Host-use patterns of saproxylic phloeophagous and xylophagous Coleoptera adults and larvae along the decay gradient in standing dead black spruce and aspen

Wood-feeding insects play important functional roles in forest ecosystems, contributing significantly to wood decay processes. However, sampling these species in a direct and quantitative way is difficult because they live most of their lives as larvae deep into the wood; knowledge of species-specific host-use patterns along the decay gradient is thus lacking in this group. To cope with these difficulties, we used a novel approach, snag dissection, to investigate occurrence patterns of such Coleoptera adults and larvae. We selected 80 snags of both black spruce and aspen along four classes of decay in five different stands distributed over the tree species' ranges within the province of Quebec, Canada, and dissected a one-meter section of each. All adults and larvae of Buprestidae, Cerambycidae and Scolytinae (Coleoptera: Curculionidae) were collected and identified to the lowest taxonomical level possible. Wood density and snag age were also calculated for each sampled snag. In black spruce, host-use was mostly concentrated at the beginning of the decay gradient. Patterns observed in aspen were opposite, as few insects were found in fresh snags, while most snags in middle to late stages of decay contained insects, often in large numbers, in some reaching densities of over 1000 cerambycid larvae m(-3). For both tree species, patterns observed were similar across regions sampled. Differences in host-use patterns between the coniferous and deciduous host species may be due to differences in secondary chemistry, mechanical defence mechanisms or the stand dynamics typically associated with each tree species.