Differences in host use efficiency of larvae of a generalist moth, Operophtera brumata on three chemically divergent Salix species

The food selection, growth, and fecundity of insect herbivores are largely dictated by the chemical composition and nutritive values of plant foliage. We studied the host-use efficiency of larvae of the generalist moth, Opero phtera brumata (Lepidoptera: Geometridae) on three chemically divergent but nutritively similar willows (Salix spp.). The 4th instars were able to use the salicylate-free leaves of S. phylicifolia efficiently. Growth was slig htly reduced on S. pentandra, which contained a moderate level of acetylate d salicylates. The high concentration of salicylates found in the leaves of S. myrsinifolia seemed to provide efficient protection against non-special ized O. brumata. We also studied assimilation of nutrients and degradation of salicylates and other secondary compounds in the digestive tract of O. b rumata larvae. Neither the assimilation of nitrogen nor of carbon were affe cted by secondary chemicals of ingested food. Salicylates were shown to be degraded to salicin and catechol, while further degradation of salicin to s aligenin was rather slow. In an artificial diet experiment, we showed that two degradation products of salicylates, catechol and saligenin markedly re duced the growth of the larvae. Neither salicin nor chlorogenic acid affect ed larval growth. We conclude that salicylates reduced the growth of the ge neralist winter moth mainly by feeding deterrence caused by 6-hydroxy-2-cyc lohexenone and catechol. Compared to the deleterious effects of salicylates the effects of other secondary compounds were minor.