Exploiting fugitive resources: How long-lived is “fugitive”? Fallen trees are a long-lasting reward for Ips typographus (Coleoptera, Curculionidae, Scolytinae)

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Because of their significant impact on forests, tree-killing bark beetles have been widely studied at epidemic population densities. However, bark-beetle outbreaks emerge from populations at low, endemic levels, where population maintenance and growth depend on scarce, ephemeral resources such as lightning-struck trees, weakened and suppressed standing trees or wind-felled trees, depending on the species. The longevity of these resources strongly influences population dynamics, but very little is known in this respect.We tested the hypothesis that the driving factor reducing the availability of fallen trees is the decrease of their nutritional quality. We analysed the qualitative changes of the phloem of spruces artificially uprooted at different times (2–17month before the experiment started) and simultaneously exposed during the same flight season to attacks of the major forest pest in Eurasia, Ips typographus. We also measured the impact of resources’ decay on the insects’ fecundity and the quality of their offspring.Our results show a surprisingly slow decrease of phloem quality over time. The beetles could develop on the oldest (17month-old) resources with no significant differences in terms of fecundity, progeny body mass and global energy acquisition.Although they retain their nutritional quality over long periods, transient resources are exposed to other factors that make them rapidly unsuitable for I. typographus. We suggest that interspecific competition is probably the most influential of them, and acts as a major driver in the beetles’ race for new resources.