Scaling the interactive effects of attractive and repellent odours for insect search behaviour

Peter A Hambäck,Thomas A Verschut,Mikael A Carlsson

doi:10.1038/s41598-019-51834-1

Peter A Hambäck, Thomas A Verschut + Show 1 more

Open Access

https://doi.org/10.1038/s41598-019-51834-1

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Journal: Scientific Reports	Publication Date: Oct 25, 2019
Citations: 13	License type: open-access

Affiliation: Stockholm University

Abstract

Insects searching for resources are exposed to a complexity of mixed odours, often involving both attractant and repellent substances. Understanding how insects respond to this complexity of cues is crucial for understanding consumer-resource interactions, but also to develop novel tools to control harmful pests. To advance our understanding of insect responses to combinations of attractive and repellent odours, we formulated three qualitative hypotheses; the response-ratio hypothesis, the repellent-threshold hypothesis and the odour-modulation hypothesis. The hypotheses were tested by exposing Drosophila melanogaster in a wind tunnel to combinations of vinegar as attractant and four known repellents; benzaldehyde, 1-octen-3-ol, geosmin and phenol. The responses to benzaldehyde, 1-octen-3-ol and geosmin provided support for the response-ratio hypothesis, which assumes that the behavioural response depends on the ratio between attractants and repellents. The response to phenol, rather supported the repellent-threshold hypothesis, where aversion only occurs above a threshold concentration of the repellent due to overshadowing of the attractant. We hypothesize that the different responses may be connected to the localization of receptors, as receptors detecting phenol are located on the maxillary palps whereas receptors detecting the other odorants are located on the antennae.

Highlights

Insects searching for resources are exposed to a complexity of mixed odours, often involving both attractant and repellent substances
While the lateral horn broadly categorizes signals into regions according to their behavioural valence[35,36,37], innate attraction and avoidance are processed in parallel pathways which may be responsible for interactive effects between odours[37,38]
For benzaldehyde (Fig. 2C), no interactive effect was found between the vinegar and benzaldehyde concentrations (GLM: χ21,246 = 0.73, P = 0.39), refuting the odour-modulation hypothesis

Summary

Introduction

Insects searching for resources are exposed to a complexity of mixed odours, often involving both attractant and repellent substances. Further interactive effects can occur in the antennal lobe, the primary olfactory processing centre in the insect brain, where each functional subunit, or glomerulus, receives signals from specific classes of ORNs18,24 At this level, the simultaneous activation of several glomeruli[25,26], or the concentration dependent activation of specific glomeruli[27,28,29], affect how the combination of signals are relayed by projection neurons to higher brain regions: the mushroom body for experience-dependent responses[30,31,32], and the lateral horn for innate responses[26,33,34]. The third hypothesis, which we coin the odour-modulation hypothesis, assumes overshadowing, but through a different mechanism where repellents and attractants have non-additive effects on the behavioural response for all odour concentrations and at a threshold. A similar hypothesis was suggested by Schröder and Hilker (2008), who argued that the ability to detect a specific odorant may depend on other odours present in the background

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