susceptible individuals and improve general stand vitality.
Insects also influence the establishment of a tree species under given site conditions. Seedlings growing on unfavourable sites are less vital and suffer more from intense insect feeding. Depending on the site conditions, this may reduce the proportion of a given tree species on a site. Other insects transmit lethal pathogens that decimate a particular tree species.
