Experimental Manipulation of Melanism Demonstrates the Plasticity of Preferred Temperature in an Agricultural Pest (Phaulacridium vittatum)

Rebecca M B Harris,Lesley Hughes,Peter Mcquillan,Gabriele Sorci

doi:10.1371/journal.pone.0080243

Abstract

Phenotypic plasticity is a key trait of successful pest species, and may increase the ability to cope with higher, more variable temperatures under climate change. We investigate the plasticity of preferred temperature in a widespread agricultural pest, the wingless grasshopper (Phaulacridium vittatum). Preferred temperature is a measure of thermoregulatory behaviour through habitat selection. It is influenced by melanism, which affects body temperature by determining the amount of radiation absorbed by the body. First we demonstrate that body temperature and preferred temperature in P. vittatum is influenced by melanism, by comparing the preferred temperature of the colour morphs in laboratory thermal gradients and field body temperatures in natural populations. We then test whether preferred temperature changes in response to changes in body temperature, by determining preferred temperature before and after manipulation of melanism by painting. When melanism was manipulated experimentally in live grasshoppers, preferred temperature changed to reflect the thermal qualities of the new colour. The preferred temperature of light grasshoppers increased after they were painted black, and decreased after being painted white. Similarly, dark individuals that were painted white behaved like a light individual, maintaining a lower body temperature. Preferred temperature in P.vittatum is a plastic thermoregulatory response to ambient temperature, mediated by the influence of melanism on body temperature.

Highlights

The role of behavioural thermoregulation in ectotherms has recently been highlighted as a mechanism for coping with climate change [1]
First we established that body temperature and preferred temperature in P. vittatum is influenced by melanism, by comparing the preferred temperature of the colour morphs in laboratory thermal gradients and field body temperatures in natural populations
Melanism in P. vittatum represents a gradation in colour, with reflectance ranging from 2.49 to 5.65% [23], we have shown elsewhere that visual separation of the colour morphs into distinct categories can reliably be made, and that these categories relate to measurable differences in average reflectance in the UV and visible range [24]

Summary

Introduction

The role of behavioural thermoregulation in ectotherms has recently been highlighted as a mechanism for coping with climate change [1]. Short-term, reactive behavioural responses enable ectotherms to regulate their body temperature under a range of environmental conditions [2] Another aspect of behavioural thermoregulation that may contribute to an ectotherm’s adaptive capacity is the phenotypic plasticity (hereafter referred to as plasticity) of preferred body temperature [3]. The preferred temperature, the ambient temperature to which an insect moves if given its choice of a temperature gradient [4], is a measure of thermoregulatory behaviour through habitat selection. As such it incorporates trade-offs between ecological thermal optima and physiological optima. A range of performance measures of insects have been shown to be maximised at the preferred temperature, such as feeding rates and development in grasshoppers [6,7] and butterflies [8], reproductive optima in beetles [4], and brood survival and caste determination in ants [9,10]

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