Evolutionary aspects of defense by recycled plant compounds in herbivorousinsects

Plant secondary compounds play a decisive role in the evolution of host ass ociations of herbivorous insects. Here, I focus on the use of secondary com pounds for the anti-predator defense of herbivores. Cardenolides, iridoid g lycosides and pyrrolizidine alkaloids are used as examples to illustrate th e stepwise adaptations that are necessary to adopt chemical defense by thes e substances. Longitarsus flea beetles provide an example where sequestrati on of plant derived pyrrolizidine alkaloids and iridoid glycosides occurs i n at least 11 and 5 species, respectively. These species feed on several di fferent plant families containing either class of compounds. A comparison w ith a phylogeny of Longitarsus based on mtDNA sequences shows that sequestr ation of pyrrolizidine alkaloids as well as of iridoid glycosides has been adopted multiple times independently. This situation contradicts the hypoth esis of a single colonization of plants with either class of deterrent comp ounds followed by radiation across plant families to chemically similar pla nts.