Role of Enemy‐Free Space and Plant Quality in Host‐Plant Selection by Willow Beetles

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<jats:p>Phratora vitellinae and Galerucella lineola are two leaf beetles that feed on willows (Salix) in central 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. In contrast, larvae of G. lineola lack dorsal glands, cannot convert salicylates to salicyl aldehyde, and no not produce such a defensive secretion. We evaluate the adult oviposition pattern, larval performances, and suceptibility to predators of these two beetles on three species of willows, two rich in salicylates (Salix fragilis and S. dasyclados) and one poor in salicylates (S. viminalis). Females of 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 not explain observed patterns of larval performance. However, larval performance was inversely related to the concentration of salicylates in the three willows. By contrast, the oviposition preferences of P. vitellinae did not correspond well to patterns of larval performance. Salix fragilis was by far preferred choice of oviposition, and females rarely placed eggs on either S. viminalis or S. dasyclados. The performance of P. vitellinae larvae differed markedly on the two salicylate—rich willows; larvae survived best and developed most rapidly on S. fragilis, but mortality was high and development slow on S. dasyclados. Poor larval performance on S. dasyclados was associated with high concentrations of simple phenolic compounds other than salicylates. Despite differences in performance, larvae fed these two salicylate—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. Females of 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. We argued that herbivores such as P. vitellinae obtain enemy—free space on hosts from which they sequesters plant—derived allelochemicals for defense. The result appears to be dietary specialization on host—plant species that provide these precursors for defense. Other related herbivores like G. lineola that do not employ plant—derived defensive secretions, are not so constrained and feed on a wider range of host willows.</jats:p>

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  • Ecology

    Ecology 71 (1), 124-137, 1990-02

    Wiley

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