Atle Wibe

Enantiomeric Composition of Monoterpene Hydrocarbons in Some Conifers and Receptor Neuron Discrimination of α-Pinene and Limonene Enantiomers in the Pine Weevil, Hylobius abietis

The enantiomeric composition of seven monoterpene hydrocarbons in headspace volatiles of spruce sawdust and seedlings (Picea abies), pine seedlings (Pinus sylvestris), and branches of juniper (Juniperus communis) was determined by gas chromatographic separation on a β-Cyclodextrin column. For the six monoterpenes, α-pinene, camphene, β-pinene, sabinene, limonene, and β-phellandrene, both enantiomers were present, whereas for 3-carene only the (+)-configuration was found. The amount of each enantiomer varied considerably both in relation to total amount of all of them, and for the six pairs also in relation to the opposite enantiomer. One olfactory receptor neuron in the pine weevil (Hylobius abietis) showed a strong response to α-pinene when stimulated with all four headspace materials via a GC equipped with a DB-WAX column. The same neuron was subsequently tested with repeated stimulations via the GC effluent containing the (+)- or (–)-enantiomer. A marked better response to (+)- than to (–)-α-pinene was elicited. Another olfactory receptor neuron that responded strongly to limonene when stimulated with the spruce volatiles was tested for enantiomers of limonene. This neuron responded more strongly to (–)- than to (+)-limonene, when stimulated alternately with each of the limonene enantiomers. Discrimination between enantiomers by plant olfactory receptor neurons suggests that the enantiomeric ratios of volatile compounds may be important in host location by the pine weevil.

IDENTIFICATION OF PLANT VOLATILES ACTIVATING SINGLE RECEPTOR NEURONS IN THE PINE WEEVIL (HYLOBIUS ABIETIS)

Abstract Naturally produced plant volatiles, eliciting responses of single olfactory receptor neurons in the pine weevil, have been identified by gas chromatography linked with mass spectrometry. The receptor neurons (n = 72) were classified in 30 types, according to the compound which elicited the strongest response in each neuron, 20 of which compounds were identified. Most potent for 14 types of neurons (n = 50) were monoterpenes, including bicyclic (e.g. α-pinene, camphor and myrtenal) for 8 types (n = 32), monocyclic (limonene, carvone, α-terpinene) for 3 types (n = 12) and acyclic (e.g. β-myrcene and linalool) for 3 types (n = 6). Other compounds eliciting strongest responses of a neuron were five sesquiterpenes, including α-copaene and a farnesene-isomer, and an anethole type which has no biosynthetic relationship with terpenes. Within one type, receptor neurons with quite selective responses to the most potent compound as well as neurons with additional responses to several, structurally similar compounds were found, indicating that the neurons may have the same functional types of membrane receptors, but different sensitivities. Response spectra of neurons within the bicyclic-, mono-cyclic and acyclic types showed more overlapping than across the neuron types. Minimal overlapping response spectra was found between monoterpene and sesquiterpene neurons. The results suggest that this structure-activity relationship is significant for encoding plant odour information in the pipe weevil.

Encoding of plant odours by receptor neurons in the pine weevil (Hylobius abietis) studied by linked gas chromatography-electrophysiology

Receptor neuron responses to plant volatiles, trapped by head-space procedures, were examined in the pine weevil Hylobius abietis, using gas chromatography linked with electrophysiological recordings from single neurons. Seventy-two receptor neurons were tested 173 times for various plant volatile mixtures, either via a polar or a non-polar column.1)All responses appeared as increased firing rates which followed the concentration profiles of the GC-eluted compounds.2)The neurons were classified separately for the two column types in 17 and 19 groups respectively, according to the compounds they responded to. It suggests that the plant odour information is encoded by a large, but limited number of receptor neuron types.3)Most neurons responded to a limited number of compounds (1–5) and showed a marked best response to one of them, whereas additional responses to several other components which seems to be structurally similar, was recorded for some neurons. It suggests that the plant odour receptor neurons are rather narrowly than broadly tuned, and that each neuron is specialized for receiving information about one or a few related compounds.4)Most neurons responded to monoterpenes, whereas the other neurons responded to compounds of other categories.5)Both major and minor plant volatile components activated specifically receptor neurons.

Molecular receptive ranges of olfactory receptor neurones responding selectively to terpenoids, aliphatic green leaf volatiles and aromatic compounds, in the strawberry blossom weevil Anthonomus rubi

Summary.An important question in insect-plant interactions is which of the numerous plant compounds contribute to the perception of odour qualities in herbivorous insects and are likely to be used as cues in host-searching behaviour. In order to identify which plant-produced volatiles the strawberry blossom weevil Anthonomus rubi detects, we have used electrophysiological recordings from single olfactory neurones linked to gas chromatography and mass spectrometry. We here present 15 receptor neurone types specialised for naturally produced compounds present in the host and nonhost plants and two types for two aggregation pheromone components. The active compounds were terpenoids, aromatic and aliphatic esters, alcohols and aldehydes, some of which are induced by feeding activity of the weevils. The neurones were characterised by a strong response to one or two primary odorants and weaker responses to a few others having similar chemical structure. With one exception, the molecular receptive range of each neurone type was within one chemical group. Enantiomers of linalool separated on a chiral column activated two neurone types with different enantioselectivity. Inhibition by linalool of another neurone type, excited by α-pinene, indicated an additional mechanism for coding the information about this compound. Altogether, detection of 54 compounds by olfactory receptor neurones is shown, of which 40 have been chemically identified in this study. Thus A. rubi has the ability to detect a large number of odorants that may be used in host selection behaviour.

Identification of plant volatiles activating the same receptor neurons in the pine weevil, Hylobius abietis

Identification of plant volatiles which activate single receptor neurons in insects is of interest for research on olfactory mechanisms as well as on insect-plant interactions. Although plant odours have been shown to be essential for host location in many herbivorous insects, little is known about the compounds serving this purpose. Previous studies involving tests of synthetic compounds on single receptor neurons, have indicated that plant odour receptor neurons are broadly tuned, generally responding to several different compounds. This suggests that the plant odour information is mediated to the brain via an across-fibre pattern mechanism (cf. Masson & Mustaparta, 1990). Gas chromatography linked to electrophysiological recordings from single receptor neurons can be used to identify compounds of blends that serve as the biological signals for insects. In this project, the pine weevil (Hylobius abietis L.)

The olfactory response of the reindeer nose bot fly,Cephenemyia trompe (Oestridae), to components from interdigital pheromone gland and urine from the host reindeer,Rangifer tarandus

SummaryThe linked gas chromatographical/electroan-tennogram (GC/EAG) technique revealed that the parasitic reindeer nose bot fly is able to specifically sense components produced by the interdigital pheromone gland of reindeer. The head-space extraction technique, with Porapak Q as the collecting polymer, was used to trap pheromone gland and urine components used to assess fly responses. One component from reindeer urine also was a potent stimulus for the sensory neurons of the fly. These components can be important chemical signals to the flies for long distance orientation towards host animals. This is the first report on EAG in Oestridae.

Specialization of receptor neurons to host odours in the pine weevil, Hylobius abietis

In previous studies of host odours, the sensitivity of single sensory cells to various compounds known to be present in host plant materials has been tested (Masson &; Mustaparta, 1990). The results suggest that individual plant odour receptor neurons respond to several compounds and are therefore less specialized than pheromone neurons. In screening tests of various synthetic compounds, it is highly probable that many constituents of the natural plant volatile complex are lacking. In our present study of the pine weevil, we linked gas chromatography to electrophysiological recordings to test single receptor neurons for all compounds present in plant volatile bouquets.

The two reindeer parasites,Hypoderma tarandi andCephenemyia trompe (Oestridae), have evolved similar olfactory receptor abilities to volatiles from their common host

SummaryUsing the linked gas chromatographical/electroantennogram (GC/EAG) technique it was revealed that the reindeer warble fly (H. tarandi) was specifically able to sense the same components from reindeer interdigital pheromone gland as the reindeer nose bot fly (C. trompe) another reindeer endoparasite. These two species belong to the same family (Oestridae), but different subfamilies, and the evolution towards an endoparasitic life cycle is thought to have been independent. The development of olfactory abilities to find reindeer from long distances is hypothesized to have taken place through convergent evolution or exists because of their common ancestry.

Semiochemical-based pest insect management in strawberry and raspberry

Introduction: For many insect species, pheromones and host plant volatiles are of major importance in mate finding and host plant location. Therefore, there is potential for using these interactions to develop new strategies and effective control measures. The strawberry blossom weevil (Anthonomus rubi), the European tarnished plant bug (Lygus rugulipennis) and the raspberry beetle (Byturus tomentosus) cause large losses (10->80%) in both conventional and organic strawberry and raspberry production. We investigated whether the natural semiochemical mechanisms of these key pests can be exploited to develop effective traps for their management through mass trapping. Methods: Attractive traps and lures with host plant volatiles and pheromones were developed and combined into a “multitrap” for management of these pests simultaneously. These traps were deployed within and around the perimeters of the soft berry fields. Insect catches were evaluated according to species, habitat and environmental variables. Plant damage was also assessed. Results/Conclusion: For both crops it was possible to combine lures so that different targeted insect species were caught in the same trap simultaneously, but optimal insect traps in strawberry were not the same as in raspberry. Perimeter traps and multiple traps placed inside the plantations showed the potential to reduce pest numbers and subsequent fruit damage. In addition, bycatch of non-targeted invertebrate species were low. Thus, it is possible to develop new plant protection strategies by using these techniques. In this paper we will discuss the benefits and difficulties in using these techniques as part of integrated or organic pest management.