Substrate selection by saprophagous wood-borer larvae within highly variable hosts

In insects completing their larval development within a single host, oviposition site is seen as a major determinant of offspring performance. However, in previous studies, the saprophagous wood-borer Anthophylax attenuatus (Haldeman) (Coleoptera: Cerambycidae) showed no strong response to between-host variations in nutritional factors influencing larval growth and survival. To explain such weak selection in adults, we hypothesized that substrate selection occurs at a smaller scale by larvae within hosts showing high variability in substrate quality. In this study, we described within-host variability in wood density and determined whether wood-boring larvae were found more often than expected in specific decay types. We characterized the variability of decay in 24 snags by producing wood density profiles for each. We then collected larvae from the same snags through wood dissection, and associated a wood density value to each by taking a wood sample around each larva found. We then compared ratios of available and used substrate types defined by wood density. We observed substantial within-snag variation in wood density. Middle decay class (0.275-0.375 g cm⁻³) was significantly overused by larvae, whereas more decayed wood was clearly avoided. High within-host variability in substrate quality and active or passive selection by larvae of specific substrate types suggest that selection pressures on adult behaviour could be lower than expected for a parasitic species, and might be linked with the weak selection observed at a larger scale by ovipositing adults.