Effects of Multi-Component Backgrounds of Volatile Plant Compounds on Moth Pheromone Perception.

Lucie Conchou,Nina Deisig,Philippe Lucas,Elodie Demondion,Michel Renou

doi:10.3390/insects12050409

Abstract

Simple SummaryIt is well acknowledged that some of the volatile plant compounds (VPC) naturally present in insect natural habitats alter the perception of their own pheromone when presented individually as a background to pheromone. However, the effects of mixing VPCs as they appear to insects in natural olfactory landscapes are poorly understood. We measured the activity of brain neurons and neurons that detect a sex pheromone component in a moth antenna, while exposed to simple or composite backgrounds of VPCs representative of the odorant variety encountered by this moth. Maps of activities were built using calcium imaging to visualize which brain areas were most affected by VPCs. In the antenna, we observed differences in VPC capacity to elicit firing response that cannot be explained by differences in stimulus intensities because we adjusted concentrations according to volatility. The neuronal network, which reformats the input from antenna neurons in the brain, did not improve pheromone salience. We postulate that moth olfactory system evolved to increase sensitivity and encode fast changes of concentration at some cost for signal extraction. Comparing blends to single compounds indicated that a blend shows the activity of its most active component, VPC salience seems more important than background complexity.The volatile plant compounds (VPC) alter pheromone perception by insects but mixture effects inside insect olfactory landscapes are poorly understood. We measured the activity of receptor neurons tuned to Z7-12Ac (Z7-ORN), a pheromone component, in the antenna and central neurons in male Agrotis ipsilon while exposed to simple or composite backgrounds of a panel of VPCs representative of the odorant variety encountered by a moth. Maps of activities were built using calcium imaging to visualize which areas in antennal lobes (AL) were affected by VPCs. We compared the VPC activity and their impact as backgrounds at antenna and AL levels, individually or in blends. At periphery, VPCs showed differences in their capacity to elicit Z7-ORN firing response that cannot be explained by differences in stimulus intensities because we adjusted concentrations according to vapor pressures. The AL neuronal network, which reformats the ORN input, did not improve pheromone salience. We postulate that the AL network evolved to increase sensitivity and to encode for fast changes of pheromone at some cost for signal extraction. Comparing blends to single compounds indicated that a blend shows the activity of its most active component. VPC salience seems to be more important than background complexity.

Highlights

Olfactory communication is essential to insects as it is involved in the identification and the location of vital resources such as a food source, a mate, or an oviposition site
Z7-12:Ac activated a large area in the antennal lobes (AL) that we identified as the macro-glomerular complex (MGC) according to its position, similar to previous observations in the same insect species [30,61] (Figure S3)
Activity patterns differed according to volatile plant compounds (VPC), with (E)-2-hexenal, (Z)-3-hexenyl acetate, and linalool activating repeatedly large areas of the ALs, while β-caryophyllene, eucalyptol, and indole showed more limited activity maps (Figure S3). α-pinene elicited a very localized increase in fluorescence in only two preparations (Figure S3). (Z)-3-hexenyl acetate, linalool, and (E)-2-hexenal triggered a strong Ca-response within the MGC (Figure 2)

Summary

Introduction

Olfactory communication is essential to insects as it is involved in the identification and the location of vital resources such as a food source, a mate, or an oviposition site. Insects have developed an exquisite olfactory sense in terms of sensitivity, specificity, and temporal dynamics. Their olfactory system enables them to discriminate the pheromones they produce, as well as the odors involved in interspecific interactions, such as, for instance, the floral compounds emitted by plants to attract specialist pollinators. Considering the hundreds of different volatile compounds released by plants (VPC) [4,5], the capacity of the insect olfactory system to extract the ecologically relevant information from that very complex chemical environment is remarkable [6,7]. Seven doses of linalool ranging from 0.00001 to 10% in mineral oil were tested for their effects on the firing activity and responses to pheromone in 5 MGC-neurons. Linalool 1% was chosen as a reference stimulus for further experiments

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