Fallen trees' last stand against bark beetles

The weakened or suppressed hosts targeted by tree-killing conifer bark beetles during endemic phases are considered to be poorly defended and available to several competing species. Hosts face trade-offs in the allocation of resources in growth and reproduction or in defense, and the defense/resources status varies with time after host's death or weakening event. Tree-killing bark beetles at endemic levels must choose along a spectrum, compromising between encountering minimal defense and facing high competition, or benefiting from high nutritional quality but facing high host defense. The relationships between resources and defenses in dead/dying trees have not been quantitatively assessed to date, because most studies focus on epidemic conditions. We measured the changes in constitutive and induced defenses in the phloem of spruce trees artificially uprooted at different times (up to 17 months prior to experiment initiation) and treated with methyl-jasmonate (MeJ) in order to induce chemical defense. We measured the effect of these changes on different steps of the host selection process and brood success of a major forest pest in Eurasia, Ips typographus (Coleoptera, Curculionidae).Our results show that uprooted trees maintain some level of constitutive defenses, in particular monoterpenes, that decrease with time after uprooting. Moreover, the trees maintain some inducibility and can produce monoterpenes in response to MeJ treatment up to 7 months after uprooting. These defenses are correlated in part with the energy available in the phloem, but are likely too weak to exert pressure on the beetles' success and selection, the latter being related to the amount of energy available. Our results confirm that wind-felled trees are poorly defended, consistent with the hypothesis of the evolution of the tree-killing behavior from saprophagy in response to an overwhelming interspecific competition on undefended hosts. Wind-felled trees still represent a useful resource, exposing endemic populations of aggressive beetles to low defense levels while facilitating reproduction, growth and development. The constraint on the beetles' populations then becomes the ability to locate these scattered and unpredictable resources.