Dose-dependent behavioural fever responses in desert locusts challenged with the entomopathogenic fungus Metarhizium acridum

Lisa M Clancy,Gareth W Griffith,Amy L Cooper,Rory Jones,Roger D Santer

doi:10.1038/s41598-018-32524-w

Abstract

Behavioural fever is a common response to immune challenge in ectotherms and confers survival benefits. However, costs accrue rapidly as body temperature rises. Thus, the magnitude of adaptive fever responses might reflect the balance of costs and benefits. We investigated behavioural fever in desert locusts, Schistocerca gregaria, infected with the entomopathogenic fungus Metarhizium acridum. We first tracked the time course of behavioural fever in infected locusts, demonstrating that body temperatures rose on the day following inoculation (day 1), and reached peak intensity on the day after that (day 2). Subsequently, the magnitude of fever responses varied during a day, and locusts tended to exhibit high-intensity fever responses in the mornings when basking was first possible. We speculate that this may have resulted from increased fungal load caused by unimpeded growth overnight when locusts could not fever. We next inoculated locusts with different M. acridum doses ranging from 0 to ca. 75,000 conidia. The magnitude of their behavioural fever responses on day 2 post-inoculation was positively related to fungal dose. Thus, we demonstrate dose-dependency in the behavioural fever responses of desert locusts and suggest that this may reflect the adaptive deployment of behavioural fever to minimize costs relative to benefits.

Highlights

Fever body temperatures are an adaptive response to infection in many species, and they can be achieved by physiological and/or behavioural means such as basking[1,2]
We investigate whether desert locust, Schistocerca gregaria, behavioural fever responses differ in intensity according to the severity of Metarhizium acridum infection
We first examined the time course of the behavioural fever response in M. acridum-infected and control-treated desert locusts housed in aluminium cages equipped with overhead basking lamps

Summary

Introduction

Fever body temperatures are an adaptive response to infection in many species, and they can be achieved by physiological and/or behavioural means such as basking[1,2]. Behavioural fever can decrease mortality and morbidity in infected animals[3,4,5,6] This occurs because high body temperatures are suboptimal for pathogen growth[5,7] and increase the mortality of some pathogens[6]; high body temperatures enhance several aspects of host immune function[8,9]. Basking to achieve fever body temperatures confers additional costs in terms of missed feeding and mating opportunities, and increased predation risk[15]. Sick individuals likely face a similar requirement to balance costs and benefits and avoid excessively high fever body temperatures, especially since behavioural fever responses are often unable to fully clear infection[5,17] (but see[6]). Additional knowledge of locust behavioural fever responses to Metarhizium infection may help inform the application of biopesticides for locust control

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Conclusion