Insectes Sociaux

Baer, B.; Schmid-Hempel, P.; Høeg, J. T.; Boomsma, J. J.

doi: 10.1007/s00040-003-0641-0pmid: N/A

Multiple insemination induces sperm competition and may select for longer, faster moving sperm in species where sperm is short-lived and egg fertilization takes place almost immediately after ejaculation. Here we report the first detailed analysis of sperm length in social insects with long-term storage of sperm, using three bumblebee species with different mating systems as models. We show that individual males produce only one size-class of sperm, but that sperm length is highly variable among brothers, among unrelated conspecific males, and among males of different species. Males of Bombus hypnorum, a species with multiple-mating queens, have longer sperm than males of B. terrestris and B. lucorum whose queens are single mated. Although the sample size on the species level was too small to perform a phylogenetic analysis, this finding supports the hypothesis that, all other things being equal, multiple mating may select for longer sperm. Sperm length was positively correlated with male body size in B. terrestris and possibly in B. hypnorum, but not in B. lucorum. The variance of sperm length within single B. terrestris males before mating was consistently higher than the variance of ejaculated sperm that was stored in a queen's spermatheca. Both longer sperm and shorter sperm could be preferentially stored, depending on the colony in which the males and queens were born and raised. These results indicate that the genotype of males may affect sperm length and that cryptic female choice of sperm length may have a genetic component as well.

Neumann, P.; Pirk, C. W. W.; Hepburn, H. R.; Moritz, R. F. A.

doi: 10.1007/s00040-003-0610-7pmid: N/A

Cape honeybee laying workers (Apis mellifera capensis) produce female diploid offspring and are facultative social parasites. In queenright host colonies, such workers have to evade worker policing (removal of worker-laid eggs by other workers) to successfully reproduce. One mechanism seems to be low removal rates of eggs laid by parasitic workers. However, because queenright colonies of other subspecies (e.g. the neighbouring A. m. scutellata) are susceptible hosts, social parasitic workers probably also use behavioural tactics to evade policing. Indeed, field observations of infested A. m. scutellata host colonies indicate that brood from parasitic workers initially appears away from the queen. The egg removal rates for queen and worker-laid eggs were recorded in the top and bottom boxes of three queenright A. m. capensis and A. m. scutellata colonies, where the queens were caged in the bottom boxes. The egg removal data show that both subspecies are able to police worker-laid eggs, because more queen-laid eggs remained than worker-laid eggs in the bottom boxes. However, fewer A. m. capensis worker-laid eggs remained in the bottom boxes than in the top boxes. Moreover, whereas A. m. capensis also polices in the top boxes, no significant differences between the removal rates of worker and queen laid eggs were found in the top boxes of the A. m. scutellata colonies. This indicates that worker policing is not always fully effective in A. m. scutellata and may explain why this subspecies is so susceptible to infestations. Our results also show that worker policing is less likely away from the queen. Therefore, queen evasion by laying social parasitic A. m. capensis workers appears to constitute a behavioural tactic to achieve successful reproduction in queenright host colonies.

Ortius-Lechner, D.; Maile, R.; Morgan, E. D.; Petersen, H. C.; Boomsma, J. J.

doi: 10.1007/s00040-003-0640-1pmid: N/A

Summary. Multiple queen-mating (polyandry) in social insects increases the genetic variability among worker offspring, which may enhance colony survival, social productivity and defence against parasites. The unique and complex symbiosis of leaf-cutting ants with a clonal mutualistic fungus makes this social system particularly vulnerable to contamination by pathogenic and unwanted saprophytic fungi and bacteria. Proper defence against such threats requires effective and flexible chemical defence mechanisms. A prime candidate for providing such defences is the metapleural gland secretion, which is known to have broad antibiotic properties. Here we use the leaf-cutting ant Acromyrmex octospinosus to specifically test the hypothesis that genetically more diverse worker-offspring produce a more variable spectrum of metapleural gland compounds. We used DNA microsatellite markers to assign workers from two colonies to the six most common patrilines in each colony, and have analysed the degree to which the observed variance in the quantitative chemical composition of the metapleural gland secretion can be explained by genetic differences among patrilines. We found a marginally significant patriline-effect on the overall variability of metapleural gland compounds in one colony, but could not detect such effect in the other colony. We discuss a number of possible reasons why the genetic variance component for quantitative variation in metapleural gland secretion may be low.

Azcárate, F. M.; Peco, B.

doi: 10.1007/s00040-003-0635-ypmid: N/A

Seed predation by harvester ants is one of the main processes involved in the seed bank dynamics of Mediterranean grassland and scrubland. We investigated the spatial patterns of seed predation by Messor spp. in these systems from a multi-level approach (nest-site selection, foraging patch selection and seed predation risk), using ten experimental plots (20 m × 40 m) set in central Spain. All habitats in grasslands are dominated by M. barbarus, while in scrubland this species shares seed exploitation with M. bouvieri. In this type of system, the nest-hole and foraging territory distribution of the two species show a certain degree of segregation. Messor bouvieri focuses on the exploitation of dry scrub, while M. barbarus also forages on subhumid and humid patches. We interpret this as a consequence of the different foraging behaviour and nest site requirements of the two species. In general, the spatial distribution of seed predation risk reflects the patterns detected for the nest-holes and foraging areas, although scrubland has a more heterogeneous predation pattern than grassland.

Pereboom, J. J. M.; Velthuis, H. H. W.; Duchateau, M. J.

doi: 10.1007/s00040-003-0639-7pmid: N/A

In pollen-storing bumblebees, the rate at which workers nourish larvae has been proposed to be the main factor influencing caste differentiation since workers feed prospective queens more frequently and longer than worker larvae during the last instars. In order to determine how the frequency of feedings is established small groups of Bombus terrestris larvae were either subjected to starvation or nourished regularly by workers. Experimentally starved larvae were fed significantly earlier and more often than control larvae. Behavioural observations provide evidence for the existence of a stimulus of larval origin that releases feeding behaviour in workers. Intentional inspections in the sense of a recognisable and functional behaviour intended to organise the feeding process were never observed. We argue that worker inspections are not required for the adequate maintenance of larvae and that a simple stimulus-response chain appears to be sufficient to regulate feeding behaviour at the individual and the colony level in bumblebees. Furthermore, hand-rearing experiments with female larvae indicate that queen rearing is not dependent on a high frequency of feedings in itself, nor is worker development induced by larval deprivation. This makes it unlikely that workers impose a caste-specific frequency of feedings on larvae in order to actively control or manipulate caste development. Since caste fate seems to be predetermined early in larval life, we propose that early caste-specific differences in development are reflected in the physiology of the larvae and the associated nutritional needs to which workers respond accordingly. Consequently, caste-specific differences in feeding frequencies are a result, but not the cause of differences in development.

Seeley, T. D.; Mikheyev, A. S.

doi: 10.1007/s00040-003-0638-8pmid: N/A

One way to understand the adaptive design of the life history of a social insect colony is to view the colony as having an investment policy whereby it allocates limited resources among the various physiological functions fostering its growth, survival, and reproduction. Prior work has shown that energy is a limited resource for a honey bee colony and that a colony faces a strong trade-off between energy investment in current reproduction and future survival. Given these facts, we hypothesized that a colony might have a flexible energy investment policy, whereby it adjusts its investment in reproduction in relation to its success in acquiring energy. To test this hypothesis, we manipulated the energy acquisition of colonies and looked for an effect on their reproductive efforts. We found that when a colony experiences difficulty building the energy reserve it needs to survive to the next reproductive season, it trims its allocation of energy to drone production and possibly also to drone maintenance. The mechanisms of social physiology which enable a colony to adjust its investment in male reproductives in relation to its energy budget remain a mystery.

Berghoff, S. M.; Gadau, J.; Winter, T.; Linsenmair, K. E.; Maschwitz, U.

doi: 10.1007/s00040-003-0642-zpmid: N/A

Army ants, although known as fierce predators, are rarely reported to prey on other army ants and most observed interspecific contacts were resolved comparatively peacefully. Of the four Asian Dorylinae, Dorylus (Dichthadia) laevigatus was believed to be the only species occurring on Borneo, sharing its habitat only with Aenictus and Leptogenys army ants and mass raiding Pheidologeton species. Using palm oil baits, we monitored the hypogaeic movements of D. laevigatus in Sabah (Malaysia, Borneo) and found a second species, i.e. D. cf. vishnui, also recruiting to the baits.¶To characterize the new species and differentiate it from D. laevigatus, we compared nine morphological traits. Hind tibia length, petiole shape, and number of antennal segments showed to differ significantly. Both species foraged predominately hypogaeicly and were found in similar habitats and altitudes. However, differences were found in bait utilization and foraging strategy. Furthermore, D. cf. vishnui seemed more likely to come to the soil surface than D. laevigatus. A phylogenetic analysis based on 385 basepairs of mtDNA confirmed the distinctiveness of both species. Interestingly, D. laevigatus showed considerable intraspecific differences between subpopulations from Java, West Malaysia, and even within Borneo, which might indicate that D. laevigatus is a species complex containing multiple cryptic species. A single male, collected from a light trap in Sabah, could be assigned to D. laevigatus using the same phylogeny, confirming this previously only assumed association.¶Encounters between the two species were observed at 11 baits, leading in ten cases to fierce fights. We provide the first detailed description of such interspecific fights, in which major workers were the fighters in both species. Experiments with intra- and interspecific mixing of workers in the laboratory showed D. laevigatus to have a higher aggressive level towards D. cf. vishnui than towards foreign conspecific colonies. No intraspecific fights between neighboring D. laevigatus colonies were observed in the field.

Hughes, D. P.; Moya-Raygoza, G.; Kathirithamby, J.

doi: 10.1007/s00040-003-0637-9pmid: N/A

We present the first record of parasitism of Dolichoderus bispinosus nests by Strepsiptera belonging to the family Myrmecolacidae. This becomes only the fourteenth species of ant and the fifth subfamily to be identified as a host to Strepsiptera. Of the three colonies examined all were parasitized. Prevalence of parasitism among adult ants was less than 2% in each case. However, among alate males of one colony, nearly 24% were parasitized. In conjunction with a reanalysis of previously published data we discuss the possibility that ant castes are differentially parasitized by Strepsiptera. We review the natural history of strepsipteran parasitism in ants, effects on host behaviour and incidences of parasitism in the hope of enabling detection of this parasite by myrmecologists.

Roulston, T. H.; Buczkowski, G.; Silverman, J.

doi: 10.1007/s00040-003-0624-1pmid: N/A

Aggression assays are commonly used to study nestmate recognition in social insects. Methods range from detailed behavioral observations on small numbers of insects to counts of individuals fighting in group interactions. These assays vary in the equipment used and the intensity and duration of observations. We used the Argentine ant, Linepithema humile, to compare four aggression bioassays for consistency between replicates, similarity between assays, and ability to predict whole colony interactions. The assays included were 1 live – 1 dead ant interactions, live 1-1 battles, live 5-5 battles, and 1 ant introduced to a foreign colony. We tested six ant colonies in all pairwise combinations using four different assays and two to three scoring methods per assay. We also conducted a colony merging experiment to see which assays were capable of predicting this ecologically important event. We found that scoring methods within assays yielded very similar results, giving us no reason to favor observationally intense procedures, such as continuous scanning, over less observationally intense systems, such as snapshot surveys. Assays differed greatly in their consistency between replicates. No two replicates of the 1 live – 1 dead assay were significantly correlated. The live 5-5 and the colony introduction assays were the most consistent across replicates. The mean scores of the live 1-1, live 5-5 and colony introduction assays were all significantly correlated with each other; only the live 5-5 assay was significantly correlated with the 1 live – 1 dead assay. Assays that utilized the greatest number of live ants were the most likely to reveal high levels of aggression. The aggression scores of all but the 1 live – 1 dead assay were positively correlated with the number of ants that died during whole colony encounters and negatively associated with colony merging. We conclude that all live ant assays tested are useful tools for analyzing aggressive interactions between colonies, but that the pairing of a live and dead ant produced inconsistent results and generally lower levels of aggression. We found relatively low consistency between trials using the live 1-1 assay, but found that with sufficient replication its results were highly correlated with the assays using more interacting ants. We suggest that isolated aggressive acts in assays do not necessarily predict whole colony interactions : some colonies that fought in bioassays merged when the entire colonies were allowed to interact.

Visscher, P. K.; Vetter, R. S.

doi: 10.1007/s00040-003-0636-xpmid: N/A

This study compares life history data from northern and southern sites within the native range of Vespula pensylvanica: new data on nests and their contents from California and previously published data from Washington and Oregon. Annual nests in southern California have more combs and cells than Washington nests and continue to rear workers later into the season. Many nests in winter in California still contain workers, brood, and gynes, which has not been reported from other areas. Some V. pensylvanica nests become polygynous and persist for more than one season. This occurs mostly in California in its native range and in other warm-winter regions where it has been introduced. The polygynous California nests reported here are similar to previous reports from Hawaii. We discuss how overwintering differences in the warm-winter regions may contribute to polygyny.