Larval parasitoid uses aggregation pheromone of adult hosts in foraging behaviour: a solution to the reliability-detectability problem

The Drosophila-parasitizing wasp Leptopilina heterotoma: A comprehensive model system in ecology and evolution

The parasitoid Leptopilina heterotoma has been used as a model system for more than 70 years, contributing greatly to diverse research areas in ecology and evolution. Here, we synthesized the large body of work on L. heterotoma with the aim to identify new research avenues that could be of interest also for researchers studying other parasitoids and insects. We start our review with a description of typical L. heterotoma characteristics, as well as that of the higher taxonomic groups to which this species belongs. We then continue discussing host suitability and immunity, foraging behaviors, as well as fat accumulation and life histories. We subsequently shift our focus towards parasitoid-parasitoid interactions, including L. heterotoma coexistence within the larger guild of Drosophila parasitoids, chemical communication, as well as mating and population structuring. We conclude our review by highlighting the assets of L. heterotoma as a model system, including its intermediate life history syndromes, the ease of observing and collecting natural hosts and wasps, as well as recent genomic advances.

Adaptations and counter-adaptations in Drosophila host-parasitoid interactions: advances in the molecular mechanisms

Both hosts and parasitoids evolved a diverse array of traits and strategies for their antagonistic interactions, affecting their chances of encounter, attack and survival after parasitoid attack. This review summarizes the recent progress that has been made in elucidating the molecular mechanisms of these adaptations and counter-adaptations in various Drosophila host-parasitoid interactions. For the hosts, it focuses on the neurobiological and genetic control of strategies in Drosophila adults and larvae of avoidance or escape behaviours upon sensing the parasitoids, and the immunological defences involving diverse classes of haemocytes. For the parasitoids, it highlights their behavioural strategies in host finding, as well as the rich variety of venom components that evolved and were partially acquired through horizontal gene transfer. Recent studies revealed the mechanisms by which these venom components manipulate their parasitized hosts in exhibiting escape behaviour to avoid superparasitism, and their counter-strategies to evade or obstruct the hosts' immunological defences.

Deployment of Aggregation-Sex Pheromones of Longhorned Beetles (Coleoptera: Cerambycidae) Facilitates the Discovery and Identification of their Parasitoids

Longhorned beetles (Coleoptera: Cerambycidae) include many species that are among the most damaging pests of managed and natural forest ecosystems worldwide. Many species of cerambycids use volatile chemical signals (i.e., pheromones) to locate mates. Pheromones are often used by natural enemies, including parasitoids, to locate hosts and therefore can be useful tools for identifying host-parasitoid relationships. In two field experiments, we baited linear transects of sticky traps with pheromones of cerambycid beetles in the subfamily Cerambycinae. Enantiomeric mixtures of four linear alkanes or four linear alkanes and a ketol were tested separately to evaluate their attractiveness to hymenopteran parasitoids. We hypothesized that parasitoids would be attracted to these pheromones. Significant treatment effects were found for 10 species of parasitoids. Notably, Wroughtonia ligator (Say) (Hymenoptera: Braconidae) was attracted to syn-hexanediols, the pheromone constituents of its host, Neoclytus acuminatus acuminatus (F.) (Coleoptera: Cerambycidae). Location and time of sampling also significantly affected responses for multiple species of parasitoids. These findings contribute to the basic understanding of cues that parasitoids use to locate hosts and suggest that pheromones can be used to hypothesize host relationships between some species of cerambycids and their parasitoids. Future work should evaluate response by known species of parasitoids to the complete blends of pheromones used by the cerambycids they attack, as well as other odors that are associated with host trees of cerambycids.

Evolution of behavioural and cellular defences against parasitoid wasps in the Drosophila melanogaster subgroup

It may be intuitive to predict that host immune systems will evolve to counter a broad range of potential challenges through simultaneous investment in multiple defences. However, this would require diversion of resources from other traits, such as growth, survival and fecundity. Therefore, ecological immunology theory predicts that hosts will specialize in only a subset of possible defences. We tested this hypothesis through a comparative study of a cellular immune response and a putative behavioural defence used by eight fruit fly species against two parasitoid wasp species (one generalist and one specialist). Fly larvae can survive infection by melanotically encapsulating wasp eggs, and female flies can potentially reduce infection rates in their offspring by laying fewer eggs when wasps are present. The strengths of both defences varied significantly but were not negatively correlated across our chosen host species; thus, we found no evidence for a trade-off between behavioural and cellular immunity. Instead, cellular defences were significantly weaker against the generalist wasp, whereas behavioural defences were similar in strength against both wasps and positively correlated between wasps. We investigated the adaptive significance of wasp-induced oviposition reduction behaviour by testing whether wasp-exposed parents produce offspring with stronger cellular defences, but we found no support for this hypothesis. We further investigated the sensory basis of this behaviour by testing mutants deficient in either vision or olfaction, both of which failed to reduce their oviposition rates in the presence of wasps, suggesting that both senses are necessary for detecting and responding to wasps.

Host gut microorganisms' cues mediate orientation behaviour in the larva of the parasitoid Mallophora ruficauda

The robber fly Mallophora ruficauda is one of the most important apicultural pests in the Pampas region of Argentina. This species is a parasitoid of scarab beetle larvae. Females lay eggs away from the host, and the larvae perform active search behaviour toward Cyclocephala signaticollis third instar larvae, parasitoid's preferred host. This behaviour is mediated by host-related chemical cues produced in hosts' fermentation chamber. Also, C. signaticollis larvae are attracted to fermentation chamber extracts. As scarab larvae have microbe-rich fermentation chamber, it has been suggested that microorganisms could be involved in the production of these semiochemicals. The aims of this work were first to ascertain the presence of microorganisms in the fermentation chamber of C. signaticollis larvae and second to determine the role of microorganisms in the orientation response of parasitoid and host larvae. We found that microorganisms-free C. signaticollis larvae showed deterioration in their development and did not produce the attractive semiochemicals. Therefore, we isolated fermentation chamber microorganisms of host larvae by means of different cultures media, and then, assayed different microorganisms' stimuli by binary choice tests. We were able to isolate microorganisms and determine that M. ruficauda larvae are attracted to semiochemicals from protein degradation in the fermentation chamber. However, C. signaticollis larvae were not attracted to any semiochemicals associated with microorganisms' activity in the fermentation chamber. Although we were unable to elucidate the exact role of gut microorganisms in host behaviour, we discuss their relevance in parasitoid host-seeking behaviour and host conspecific interaction in M. ruficauda-C. signaticollis system.

Aggregation pheromones ofDrosophila immigrans, D. phalerata, andD. subobscura

Aggregation pheromones ofDrosophila immigrans, D. phalerata andD. subobscura were demonstrated by testing attraction of adult flies to hexane extracts of the flies in a windtunnel bioassay. Extracts of adult males of all species attracted conspecific males and females. However,D. subobscura flies were attracted only when the extract (cVA) in the extracts of adult maleD. immigrans andD. phalerata. Both species were attracted to synthetic cVA. Male and femaleD. phalerata. Both species were attracted to synthetic cVA. Male and femaleD. subobscura produced 5,9-pentacosadiene, 5-pentacosene, 2-methylhexacosene and 5,9-heptacosadiene, while only maleD. subobscura produced (Z)-5-tricosene and minor amounts of cVA.

Exploitation of chemical signaling by parasitoids: impact on host population dynamics

Chemical information mediates species interactions in a wide range of organisms. Yet, the effect of chemical information on population dynamics is rarely addressed. We designed a spatio-temporal parasitoid--host model to investigate the population dynamics when both the insect host and the parasitic wasp that attacks it can respond to chemical information. The host species, Drosophila melanogaster, uses food odors and aggregation pheromone to find a suitable resource for reproduction. The larval parasitoid, Leptopilina heterotoma, uses these same odors to find its hosts. We show that when parasitoids can respond to food odors, this negatively affects fruit fly population growth. However, extra parasitoid responsiveness to aggregation pheromone does not affect fruit fly population growth. Our results indicate that the use of the aggregation pheromone by D. melanogaster does not lead to an increased risk of parasitism. Moreover, the use of aggregation pheromone by the host enhances its population growth and enables it to persist at higher parasitoid densities.

Decision-making dynamics in parasitoids of Drosophila

Drosophilids and their associated parasitoids live in environments that vary in resource availability and quality within and between generations. The use of information to adapt behavior to the current environment is a key feature under such circumstances and Drosophila parasitic wasps are excellent model systems to study learning and information use. They are among the few parasitoid model species that have been tested in a wide array of situations. Moreover, several related species have been tested under similar conditions, allowing the analysis of within and between species variability, the effect of natural selection in a typical environment, the current physiological status, and previous experience of the individual. This holds for host habitat and host location as well as for host choice and search time allocation. Here, we review patterns of learning and memory, of information use and updating mechanisms, and we point out that information use itself is under strong selective pressure and thus, optimized by parasitic wasps.

Ecology and life history evolution of frugivorous Drosophila parasitoids

Parasitoids and their hosts are linked by intimate and harmful interactions that make them well suited to analyze fundamental ecological and evolutionary processes with regard to life histories evolution of parasitic association. Drosophila aspects of what parasitoid Hymenoptera have become model organisms to study aspects that cannot be investigated with other associations. These include the genetic bases of fitness traits variations, physiology and genetics of resistance/virulence, and coevolutionary dynamics leading to local adaptation. Recent research on evolutionary ecology of Drosophila parasitoids were performed mainly on species that thrive in fermenting fruits (genera Leptopilina and Asobara). Here, we review information and add original data regarding community ecology of these parasitoids, including species distribution, pattern of abundance and diversity, host range and the nature and intensity of species interactions. Biology and the evolution of life histories in response to habitat heterogeneity and possible local adaptations leading to specialization of these wasps are reported with special emphasis on species living in southern Europe. We expose the diversity and intensity of selective constraints acting on parasitoid life history traits, which vary geographically and highlight the importance of considering both biotic and abiotic factors with their interactions to understand ecological and evolutionary dynamics of host-parasitoid associations.

Dynamic use of fruit odours to locate host larvae: individual learning, physiological state and genetic variability as adaptive mechanisms

This chapter presents a series of behavioral studies designed to document how Leptopilina spp. learn fruit odours in order to find and explore host-infested fruits. Experimental analyses of conditioned responses explored individual learning, physiological changes and genetic variability as adaptive mechanisms of the host searching behavior. Both oriented walking and substrate probing can be easily observed and quantified in laboratory devices. We studied walking in a four-arm olfactometer and probing in an agar substrate in response to olfactory stimulation by fruit odours. We analyzed the odour learning process and the dynamics of the memory. We next investigated how odour memory is influenced by motivation factors such as mating or egg-load, and how much variation is due to inheritance, using isofemale lines. Next, we addressed the adaptive significance of innate and conditioned responses to fruit odour by comparing and crossing populations originating from areas with contrasted levels of host availability.

Hierarchical order of host cues in parasite foraging strategies

The importance of host cues to three species of steinernematid nematodes (Rhabdita: Steinernematidae) with different foraging strategies was compared. We presented host materials to nematodes in series to test responses to combinations of host cues. If a fixed hierarchy of cues is followed during foraging, parasites should respond most strongly to cues offered in natural order. Steinernema carpocapsae, an ambush forager, aggregated at the source of volatile host cues only after attachment to host cuticle. They also parasitized hosts more efficiently after contact with cuticle. Steinernema glaseri, a cruise forager, was unaffected by exposure to combinations of host cues. Steinernema feltiae, a nematode with characteristics of both ambushing and cruising, was affected by cue hierarchies when either contact or volatile cues were presented first. Host-associated materials encountered out of the context may not qualify as host cues for the ambush forager, S. carpocapsae. Perhaps the order in which cues are encountered is more predictable for ambushers than for cruisers. Therefore an ambusher's response to host materials has a more fixed contextual framework.

The effect of chemical information on the spatial distribution of fruit flies: I Model results

Animal aggregation is a general phenomenon in ecological systems. Aggregations are generally considered as an evolutionary advantageous state in which members derive the benefits of protection and mate choice, balanced by the costs of limiting resources and competition. In insects, chemical information conveyance plays an important role in finding conspecifics and forming aggregations. In this study, we describe a spatio-temporal simulation model designed to explore and quantify the effects of these infochemicals, i.e., food odors and an aggregation pheromone, on the spatial distribution of a fruit fly (Drosophila melanogaster) population, where the lower and upper limit of local population size are controlled by an Allee effect and competition. We found that during the spatial expansion and strong growth of the population, the use of infochemicals had a positive effect on population size. The positive effects of reduced mortality at low population numbers outweighed the negative effects of increased mortality due to competition. At low resource densities, attraction toward infochemicals also had a positive effect on population size during recolonization of an area after a local population crash, by decreasing the mortality due to the Allee effect. However, when the whole area was colonized and the population was large, the negative effects of competition on population size were larger than the positive effects of the reduction in mortality due to the Allee effect. The use of infochemicals thus has mainly positive effects on population size and population persistence when the population is small and during the colonization of an area.

Intra- and interspecific density-dependent dispersal in an aquatic prey-predator system

1. Dispersal intensity is a key process for the persistence of prey-predator metacommunities. Consequently, knowledge of the ecological mechanisms of dispersal is fundamental to understanding the dynamics of these communities. Dispersal is often considered to occur at a constant per capita rate; however, some experiments demonstrated that dispersal may be a function of local species density. 2. Here we use aquatic experimental microcosms under controlled conditions to explore intra- and interspecific density-dependent dispersal in two protists, a prey Tetrahymena pyriformis and its predator Dileptus sp. 3. We observed intraspecific density-dependent dispersal for the prey and interspecific density-dependent dispersal for both the prey and the predator. Decreased prey density lead to an increase in predator dispersal, while prey dispersal increased with predator density. 4. Additional experiments suggest that the prey is able to detect its predator through chemical cues and to modify its dispersal behaviour accordingly. 5. Density-dependent dispersal suggests that regional processes depend on local community dynamics. We discuss the potential consequences of density-dependent dispersal on metacommunity dynamics and stability.

Spatial aggregation and association in different resource-patch distributions: experimental analysis with Drosophila

1. Laboratory experiments using homogeneous resources were conducted to examine intra- and interspecific spatial egg distribution of D. simulans, D. auraria and D. immigrans in three different resource-patch distribution patterns: patchy, even and clustered. 2. Individuals of each species were introduced separately or simultaneously into the cage, into which artificial substrates were placed and allowed to oviposit for 24 h. Spatial analyses were performed with indices of intraspecific aggregation (J), interspecific association (C) and L-function based on Ripley's K-function. 3. Eggs were always spatially aggregated irrespective of species and the resource-patch distribution patterns. Spatial egg aggregation was influenced significantly by the resource-patch distribution pattern and tended to be weaker in the clustered resource-patch distribution than in the patchy or even resource-patch distribution. 4. Spatial extent of egg aggregation was always beyond the single resource patch scale, indicating aggregation of ovipositing females. 5. Interspecific association of egg distribution was absent or very weak. Thus, these results present experimental evidence of independent egg aggregation among drosophilids.

Olfactory Responses of Banana Weevil Predators to Volatiles from Banana Pseudostem Tissue and Synthetic Pheromone

As a response to attack by herbivores, plants can emit a variety of volatile substances that attract natural enemies of these insect pests. Predators of the banana weevil, Cosmopolites sordidus (Germar) (Coleoptera: Curculionidae) such as Dactylosternum abdominale (Coleoptera: Hydrophilidae) and Pheidole megacephala (Hymenoptera: Formicidae), are normally found in association with weevil-infested rotten pseudostems and harvested stumps. We investigated whether these predators are attracted to such environments in response to volatiles produced by the host plant, by the weevil, or by the weevil plant complex. We evaluated predator responses towards volatiles from banana pseudostem tissue (synomones) and the synthetic banana weevil aggregation pheromone Cosmolure+ in a two-choice olfactometer. The beetle D. abdominale was attracted to fermenting banana pseudostem tissue and Cosmolure+, whereas the ant P. megacephala was attracted only to fermented pseudostem tissue. Both predators were attracted to banana pseudostem tissue that had been damaged by weevil larvae irrespective of weevil presence. Adding pheromone did not enhance predator response to volatiles from pseudostem tissue fed on by weevils. The numbers of both predators recovered with pseudostem traps in the field from banana mats with a pheromone trap were similar to those in pseudostem traps at different distance ranges from the pheromone. Our study shows that the generalist predators D. abdominale and P. megacephala use volatiles from fermented banana pseudostem tissue as the major chemical cue when searching for prey.

Identification of the Larval Aggregation Pheromone of Codling Moth, Cydia pomonella

Mature larvae of the codling moth, Cydia pomonella L. (Lepidoptera: Olethreutidae), exit the fruit and seek sites suitable for pupation. Spinning cocoons in such sites, larvae produce a complex, cocoon-derived blend of volatiles recently shown to attract and/or arrest both conspecific larvae and the prepupal parasitoid Mastrus ridibundus Gravenhorst (Hymenoptera: Ichneumonidae). Here we report components of this blend that constitute the pheromone of fifth-instar C. pomonella larvae. Thirty-one two-choice olfactometer experiments showed that a blend of synthetic (E)-2-octenal, (E)-2-nonenal, sulcatone, and geranylacetone, in combination with either 3-carene and/or three saturated aldehydes (octanal, nonanal, decanal), elicited behavioral responses from C. pomonella larvae. In on-tree experiments with corrugated cardboard bands as pupation sites for larvae affixed to tree trunks, and with laboratory-reared larvae released onto such trees, more larvae cocooned in those halves of cardboard bands baited with cocoon-spinning conspecific larvae, or with synthetic pheromone components, than in unbaited control halves of the bands. With the larval aggregation pheromone identified in this study, there might be an opportunity to manipulate C. pomonella larvae in commercial fruit or nut orchards.

Pheromone-mediated aggregation in nonsocial arthropods: an evolutionary ecological perspective

Although the use of aggregation pheromones has been reported for hundreds of nonsocial arthropod species, the evolutionary ecological aspects of this behavior have received little attention. Despite the elaborate literature on mechanisms, robust data on costs and benefits of aggregation pheromones are scant. Existing literature indicates that, in contrast to the diversity of mechanisms, the ecological conditions in which aggregation pheromones are used are more alike. This points to a few general categories for costs and benefits of aggregation pheromones, and these are discussed. We subsequently review interspecific interactions that may be affected by the use of aggregation pheromones. We encounter a strikingly frequent association of aggregation pheromones with fungi and microorganisms and address cross-attraction by competitor species and exploitation by natural enemies. We show that aggregative behavior by individuals through the use of pheromones can profoundly affect ecological interactions and advocate further evolutionary and ecological investigations of pheromone-mediated aggregation.

Mastrus ridibundus parasitoids eavesdrop on cocoon-spinning codling moth, Cydia pomonella, larvae

Cocoon-spinning larvae of the codling moth, Cydia pomonella L. (Lepidoptera: Olethreutidae) employ a pheromone that attracts or arrests conspecifics seeking pupation sites. Such intraspecific communication signals are important cues for illicit receivers such as parasitoids to exploit. We tested the hypothesis that the prepupal C. pomonella parasitoid Mastrus ridibundus Gravenhorst (Hymenoptera: Ichneumonidae) exploits the larval aggregation pheromone to locate host prepupae. In laboratory olfactometer experiments, female M. ridibundus were attracted to 3-day-old cocoons containing C. pomonella larvae or prepupae. Older cocoons containing C. pomonella pupae, or larvae and prepupae excised from cocoons, were not attractive. In gas chromatographic-electroantennographic detection (GC-EAD) analyses of bioactive Porapak Q extract of cocoon-derived airborne semiochemicals, ten compounds elicited responses from female M. ridibundus antennae. Comparative GC-mass spectrometry of authentic standards and cocoon-volatiles determined that these compounds were 3-carene, myrcene, heptanal, octanal, nonanal, decanal, (E)-2-octenal, (E)-2-nonenal, sulcatone, and geranylacetone. A synthetic 11-component blend consisting of these ten EAD-active compounds plus EAD-inactive (+)-limonene (the most abundant cocoon-derived volatile) was as effective as Porapak Q cocoon extract in attracting both female M. ridibundus and C. pomonella larvae seeking pupation sites. Only three components could be deleted from the 11-component blend without diminishing its attractiveness to M. ridibundus, which underlines the complexity of information received and processed during foraging for hosts. Mastrus ridibundus obviously "eavesdrop" on the pheromonal communication signals of C. pomonella larvae that reliably indicate host presence.