How the social parasitic bumblebee <Emphasis Type="Italic">Bombus bohemicus</Emphasis> sneaks into power of reproduction

Social parasitism is widespread in many groups of social living hymenopteran species and has also evolved in the genus Bombus. Cuckoo bumblebees (subgenus Psithyrus) are obligate brood parasites in nests of other bumblebee species. After nest usurpation and the killing of the host queen, the parasite female has to control worker reproduction in order to accomplish and maintain reproductive dominance and to ensure her reproductive success. The aim of our study was to examine whether the generalist parasitic bumblebee Bombus bohemicus monopolizes and prevents worker reproduction by physical or chemical means and to identify possible odor compounds involved therein. We performed bioassays with callow workers of the host Bombus terrestris and have shown that B. bohemicus females are able to suppress host worker ovarian development, when these host workers are under the direct influence of the parasite female. Furthermore, by chemical analyses, we have demonstrated that the parasite females adjust to the odor profiles of their host queens in order to maintain the level of fertility signaling inside the host colony although the host queen is absent. We also found that host workers change their odor profile after nest usurpation by the parasite female and consequently, we suggest that the host and parasite are caught up in a chemical arms race.     

Chemical ecology involved in invasion success of the cuckoo bumblebee <Emphasis Type="Italic">Psithyrus vestalis</Emphasis> and in survival of workers of its host <Emphasis Type="Italic">Bombus terrestris</Emphasis>

In bumblebees all species of the subgenus Psithyrus are social parasites in the nests of their Bombus hosts. In the bumblebee B. terrestris we investigated how colony size influences survival rates of nest entering females of the social parasite Psithyrus vestalis. Furthermore, we studied whether the host worker’s dominance status and age are reflected in its individual scent and whether Psithyrus females use volatiles to selectively kill host workers. The survival rate of Psithyrus vestalis females drops from 100%, when entering colonies with five workers, to 0% for colonies containing 50 host workers. Older host workers, born before the nest invasion, were selectively killed when Psithyrus females entered the nest. In contrast, all workers born after the nest invasion survived. The host workers’ dominance status and age are reflected by their individual odours: newly emerged workers produced a significantly lower total amount of secretions than 4-day-old workers. In chemical analyses of female groups we identified saturated and unsaturated hydrocarbons, aldehydes, and unsaturated wax-type esters of fatty acids. In a discriminant function analysis different worker groups were mainly separated by their bouquets of hydrocarbons. Killed workers release significantly more scent and of a different chemical composition, than survivors. Survivors alter scent production and increase it beyond the level of the killed workers within 1 day of the invasion. The Psithyrus female clearly maintains reproductive dominance utilizing these differences in the odour bouquets as criteria for killing workers that compete for reproduction.     

Transmission of a pathogen in Bombus terrestris,with a note on division of labour in social insects

Summary Parasites of social insect workers can be transmitted within the colony to other, related host individuals or, alternatively, to unrelated workers of other colonies. Division of labour affects the probability of transmission, as young individuals often work inside the nest whereas older ones often leave the nest to forage. Therefore, the relative probabilities of transmission within-vs. between-nests is also affected by the delay between host infection and the shedding of propagules, i.e. the latent period of the parasite strain. We therefore hypothesized that strains of the flagellate parasite Crithidia bombi (Trypanosomatidae, Zoomastigophorea) infecting workers of the bumble bee Bombus terrestris (Hymenoptera, Apidae) could differ in their delays and coexist in a population. This would be the case if strains that are shed after a short time delay were more efficiently transmitted to other colony members, whereas strains with long delays were more efficiently transmitted to non-related workers in the population. We tested this hypothesis by experimentally varying time delay and by allowing transmission to either sister workers from the same nest or unrelated workers from other nests. Transmission of C. bombi was measured as the number of parasitic cells shed by the exposed workers after a standard period. The results showed that relatedness as such had no effect, but that delay and nest identity were highly significant effects to explain variation in transmission success. There was a significant interaction between nest identity and delay, such that bees of some colonies acted as efficient transmitters for C. bombi under short delays and vice versa. We discuss how division of labour may affect parasitism in social insects and, vice versa, how division of labour may be under selection from the effects of parasitism, using available evidence from the literature. Correspondence to: P. Schmid-Hempel     

Mother’s choice of the oviposition site: balancing risk of egg parasitism and need of food supply for the progeny with an infochemical shelter?

Summary. Oviposition site selection of herbivorous insects depends primarily on host plant presence which is essential for offspring survival. However, parasitoids can exploit host plant cues for host location. In this study, we hypothesised that herbivores can solve this dilemma by ovipositing within high plant diversity. A diverse plant species composition might represent an ‘infochemical shelter’, as a potentially complex volatile blend can negatively affect the host location ability of parasitoids. We examined this exemplarily for the egg-laying response of the generalist leaf beetle, Galeruca tanaceti, in relation to (1) host plant availability and (2) plant species diversity in the field. Further, we investigated the effect of odours from mixed plant species compositions on (3) leaf beetle oviposition site selection and on (4) the orientation of its specialised egg parasitoid, Oomyzus galerucivorus. In the field, egg clutch occurrence was positively related to the presence and quantity of two major host plants, Achillea millefolium (yarrow) and Centaurea jacea, and to the number of herbaceous plant species. In two-choice bioassays, female beetles oviposited more frequently on sites surrounded by an odour blend from a diverse plant species composition (including yarrow) than on sites with a pure grass odour blend. In the presence of yarrow odour and an odour blend from a diverse plant mixture (including yarrow) no difference in the oviposition response was recorded. Experienced parasitoid females were attracted to yarrow odours, but showed no response when yarrow odours were offered simultaneously with odours of a non-host plant. In conclusion, it could be shown in laboratory bioassays that the parasitoid responds only to pure host plant odours but not to complex odour blends. In contrast, the herbivore prefers to oviposit within diverse vegetation in the field and in the laboratory. However, the laboratory results also point to a priority of host plant availability over the selection of a potential ‘infochemical shelter’ for oviposition due to high plant diversity.     

Volatile attractants for three Pteromalid parasitoids attacking concealed spruce bark beetles

Summary. The odour perceptive abilities, and preferences, of three bark beetle parasitoid species; Rhopalicus tutela (Walker), Roptrocerus mirus (Walker), and Roptrocerus xylophagorum (Ratzeburg) (Hymenoptera: Pteromalidae), were investigated to isolate and identify the essential compounds involved in host location. These parasitoids attack several economically important bark beetle species and oviposit preferentially on late larval stages concealed under the bark of conifers. Odours were collected from Norway spruce logs (Picea abies L. Karst.) containing Ips typographus L. (Coleoptera: Scolytidae) larvae. Biologically active compounds were isolated by coupled gas chromatographic-electroantennographic detection (GC-EAD), and identified by GC-mass spectrometry (GC-MS). Based on these analyses, four different synthetic baits were prepared and tested in a Y-tube walking bioassay. In the complex odour samples from spruce logs, only 16 compounds were EAD-active. The tested R. tutela and R. mirus females displayed similar trends in antennal activity to EAD-active compounds, responding mainly to oxygenated monoterpenes that indicate damaged conifers. Consequently, the synthetic baits were exclusively prepared with oxygenated monoterpenes. Parasitoid females (R. tutela and R. mirus) preferred spruce logs containing susceptible hosts over fresh logs, while male parasitoids (R. mirus) did not show any preference. However, when odours from fresh logs were mixed with synthetic baits (mimicking the odour composition of logs containing susceptible hosts), these combinations attracted female parasitoids (R. tutela, R. mirus, and R. xylophagorum). All synthetic baits seemed to be equally attractive to female parasitoids. Received 12 October 2000; accepted 18 January 2001     

Repellent scent-marking of flowers by a guild of foraging bumblebees (Bombus spp.)

We have found that foraging bumblebees (Bombus hortorum, B. pascuorum, B. pratorum and B.␣terrestris) not only avoid flowers of Symphytum officinale that have recently been visited by conspecifics but also those that have been recently visited by heterospecifics. We propose that the decision whether to reject or accept a flower is influenced by a chemical odour that is left on the corolla by a forager, which temporarily repels subsequent foragers. Honeybees and carpenter bees have previously been shown to use similar repellent forage-marking scents. We found that flowers were repellent to other bumblebee foragers for approximately 20 min and also that after this time nectar levels in S. officinale flowers had largely replenished. Thus bumblebees could forage more efficiently by avoiding flowers with low rewards. Flowers to which extracts of tarsal components were applied were more often rejected by wild B. terrestris workers than flowers that had head extracts applied, which in turn were more often rejected than flowers that had body extracts applied. Extracts from four Bombus species were equally repellent to foragers. The sites of production of the repellent scent and its evolutionary origins are discussed. Received: 24 November 1997 / Accepted after revision: 8 March 1998     

Behaviorally mediated spatial and temporal refuges from a cleptoparasite,Argochrysis armilla (Hymenoptera: Chrysididae), attacking a ground-nesting wasp,Ammophila dysmica (Hymenoptera: Sphecidae)

Summary It has been suggested that parasite pressure favors the evolution of sociality within the Hymenoptera. I analyzed the impact of a chrysidid nest parasite, Argochrysis armilla, on its host, the solitary ground-nesting wasp, Ammophila dysmica, to assess the role of parasitism in favoring two steps towards sociality: aggregated nesting and delayed nest provisioning. The foraging strategy of Argochrysis armilla involves discovering host nests during excavation, learning the locations of discovered nest, and returning to oviposit in nests during nest provisioning; I therefore assessed the influence of host behavior on (1) parasite discovery of nests and (2) parasite oviposition in nests. Significantly fewer parasites discovered host nests that were excavated during the early morning and late afternoon, due to partial asynchrony of host and parasite activity. Nests excavated in areas of low nest density were also less likely to be discovered; use of low density nest sites increased during periods of high parasite activity. Due to a rapidly decelerating rate of parasite recruitment to nests under excavation, the duration of nest digging had only a limited influence on nest discovery by parasites. The probability of parasite oviposition in a host nest was determined by the number of parasites discovering the nest during excavation and by the time between nest excavation and provisioning; delaying nest provisioning reduced the risk of parasite oviposition. Delayed provisionings primarily appeared to be a result of the stochastic process of hunting and prey encounter. The number of provisions placed in a nest (one vs two) had no effect on the probability of nest parasitism. Spatial patterns of parasitism were directly density dependent in 1984 and density independent in 1986. In this system parasite pressure acts against the formation of nesting aggregations and in favor of delayed nest provisioning. The dependence of these results on species-specific aspects of the parasite's foraging strategy and the host's defensive strategy suggests, however, that different parasite species may generate qualitatively different selection pressures, potentially contributing to the diversity of nesting behavior in the Hymenoptera.     

The role of wax and resin in the nestmate recognition system of a stingless bee, Tetragonisca angustula

Recent research has shown that entrance guards of the stingless bee Tetragonisca angustula make less errors in distinguishing nestmates from non-nestmates than all other bee species studied to date, but how they achieve this is unknown. We performed four experiments to investigate nestmate recognition by entrance guards in T. angustula. We first investigated the effect of colony odours on acceptance. Nestmates that acquired odour from non-nestmate workers were 63% more likely to be rejected while the acceptance rate of non-nestmates treated with nestmate odour increased by only 7%. We further hypothesised that guards standing on the wax entrance tube might use the tube as an odour referent. However, our findings showed that there was no difference in the acceptance of non-nestmates by guards standing on their own colony’s entrance tube versus the non-nestmate’s entrance tube. Moreover, treatment of bees with nestmate and non-nestmate resin or wax had a negative effect on acceptance rates of up to 65%, regardless of the origin of the wax or resin. The role of resin as a source of recognition cues was further investigated by unidirectionally transferring resin stores between colonies. Acceptance rates of nestmates declined by 37% for hives that donated resin, contrasting with resin donor hives where acceptance of non-nestmates increased by 21%. Overall, our results confirm the accuracy of nestmate recognition in T. angustula and reject the hypothesis that this high level of accuracy is due to the use of the wax entrance tubes as a referent for colony odour. Our findings also suggest that odours directly acquired from resin serve no primary function as nestmate recognition cues. The lack of consistency among colonies plus the complex results of the third and fourth experiments highlight the need for further research on the role of nest materials and cuticular profiles in understanding nestmate recognition in T. angustula.     

Strategic exploitation in a socially parasitic bee: a benefit in waiting?

Social parasitism has evolved at least ten times in the allodapine bees but studies that explore the parasite’s integration and exploitation of host colonies are lacking. Using colony content and dissection data, we examine how Inquilina schwarzi affects the social organisation of its host Exoneura robusta. Our samples include three critical periods in the host life cycle: initial formation of dominance hierarchies in late autumn, commencement of oviposition by host queens in late winter, and development of secondary reproductives in late spring. I. schwarzi preferentially parasitises larger host colonies in autumn, but during autumn and winter, the parasite appears to be socially invisible, living in the nest without disrupting the normal functioning of these colonies. Inquilines begin egg laying much later than their hosts, and by late spring, they have disrupted host reproductive hierarchies, leading to lower skew in ovarian sizes of their host nestmates. Living invisibly within the host nest for the first 6 months and waiting until well after host reproduction has begun before disrupting their social organisation appear to be unique among social insects. Such a change in strategy may be facilitated by the different social systems found in allodapine bees, with the social parasites possibly disrupting the reproductive hierarchies during spring to prevent or reduce the normal dispersal of some host females from their natal nests.     

Nestmate recognition in three species of stenogastrine wasps (Hymenoptera,Vespidae)

The capacity to recognise a conspecific intruder was investigated in Parischnogaster jacobsoni, Liostenogaster flavolineata and L. vechti, three species of primitively social wasps of the subfamily Stenogastrinae. Results of behavioural experiments carried out in the field showed that females of all three species react pacifically if presented with female nestmates, but aggressively reject an intruder from a conspecific colony. As L. flavolineata and L. vechti both build large clusters of nests, often very close to each other, the recognition capacity among females from different nests, but in the same conspecific cluster, was also investigated. Females of both species were more aggressive towards females from a different colony in the same cluster than towards their female nestmates. Additional experiments on L. flavolineata showed that there was no difference in reaction towards females from colonies nearer or further from the tested colony but within the same cluster, nor towards females from a different cluster. The capacity to recognise an alien conspecific nest containing immature brood was investigated in P. jacobsoni. Adult females of this species, invited to land on an alien nest which had experimentally been exchanged for their own, accepted the new nest and partially destroyed the immature brood. The behaviour of the females when they land on an alien nest, however, suggests that they do recognise the nest as foreign. Acceptance of foreign nests coupled with low immature brood destruction is probably due to the high energetic costs of egg-deposition and larval rearing in stenogastrine wasps. These results suggest that nestmate recognition in these wasps is very efficient, even though they belong to the most primitive subfamily of social wasps. Received: 16 April 1996/Accepted after revision: 9 August 1996     

Experimental analysis of worker division of labor in bumblebee nest thermoregulation (<Emphasis Type="Italic">Bombus huntii</Emphasis>, Hymenoptera: Apidae)

Bumblebee colonies experience daily and seasonal fluctuations in ambient temperature, but proper brood development requires a stable nest temperature. This study examined how adaptive colony responses to changing ambient temperature are achieved through the in-nest workers’ behavioral plasticity. We studied three Bombus huntii colonies in the laboratory. In the first experiment, we manipulated ambient temperature and recorded brood cell incubation and wing fanning by individually marked, known-age bees. The colonies maintained their nests closer to appropriate brood development temperatures (28 to 32°C) when exposed to a range of ambient temperatures from 10.3 to 38.6°C. Incubation activity was greater in cooler treatment conditions, whereas in the highest temperature treatment, some bees fanned and others moved off the brood. As the ambient temperature dropped, workers increased the duration of their incubating bouts, but, except at the highest temperature, the number of workers that incubated did not differ significantly among treatments. A subset of the bees incubated significantly more than their nest mates, some of which never incubated. Worker body size, but not age, was a good predictor of incubation rates, and smaller bees incubated at higher rates. In the second experiment, we removed the most actively incubating workers. Immediately after removals, the total colony incubation effort was lower than pre-removal levels, but incubation effort rebounded toward pre-removal levels after 24 h. The increased thermoregulatory demand after removals was met primarily by bees increasing their rates of incubation rather than by bees switching from a different task to incubation. We conclude that some B. huntii workers specialize on nest thermoregulation, and that changes in work rates are more important than task switching in meeting thermal challenges.     

The effect of queen-worker conflict on caste determination in the bumblebee Bombus terrestris

Endocrine analyses were used to investigate the well-known association between queen production and the onset of worker reproduction (termed the competition phase, CPh) in Bombus terrestris. Larvae that reached the age of 5 days before the CPh had a worker-like profile: low juvenile hormone (JH) biosynthesis rates and low JH hemolymph titers. In contrast, larvae that reached the age of 5 days during the CPh had a queen-like profile: high JH biosynthesis rates and high hemolymph JH levels. Larval fate could be manipulated by transplanting egg cells into host colonies with different social structures. There was a steep rise in JH production in larvae transplanted into colonies near or during the CPh. This indicates that during colony development, larvae switch from the ”worker developmental pathway” to the ”queen developmental pathway,” and that the switch is socially regulated. In small rearing groups, larvae reared with queens before the CPh developed into workers, whereas those reared with queens after the CPh developed into queens. Variation in worker type (naive or experienced) did not affect caste determination. Therefore, we hypothesize that queens produce a pheromone that directly inhibits queen differentiation by larvae. We also present two alternative scenarios that explain the timing of gyne production in B. terrestris, one based on ecological constraints and the other based on queen-worker competition. Received: 20 October 1999 / Received in revised form: 18 December 1999 / Accepted: 23 January 2000     

Fight or flight? A geographic mosaic in host reaction and potency of a chemical weapon in the social parasite <Emphasis Type="Italic">Harpagoxenus sublaevis</Emphasis>

Ant social parasites use chemical warfare to facilitate host colony takeover, which is a critical but recurring step in their life cycle. Many slave-making ants use the secretion of the Dufour gland to manipulate host behaviour during parasitic nest foundation and slave raids. Harpagoxenus sublaevis applies this chemical weapon onto defending Leptothorax host workers, which elicits deadly fights amongst them. Host species are expected to evolve counter-adaptations against this behavioural manipulation and in this study we investigated the geographic structure of this co-evolving trait. We compared the effectiveness of the parasitic gland secretion from different H. sublaevis populations in host colonies from various sites and analysed the occurrence of local adaptation. The two host species L. muscorum and L. acervorum generally showed different responses to the parasites’ chemical weapon: L. acervorum attacked nestmates treated with Dufour gland secretion, while L. muscorum workers fled. Flight, instead of intraspecific fights, is an adaptive host reaction as it results in fewer host fatalities during raids. Beside interspecific host differences, we found a geographic mosaic of host resistance: parasites from a German population strongly manipulated the behaviour of both sympatric Leptothorax populations. Russian or Italian hosts instead did not react with intracolonial aggression, but fled when confronted with the gland secretion of their sympatric parasite. Not only variation in host resistance explains differences in the effectiveness of the parasitic gland secretion but also interpopulational differences in its chemical composition, which were revealed by gas chromatography and mass spectrometry.     

Utilization of a new host in the screaming cowbird <Emphasis Type="Italic">Molothrus rufoaxillaris</Emphasis>, a host specialist brood parasite: host switch or host acquisition?

The screaming cowbird Molothrus rufoaxillaris has been long known as a host specialist brood parasite. However, in the past years, the utilization of two new hosts has been documented. We examined the variation in mitochondrial control region sequences from screaming cowbird chicks found in the nests of two hosts, the bay-winged cowbird (Agelaioides badius), which is its regular host, and the chopi blackbird (Gnorimopsar chopi), which is a new host, in Formosa Province, Argentina. If a group of females switched to this new host, we expected to find an association between host use and haplotype frequency distribution, indicating the presence of host-specific female lineages, whereas we expected no such association if the cowbird population incorporated this new host and females use both hosts simultaneously. Haplotype frequency distributions differed between cowbird chicks from the nests of both hosts. This indicates that nest choice by females of this brood parasite is not random and that they preferentially parasitize the nests of the same host species.     

Antennal response to fragrance compounds in male orchid bees

Summary. Male orchid bees (Euglossini) are attracted to floral and non-floral odours, which they collect and accumulate in hind tibial cavities for subsequent exposure during courtship. Fragrance preferences are species-specific, leading to relatively specialised pollination of euglossophilous plants. We tested the hypothesis that preferences for attractive compounds have led to species-specific sensory adaptations that are measurable by electroantennography (EAG). All of 16 synthetic fragrance compounds elicited significant responses on male bee antennae, with some difference of response spectra between individuals of Euglossa spp. and bumblebee (Bombus terrestris) controls, but no difference between three different species of Euglossa. There was no correspondence between a compounds’ attractiveness in baiting assays and the size of its electrophysiological response. Our results strengthen the view that fragrance preferences are largely mediated by processes in higher nervous centres. Peripheral sensory tuning to single attractive odorants may be constrained by the need to detect and discriminate between many fragrances, including many that have repellent effects on male bees.     

Kin discriminators in the eusocial sweat bee <Emphasis Type="Italic">Lasioglossum malachurum</Emphasis>: the reliability of cuticular and Dufour’s gland odours

The ability to discriminate degrees of relatedness may be expected to evolve if it allows unreciprocated altruism to be preferentially directed towards kin (Hamilton in J Theor Biol 7:1–16, 1964). We explored the possibility of kin recognition in the primitively eusocial halictid bee Lasioglossum malachurum by investigating the reliability of worker odour cues that can be perceived by workers to act as indicators of either nest membership or kinship. Cuticular and Dufour’s gland compounds varied significantly among colonies of L. malachurum, providing the potential for nestmate discrimination. A significant, though weak, negative correlation between chemical distance and genetic relatedness (r = −0.055, p < 0.001) suggests a genetic component to variation in cuticular bouquet, but odour cues were not informative enough to discriminate between different degrees of relatedness within nests. This pattern of variation was similar for Dufour’s gland bouquets. The presence of unrelated individuals within nests that are not chemically different from their nestmates suggests that the discrimination system of L. malachurum is prone to acceptance errors. Compounds produced by colony members are likely combined to generate a gestalt colony chemical signature such that all nestmates have a similar smell. The correlation between odour cues and nest membership was greater for perceived compounds than for non-perceived compounds, suggesting that variability in perceived compounds is a result of positive selection for nestmate recognition despite potentially stabilising selection to reduce variability in odour differences and thereby to reduce costs derived from excessive intracolony nepotistic behaviour.     

Colony take-over and brood survival in temporary social parasites of the ant genus Formica

Parasites reduce host fitness, and so instigate counter adaptations by their hosts. In temporary social parasitism, usurpers must not only enter the colony unharmed, but also have their eggs reared by the host workers. We introduced parasitic Formica lugubris and Formica aquilonia queens into queen right and orphaned fragments of three host species, Formica cinerea, Formica picea and Formica fusca, and show that workers of all three host species kill over 40 % of the introduced queens within 10 days, regardless of the presence/absence of a resident queen, and parasite species. More parasite queens died in F. cinerea than in F. picea and F. fusca. There were no major differences in survival between the parasite species (except that F. lugubris survived longer than F. aquilonia in F. fusca colonies compared to F. picea colonies), but parasite queens survived longer in orphaned than in queen right fragments of F. fusca. Experimental introduction of parasite (F. aquilonia) eggs into orphaned colonies of F. fusca showed that none of the parasite eggs were reared until pupation; whereas on average, 12 % of the con-specific hetero-colonial eggs introduced in the same manner were reared until pupation. In all colonies that received parasite brood, all offspring consisted of worker-laid males, whereas the corresponding value was 50 % for colonies that received con-specific hetero-colonial brood. Thus, when the risks of entering host colonies and brood failure are combined, the rate of successful colony take-over is very low. Moreover, the host workers can to some extent alleviate the costs of parasitism by producing a final batch of own offspring.     

Influence of dung volatiles on the process of resource selection by coprophagous beetles

Summary. Most dung beetles colonize the faeces of several vertebrate species without much discrimination, and are thus often considered as polyphagous. Recent studies have provided evidence for clear feeding preferences in scarab beetles colonizing dung of herbivore species, but little is known about these insects’ abilities to discriminate among odours from faeces of various herbivores. In this study, trophic preferences were examined using blocks of pitfall traps baited with dung from four different herbivore species, i.e., sheep, cattle, horse, and red deer, in a mountainous area of south-central France. 4941 coprophagous scarabs, belonging to 27 species, were captured. Beetles were more attracted to dung of sheep (2257 individuals) than that of cattle (1294 individuals), followed by deer dung (768 individuals) and horse dung (622 individuals). Eleven of the 27 beetle species collected had significant feeding preferences for one of the four dung types. For each insect species, trophic habits did not vary between the two different sites of trapping, an open pasture and a wooded habitat. In laboratory olfactometer bioassays, scarab beetles orientated preferentially towards the dung volatiles from the dung type they preferred in the field. Trypocopris pyrenaeus, Anoplotrupes stercorosus, and Aphodius rufipes were more attracted to volatile compounds from sheep dung, Onthophagus fracticornis significantly preferred horse dung volatiles, and Aphodius haemorrhoidalis responded positively to deer dung odours. The role of dung olfactory cues in the process of resource selection by dung beetles is discussed.     

Black spots and female association preferences in a sexual/asexual mating complex (Poecilia, Poeciliidae, Teleostei)

We investigated whether female association preferences for males are influenced by black spot disease (BSD), a parasite induced change of the host phenotype. We compared three different species of fish: a gynogenetic hybrid species, Poecilia formosa (amazon molly) and two sexual species (Poecilia latipinna and Poecilia mexicana), which were involved in the natural hybridisation leading to the amazon molly. Contrary to their sexual relatives, asexual amazon mollies significantly avoided images of males infected with black spot disease. We propose that amazon molly females have direct fitness benefits from choosing healthy males. The adaptive significance of the preference for BSD-uninfected males in the asexual amazon molly is yet unclear but may involve avoidance of predation or parasite infection as well as increased sperm availability.     

Genetic relatedness and eusociality: parasite-mediated selection on the genetic composition of groups

Summary Contrary to the expectations of kin selection theory, intracolony relatedness in eusocial insects is often low. We examined the idea that associations of low relatedness (high genetic variability) may be advantageous because of negative frequency-dependent selection on common host phenotypes by rapidly evolving parasites and pathogens. Using the natural host-parasite system of the bumble bee Bombus terrestris and its intestinal trypanosome Crithidia bombi, we investigated the transmission properties of parasites in host groups. Within naturally infested nests and in artificially constructed groups of workers, prevalence of infestation increased with time of exposure (Table 1). The susceptibility of isolated groups of workers to the parasites to which they were exposed differed with identity and natural infestation of their nest of origin (Table 2). In addition, those workers that were related to the individual introducing an infection to their group were more likely to become infested than were unrelated workers (Table 3). Although the bumble bee workers in experimental boxes appeared to differ in behavior toward kin and non-kin, making more physical contacts with kin, we found no discernible relationship between number of physical contacts and prevalence of infestation in a group. Therefore, we conclude that differences in parasite transmission reflected interactions among different host and parasite phenotypes. This system thus demonstrates the factors necessary for negative frequency-dependent selection by parasites on common host phenotypes - variability for susceptibility and infectiousness in host and parasite populations and similarity for these traits among related individuals. If, as we show here, high genetic relatedness within groups enhances parasite transmission, kin directed altruism may increase the risk of contracting parasites and infectious diseases. Therefore, parasites and pathogens may be an important force moderating the genetic structure of social groups. Offprint requests to: J.A. Shykoff at the present address