While specialist insects are able tolerate the defensive secondary chemistry of their hosts, sometimes even co-opting host chemistry for their own defense, few studies have examined the effects of a chemically variable host. Chrysomela leaf beetles specialize on trees in the Salicaceae and their larvae secrete salicylaldehyde, a potent predator deterrent, which they synthesize from the host-derived phenolic glycoside (PG) salicin. Besides salicin, host trees such as Eurasian aspen (Populus tremula), contain large amounts of other PGs known to degrade to salicin. Chrysomela tremula larvae feed on P. tremula, which has four defined PG chemotypes with individuals high in either 2’-acetyl PGs, 2’-cinnamoyl PGs, both of these, or neither. Work in other aspen systems has shown that the toxicity of PGs toward insect herbivores increases with greater structural complexity. To determine whether larval performance and survival of this specialist insect varies with chemotype, we conducted feeding trails where we fed newly hatched larvae separately on the various chemotypes and measured their performance and survival. Next, to see if larvae can use more complex PGs to produce salicylaldehyde, we acclimatized larvae on a host without PGs and then fed them leaves painted with salicin, salicortin, and tremulacin and monitored salicylaldehyde secretion.
Results/Conclusions
Initial analyses of the feeding trail experiments showed no differences for larval relative growth rates or mortality between chemotypes, but developmental time and final adult weight did differ with faster development and higher adult weights of larvae feeding on chemotypes with more complex PGs. After about a week, all larvae fed on a non PG-containing host ceased to produce salicylaldehyde secretions, but this activity was restored for groups of larvae subsequently fed leaves painted with salicin, and the two more complex PGs, salicortin and tremulacin. Results of the feeding trials demonstrate that the relationship between specialist insects and their host plants may be more complex than initially thought, especially when the host has variable secondary chemistry. Intraspecific variation in plant resistance that may lead to herbivore specialization on distinct host chemistry which has implications for the evolutionary trajectory of both the insect and plant species. In addition, in this particular system, we found that leaf beetle larvae are more efficient than previously thought in exploiting host chemistry for their own defense.