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

Abstract

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.

Dietary specialization influences the efficacy of larval tortoise beetle shield defenses

“…In two studies comparing caterpillars reared on the same plant, the specialist caterpillar was actually more vulnerable (Stamp 1992) and no relationship was found between host concentration of sequesterable compounds and host use by generalist or specialists in the presence of predators (Stamp and Bowers 2000). In the Phratora-Galerucella study cited previously, Denno et al (1990) compared a specialist and generalist on the same host, but did not directly asses their vulnerabilities to predation.…”

“…Although this research has revealed a positive relationship between diet breadth and susceptibility to predator attack (Bernays and Graham 1988;Bernays and Cornelius 1989), it is not clear if the advantage of specialization derives from the characteristics of the insect species or from their host plants, since the specialists and generalists in all studies to date were collected or reared from different plant species. For example, among willow-feeding leaf beetles, Phratora vitellinae exudes defensive secretions derived from salicylate-rich willows upon which it specializes, whereas Galerucella lineola produces no such secretions but uses a wider variety of willow hosts (Denno et al 1990). Although the specialist is likely better protected, there was no direct assessment of susceptibility to predation for both species on the same host.…”

Invasive Spiraea tomentosa: a new host for monophagous Earias clorana?

“…This creates a favourable opportunity for the population of E. clorana to avoid competition with other folivorous species, as well as its natural enemies. For E. clorana, the use of Spiraea tomentosa could therefore result in an enemy-free space (Denno et al 1990), which is potentially an important factor affecting host plant choice by phytophagous insects (Mulatu et al 2004). Utilising Spiraea tomentosa may also be beneficial for E. clorana as an opportunity to improve geographical and ecological expansion.…”

Detection of refuge from enemies through phenological mismatching in multitrophic interactions requires season-wide estimation of host abundance

“…maximized given constraints. Some herbivores indeed show a positive correlation between adult oviposition preference and larval performance (Denno et al 1990). It has also been frequently found, however, that herbivorous insects use plants or plant parts on which larval performance is sub-optimal (Gratton and Welter 1999;Mulatu et al 2004).…”