Joachim Ruther

Rich in phenomena-lacking in terms. A classification of kairomones

Summary. Although the usefulness of the term kairomone was discussed controversially after its introduction, it is now widely accepted in chemical ecology. It is commonly used to describe a chemical that is pertinent to the biology of an organism (organism 1) and that when it contacts an individual of another species (Organism 2) evokes in the receiver a behavioural or physiological response that is adaptively favourable to organism 2 but not to organism 1. A look at the chemoecological literature reveals that chemicals classified by the mere term kairomone may have completely different biological functions for the receiving organism. Chemicals meeting the definition of a kairomone are used for the location of food sources and sexual mates, or may be used by potential prey or host organisms to decrease the negative impact of natural enemies. Thus, by describing those diverse mediators merely as kairomones, no information on the actual function of the chemical is given. When considering the terminology of pheromones, another diverse group of infochemicals mediating intraspecific interactions, further subdivision is common practice and useful to describe the multitude of different functions and thus, to prevent terminological confusion.¶The present paper demonstrates the diversity of kairomone-related ecological phenomena by several examples and proposes a further classification of kairomones according to the function for the benefiting organism, by introducing the terms foraging kairomone (used in the context of food location), enemy-avoidance kairomone (used to reduce the negative impact of natural enemies), sexual kairomone (used for sexual purposes), and aggregation kairomone (attracting/arresting both sexes of an organism). Additionally, discrimination of two groups of kairomones according to the effect on the benefiting organism is proposed leading to the terms primer kairomone (inducing physiological responses) and releaser kairomone (inducing behavioural responses). The intention of the proposed classification is to allow a more precise description of kairomones and thus, to aid the discussion of these compounds and to improve the readability of kairomone-related papers.

Mate finding in the forest cockchafer, Melolontha hippocastani, mediated by volatiles from plants and females.

The response of the forest cockchafer, Melolontha hippocastani F. (Coleoptera, Scarabaeidae), towards volatiles emitted by different host plants and conspecifics was tested in field experiments during the flight period at dusk. Funnel traps containing artificially damaged leaves from the host plants Carpinus betulus L. and Quercus rubra L., as well as from the non‐host plant Prunus serotina Ehrh. caught significantly more beetles than empty control traps. On the other hand, traps baited with undamaged leaves from Q. rubra did not catch significantly more beetles than empty controls. Leaves from C. betulus damaged by beetle feeding did not attract more beetles than artificially damaged leaves. By use of gas chromatography coupled with electroantennographic detection (GC‐EAD) electrophysiological responses of males and females were shown for 18 typical plant volatiles. A synthetic mixture of selected typical green plant volatiles was also highly attractive in the field. A total of 9982 beetles was caught during the field experiments, among them only 33 females. This suggests that attraction to damaged foliage during flight period at dusk is male‐specific. Field experiments testing the attractiveness of female M. hippocastani towards conspecific males by employing caged beetles and beetle extracts indicated that males of M. hippocastani use a female‐derived sex pheromone for mate location. On wired cages containing either unmated feeding females, or unmated females without access to foliage, or feeding males in combination with extracts from unmated females, significantly more males landed during the flight period than on comparable control cages containing feeding males or male extracts. A possible scenario of mate location in M. hippocastani involving feeding‐induced plant volatiles and a female‐derived sex pheromone is discussed.

Behavioural and genetic analyses of Nasonia shed light on the evolution of sex pheromones

Sex pheromones play a pivotal role in the communication of many sexually reproducing organisms. Accordingly, speciation is often accompanied by pheromone diversification enabling proper mate finding and recognition. Current theory implies that chemical signals are under stabilizing selection by the receivers who thereby maintain the integrity of the signals. How the tremendous diversity of sex pheromones seen today evolved is poorly understood. Here we unravel the genetics of a newly evolved pheromone phenotype in wasps and present results from behavioural experiments indicating how the evolution of a new pheromone component occurred in an established sender–receiver system. We show that male Nasonia vitripennis evolved an additional pheromone compound differing only in its stereochemistry from a pre-existing one. Comparative behavioural studies show that conspecific females responded neutrally to the new pheromone phenotype when it evolved. Genetic mapping and gene knockdown show that a cluster of three closely linked genes accounts for the ability to produce this new pheromone phenotype. Our data suggest that new pheromone compounds can persist in a sender’s population, without being selected against by the receiver and without the receiver having a pre-existing preference for the new pheromone phenotype, by initially remaining unperceived. Our results thus contribute valuable new insights into the evolutionary mechanisms underlying the diversification of sex pheromones. Furthermore, they indicate that the genetic basis of new pheromone compounds can be simple, allowing them to persist long enough in a population for receivers to evolve chemosensory adaptations for their exploitation.

Nestmate recognition in social wasps: manipulation of hydrocarbon profiles induces aggression in the European hornet

Abstract. The influence of individual cuticular hydrocarbons on nestmate recognition in the European hornet, Vespa crabro L., was investigated. We observed the behavioural response of workers towards differently treated dead conspecifics in a bioassay. Dummies were extracted with dichloromethane and extracts were spiked with microgram amounts of synthetic hydrocarbons naturally occurring on the cuticle of V. crabro. These modified extracts were reapplied to extracted workers that were subsequently tested in the bioassay. Non-spiked nestmate dummies (negative control) and untreated non-nestmate dummies (positive control) were tested in control experiments. The addition of only heneicosane or a mixture of heneicosane, tricosane, and (Z)-9-tricosene to the extracts led to a significant increase of agonistic behaviour in workers leaving the nest for foraging flights. Returning workers reacted much less aggressively than those leaving. This is one of the first behavioural proofs that manipulation of cuticular hydrocarbon profiles can be perceived by a social insect species. The results support the hypothesis that colony-specific cuticular hydrocarbon profiles are involved in the phenomenon of nestmate recognition among social insects.

A male sex pheromone in a parasitic wasp and control of the behavioral response by the female's mating status.

SUMMARY Male insects may increase their chance of successful reproduction by releasing pheromones that attract females or elicit sexual acceptance. In parasitic wasps, male pheromones have been suggested for a few species but no chemicals have been identified so far. Here we report the first identification of a male sex pheromone in parasitic Hymenoptera. In abdomens of male jewel wasps, Nasonia vitripennis Walker, we found a mixture of (4R,5R)- and (4R,5S)-5-hydroxy-4-decanolide (HDL), which was released intermittently and attracted virgin females, but no males, in an olfactometer bioassay. However, only a few minutes after copulation mated females avoided the male-derived pheromone. Neither preference nor avoidance was shown by mated females after 24 h and even after they had been allowed to oviposit for 6 days. Nasonia vitripennis females normally mate only once. Thus, their variable response to the sex attractant depending on the mating status makes sense from an evolutionary perspective. Firstly, it increases the chance of virgins to be inseminated. Secondly, by terminating the response or even avoiding the male pheromone, mated females decrease the probability of encountering males and being disturbed by their courtship activities when searching for new oviposition sites.

Plant volatiles in the sexual communication of Melolontha hippocastani: response towards time-dependent bouquets and novel function of (Z)-3-hexen-1-ol as a sexual kairomone

1. Swarming males of Melolontha hippocastani are known to locate females that stay feeding within the host trees by orienting towards damage‐induced plant volatiles (green leaf volatiles) and a sex pheromone. Thus, volatiles emitted by freshly damaged leaves might indicate to a male the presence of currently feeding females.

Quantity matters: male sex pheromone signals mate quality in the parasitic wasp Nasonia vitripennis

Sexual selection theory asserts that females are well adapted to sense signals indicating the quality of potential mates. One crucial male quality parameter is functional fertility (i.e. the success of ejaculates in fertilizing eggs). The phenotype-linked fertility hypothesis (PLFH) predicts that functional fertility of males is reflected by phenotypic traits that influence female mate choice. Here, we show for Nasonia vitripennis, a parasitic wasp with haplodiploid sex determination and female-biased sex ratios, that females use olfactory cues to discriminate against sperm-limited males. We found sperm limitation in newly emerged and multiply mated males (seven or more previous matings) as indicated by a higher proportion of sons in the offspring fathered by these males. Sperm limitation correlated with clearly reduced pheromone titres. In behavioural bioassays, females oriented towards higher doses of the synthetic pheromone and were attracted more often to scent marks of males with a full sperm load than to those of sperm-limited males. Our data support the PLFH and suggest that N. vitripennis females are able to decrease the risk of getting constrained to produce suboptimal offspring sex ratios by orienting towards gradients of the male sex pheromone.

Female‐derived sex pheromone mediates courtship behaviour in the parasitoid Lariophagus distinguendus

Courtship behaviour of the polyphagous ectoparasitoid Lariophagus distinguendus Först. (Hymenoptera, Pteromalidae) was studied. The initial behavioural element of the courtship sequence is a characteristic wing fanning shown by the males at encounters with females. Wing fanning and arrestment of the males was elicited by paper discs treated with dichloromethane extracts from virgin females showing the existence of a female‐derived sex pheromone. The pheromone is only active at a distance between 0–5 mm suggesting low volatility of the active compound(s). Females mate only once, and the pheromone is still perceived by males at least 5 days after female mating. Males exposed to dissected female heads, thoraces and abdomens, showed wing fanning towards all segments. However, extracts from female abdomens were significantly more active than those from heads or thoraces suggesting the pheromone source to be located in the abdomen.

Emission of Herbivore-induced Volatiles in Absence of a Herbivore - Response of Zea mays to Green Leaf Volatiles and Terpenoids

Abstract Green leaf volatiles (GLV), a series of saturated and monounsaturated six-carbon aldehydes, alcohols, and esters are emitted by plants upon mechanical damage. Evidence is increasing that intact plants respond to GLV by activating their own defense mechanisms, thus suggesting that they function in plant-plant communication. The present paper demonstrates that exposure of maize plants to naturally occurring GLV, including (Z)-3-, (E)-2- and saturated derivatives, induce the emission of volatile blends typically associated with herbivory. Position or configuration of a double bond, but not the functional group of the GLV influenced the strength of the emissions. (Z)-3-Configured compounds elicited stronger responses than (E)-2- and saturated derivatives. The response to (Z)-3-hexen-1-ol increased linearly with the dose between 200 and 1000 nmol per plant. Not only the naturally occurring (E)-2- hexenal, but also (E)-2-pentenal and (E)-2-heptenal induced maize plants, although to a lesser extend. Externally applied terpenoids [(3E)-4,8-dimethyl-1,3,7-nonatriene, β-caryophyllene, and (E)- β-farnesene] did not significantly increase the total amount of inducible volatiles in maize. Of three tested maize cultivars Delprim and Pactol responded much stronger than Attribut. Recovery experiments in the presence and absence of maize plants demonstrated that large proportions of externally applied GLV were assimilated by the plants, whereas (3E)-4,8-dimethyl-1,3,7-nonatriene was recovered in much higher amounts. The results furthermore suggested that plants converted a part of the assimilated leaf aldehydes and alcohols to the respective acetates. We propose that GLV not only can alert neighboring plants, but may facilitate intra-plant information transfer and can help mediate the systemic defense response in a plant.

Alcoholism in cockchafers: orientation of male Melolontha melolontha towards green leaf alcohols

Abstract. Chemical orientation of the European cockchafer, Melolontha melolontha L., a serious pest in agriculture and horticulture, was investigated by field tests and electrophysiological experiments using plant volatiles. In total, 16 typical plant volatiles were shown to elicit electrophysiological responses in male cockchafers. Funnel trap field bioassays revealed that green leaf alcohols (i.e. (Z)-3-hexen-1-ol, (E)-2-hexen-1-ol and 1-hexanol) attracted males, whereas the corresponding aldehydes and acetates were behaviourally inactive. Furthermore, male cockchafers were attracted by volatiles from mechanically damaged leaves of Fagus sylvatica L., Quercus robur L. and Carpinus betulus L. However, volatiles emitted by damaged leaves of F. sylvatica attracted significantly more males than those from the other host plants. Odour from intact F. sylvatica leaves was not attractive to M. melolontha males. Females were not attracted by any of the tested volatile sources. The results suggest that plant volatiles play a similar role as a sexual kairomone in mate finding of M. melolontha, as has been shown for the forest cockchafer, Melolontha hippocastani F. Nevertheless, both species show remarkable differences in their reaction to green leaf alcohols.