Host defence predicts host specificity in a long-lived arboreal parasite

Abstract

Some parasites exploit many host species, while others are confined to only a few. Despite decades of study, the mechanisms that determine parasite host specificity remain poorly understood. We investigated host specificity in New Zealand’s largest endemic moth, Aenetus virescens (Lepidoptera: Hepialidae), which as larvae excavate tunnels into tree trunks and feed on host tree sap. We sampled host use in 2213 trees belonging to 24 tree species in three sites on the North island of New Zealand to test whether (1) larvae preferentially use particular tree species rather than the most abundant, (2) larvae preferentially use tree species with high nutrient rewards (Phloem turnover and sugar content), and (3) larvae preferentially use tree species with reduced defences (wood density and bark thickness). Overall, 180 individual trees from 15 species were used by larvae. However, results indicated larvae preferentially used particular tree species rather than the most abundant. In particular, larvae preferentially used Aristotelia serrata and Carpodetus serratus, despite being rare at each site. Larvae did not preferentially use tree species with high nutrient rewards nor low wood density. Instead, we found a significant negative relationship between the number of trees per species used by larvae and bark thickness. Overall results indicate bark thickness may be the primary mechanism driving host specificity for A. virescens.