Phratora vitellinae and Galerucella lineola feed on Salix in Sweden. When disturbed, larvae of P. vitellinae exude droplets of a defensive secretion from dorsal glands. The secretion contains salicyl aldehyde, the precursors for which are plant-derived salicylates like salicin and other chemically similar phenolic glycosides. Larvae of G. lineola lack dorsal glands, cannot convert salicylates to salicyl aldehyde, and do not produce such a defensive secretion. Oviposition patterns, larval performances and susceptibility to predators of these beetles were evaluated on willows, 2 rich in salicylates (S. fragilis, S. dasyclados), one poor in salicylates (S. viminalis). Females G. lineola oviposited preferentially on S. viminalis and S. fragilis where larvae developed rapidly and survival was high, and avoided S. dasyclados where larval performance was poor. Variation in leaf pubescence, toughness, and nitrogen content across willow species did not explain observed patterns of larval performance, but larval performance was inversely related to the concentration of salicylates in the 3 willows. Oviposition preferences of P. vitellinae did not correspond well to patterns of larval performance. S. fragilis was the preferred choice for oviposition. Performance of P. vitellinae larvae differed markedly on the 2 salicylate-rich willows; larvae survived best and developed most rapidly on S. fragilis, but mortality was high and development slow on S. dasyclados. Despite differences in performance, larvae fed these 2 silicylate-rich willows produced a defensive secretion which effectively repelled coccinellid predators. Larvae fed the salicylate-poor S. viminalis survived and developed well in the absence of enemies, but lacked the salicylate-based defensive secretion and were readily eaten by coccinellids. Female P. vitellinae apparently avoided S. viminalis as an oviposition host, not because larvae performed poorly, but because larvae were defenseless against some predators when raised on this salicylate-poor willow. Herbivores such as P. vitellinae thus obtain enemy-free space on hosts from which they sequester plant-derived allelochemicals for defense. -from Authors
