Abstract

Leaf colour has been proposed to signal levels of host defence to insect herbivores, but we lack data on herbivory, leaf colour and levels of defence for wild host populations necessary to test this hypothesis. Such a test requires measurements of leaf spectra as they would be sensed by herbivore visual systems, as well as simultaneous measurements of chemical defences and herbivore responses to leaf colour in natural host-herbivore populations. In a large-scale field survey of wild cabbage (Brassica oleracea) populations, we show that variation in leaf colour and brightness, measured according to herbivore spectral sensitivities, predicts both levels of chemical defences (glucosinolates) and abundance of specialist lepidopteran (Pieris rapae) and hemipteran (Brevicoryne brassicae) herbivores. In subsequent experiments, P. rapae larvae achieved faster growth and greater pupal mass when feeding on plants with bluer leaves, which contained lower levels of aliphatic glucosinolates. Glucosinolate-mediated effects on larval performance may thus contribute to the association between P. rapae herbivory and leaf colour observed in the field. However, preference tests found no evidence that adult butterflies selected host plants based on leaf coloration. In the field, B. brassicae abundance varied with leaf brightness but greenhouse experiments were unable to identify any effects of brightness on aphid preference or performance. Our findings suggest that although leaf colour reflects both levels of host defences and herbivore abundance in the field, the ability of herbivores to respond to colour signals may be limited, even in species where performance is correlated with leaf colour.

Highlights

  • Interactions between plants and their specialist herbivores are frequently the result of close coevolutionary relationships [1]

  • This study examines the hypothesis that leaf colour provides information about host defences to herbivores by exploring the relationship between leaf colour, defence and herbivory in wild cabbage, Brassica oleracea L. (Brassicaceae)

  • Measures of leaf coloration based on P. rapae spectral sensitivities were significantly correlated

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Summary

Introduction

Interactions between plants and their specialist herbivores are frequently the result of close coevolutionary relationships [1]. There has been less work on the importance of visual stimuli in host selection [6], but leaf coloration has attracted increasing interest following Hamilton and Brown’s [7] proposal that the autumn colours of trees have evolved to communicate host defence levels to aphids. Under this scenario, correlations between defence levels and leaf colour allow herbivores to use colour as a cue to defence [8]. This idea has been broadened to consider the role of leaf colour as a cue or signal of other host traits besides defence that are relevant to searching herbivores, for example nitrogen levels [1, 9, 10]

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