Biological constraints and colony founding in the polygynous invasive ant Lasius neglectus (Hymenoptera, Formicidae)

Summary. The polygynous invasive ant Lasius neglectus was described from Budapest, Hungary, as an unicolonial species, with no apparent colony barriers, and inferred intra-nidal mating without a nuptial flight. Here we analyze additional morphological characteristics of gynes, their physiological condition at emergence and at the time of mating and we describe the productivity of different types of colony founding in the laboratory. A low increase in dry weight and in fat content from emergence to mating indicates that gynes can only succeed in dependent colony founding. However, the queen-worker thorax volume ratio is typical of a species with independent colony founding and we were able to demonstrate independent colony founding in the laboratory (both haplo- and pleometrotic). Brood development in independent founding is rapid and the number of nanitics higher than in other Lasius species. Both colony budding and dependent colony founding could also be demonstrated in the laboratory. Worker oviposition was absent. The carbohydrate content of newly mated queens is consistent with the observed loss of mating flight of this species. However the relative wing area clearly indicates that L. neglectus queens should be able to fly. Therefore, both queen morphology and physiological state at maturity show that L. neglectus is intermediate between a monogynous, free-living, non pest ant and a multiqueened (polygynic) invasive tramp ant. This neglected ant thus offers interesting opportunities to study the origin of unicoloniality and the spread of this species in northeast Spain.     

Size and fat content of gynes in relation to the mode of colony founding in ants (Hymenoptera; Formicidae)

Summary In ants, there are two main processes of colony founding, the independent and the dependent modes. In the first case young queens start colony founding without the help of workers, whereas in the second case they are accompanied by workers. To determine the relation between the mode of colony founding and the physiology of queens, we collected mature gynes of 24 ant species. Mature gynes of species utilizing independent colony founding had a far higher relative fat content than gynes of species employing dependent colony founding. These fat reserves are stored during the period of maturation, i.e. between the time of emergence and mating, and serve as fuel during the time of colony founding to nurture the queen and the brood. Gynes of species founding independently but non claustrally were found to have a relative fat content intermediate between the values found for gynes founding independently and those founding dependently. This suggests that such gynes rely partially on their fat reserves and partially on the energy provided by prey they collect to nurture themselves and the first brood during the time of colony founding. Study of the fat content of mature gynes of all species has shown that it gives a good indication of the mode of colony founding.     

Colony founding and social parasitism in <Emphasis Type="Italic">Lasius</Emphasis> (<Emphasis Type="Italic">Acanthomyops)</Emphasis>

We investigated colony foundation behavior of three species of the Lasius claviger group, L. latipes, L. interjectus and L. claviger, using field observations and laboratory experiments. Laboratory studies included observing gyne behavior prior to interaction with host colonies (e.g. grouping, overwintering, feeding) and experiments in which gynes were introduced to putative host colonies. Observations of gyne parasitic behavior immediately after mating flights in the field and the discovery of mixed colonies both confirm the parasitic behavior of these species and point toward a wide host species range including two cases of hyperparasitism. L. interjectus and L. latipes enter the host colony immediately after the nuptial flight, while L. claviger may hibernate and enter in the spring. Laboratory experiments showed that L. interjectus gynes prefer to group and suffer lower mortality when in a group, which is consistent with field observations of L. interjectus gynes entering host colonies in large numbers. Details on behavior of L. latipes alpha and beta form gynes are also given.     

Diploid male production in a leaf‐cutting ant

1. In haplodiploid social insects where males are haploid and females are diploid, inbreeding depression is expressed as the production of diploid males when homozygosity at the sex‐determining locus results in the production of diploid individuals with a male phenotype. Diploid males are often assumed to have reduced fitness compared with their haploid brothers. 2. While studying the reproductive biology of a leaf‐cutting ant, Atta sexdens, in Gamboa, Republic of Panama, we detected the presence of a larger male morph. Using microsatellite markers we were able to confirm that the large male morph was diploid in 87% of cases. 3. We infer that the Gamboa population of A. sexdens experiences inbreeding depression because diploid males were found in three out of five mature colonies. However, their frequencies were relatively low because queens were multiply mated and our estimates suggest that many diploid male larvae may not survive to adulthood. 4. We measured two traits potentially linked to male reproductive success: sperm length and sperm number, and showed that diploid males produced fewer but longer sperm. These results provide indirect evidence that diploid male reproductive success would be reduced compared with haploid males if they were able to copulate. 5. We conclude that diploid male production is likely to affect the fitness of A. sexdens queens with a matched mating, as these males are produced at the cost of workers and, if the colony survives to reach mature size, also gynes.     

Mating frequency in dispersing potato tuber moth, Phthorimaea operculella, and its influence on the design of refugia to manage resistance in Bt transgenic crops

Mating of potato tuber moth, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae), was investigated in relation to the dispersal of males in laboratory and field trials. The effect of stimulating the flight of males to light sources in a large cage on their mating ability was estimated for three age groups, and compared with similar estimates for confined moths. Although the mating of males declined with ages of up to 15 days, simulated dispersal had no effect on subsequent mating when the males were paired with virgin females. The dispersal of male moths was also categorised by the initial flight activity of untethered moths to a light source. Scores for poor, moderate, and good flight provided a repeatable measure of initial male flight activity, but the degree of activity was not related to their subsequent mating ability. In the field, virgin female potato tuber moths were tethered at various distances from the edge of isolated potato crops and then dissected to determine their mating status. Female mating frequency averaged 75% at the crop margin, remained above 50% up to 200 m, and then declined to 19% at 360 m from the margin. Derivation of the mating probability for an individual male potato tuber moth confirmed earlier work by other researchers that has indicated a tendency for dispersal prior to mating, and that males retain their ability to mate as they disperse from a crop. The influence of dispersal and mating on gene flow between crops, and its potential effects on refuge size required to minimise the development of resistance to Bt transgenic potato crops was examined.     

Sex-ratio dependent execution of queens in polygynous colonies of the ant Formica exsecta

Formica exsecta has become an important model system for studying intraspecific variation in sex ratios. Patterns of sex allocation in polygynous (multiple queen per nest) populations of F. exsecta are generally consistent with the queen-replenishment hypothesis. This hypothesis states that colonies produce gynes (reproductive females) in order to increase queen number and enhance colony survival and/or productivity when the number of resident queens is low. However, the small proportion of colonies that raise gynes produce more than necessary for simple queen replenishment. It has been hypothesized that excess production of gynes may occur to reduce the frequency of accepting foreign unrelated gynes into the colony when workers cannot distinguish nestmate from non-nestmate queens. This explanation for excess gynes requires weak or no aggression between non-nestmates and is expected to lead to the selective execution of new queens by colonies that do not invest in the production of gynes. Experimental studies where gynes were introduced into natal and foreign colonies indeed suggested that polygynous populations of F. exsecta have a poor nestmate recognition system. Although gynes were significantly more likely to be accepted in their parental colony compared to another foreign female-producing colony, the difference was small. Moreover, encounters between workers from different colonies within the population showed very little aggression and were no more aggressive than encounters between nestmates, again suggesting a weak capacity for nestmate recognition. Our experiment also showed that colonies that produced only males executed most of the gynes that were experimentally introduced into the colony, whereas female-producing colonies accepted most gynes. This is consistent with ants using a simple rule of thumb to decrease parasitism by unrelated queens, whereby colonies selectively destroy gynes whenever gynes are not produced in the colonies.     

Natural patterns of caste and sex allocation in the stingless bees <Emphasis Type="Italic">Melipona favosa</Emphasis> and <Emphasis Type="Italic">M. trinitatis</Emphasis> related to worker behaviour

Summary. This paper is the report of a long-duration study of the production of gynes and males in Melipona stingless bees in Trinidad and Tobago, including a behavioural study of laying workers. In 167 brood samples (16342 pupae) of M. favosa, taken from 78 colonies in the natural environment over the years 1993-2001, workers represented 78.4% and males represented 17.3%. Gynes represented 5.1% of all female brood. Gyne proportion never exceeded 22.5%. Gynes were found to be common throughout the year. Like the gynes, males were present all year, but their occurrence varied over the months. Males were most common in the months of July and August. At the colony level male occurrence in the brood was characterized by very high values (maximum 74.2%), next to a frequent complete absence (in 26.5% of all samples). The finding of bout-like production of males at the colony level is supported by the observations on reproductive worker behaviour. The colonies appeared to be in different phases of male production. When male producing laying workers were present, they competed heavily for egg-laying opportunities. Various behaviours of laying workers and the queen are described and interpreted on the basis of reproductive competition and kin conflict theories. From these results, we conclude that in M. favosa, workers have a distinct influence on reproduction, including the production of gynes and males.     

Honey Bee Queens Do Not Count Mates to Assess their Mating Success

The mating system of honey bees (genus Apis) is extremely polyandrous, where reproductive females (queens) typically mate with 12 or more males (drones) during their mating flight(s). The evolutionary implications for hyperpolyandry have been subject to considerable debate and empirical testing because of the need to understand the proximate mechanisms that drive such extreme mating behavior despite the potential costs. The ability of queens to gauge and adjust their reproductive success is therefore important for selection to act on queen mating number at both the evolutionary (colony-level) and proximate (individual-level) timescales. We observed the mating flight activities of 80 queens in their respective mating nucleus hives each with a modified entrance that restricts flight attempts. We also attached a small weight (0, 16, or 38 mg) onto each queen’s thorax as a means of imposing additional flight costs. We then compared queens that were restricted from taking multiple mating flights to those that started oviposition after a single flight for their mating numbers as quantified by microsatellite analyses of their respective worker offspring. We found that neither additional weight nor restricted mating attempts had any significant effect on the effective mating frequencies of the experimental queens during their single mating flight. This observation suggests that queens are not adjusting their nuptial flight activity according to their precise mating number during their flight. These findings provide insights into the proximate regulation of honey bee queen mating behavior and the fitness consequences of hyperpolyandry at the colony level.     

Inbreeding and reproductive investment in the ant Formica exsecta

In social animals, inbreeding depression may manifest by compromising care or resources individuals receive from inbred group members. We studied the effects of worker inbreeding on colony productivity and investment in the ant Formica exsecta. The production of biomass decreased with increasing inbreeding, as did biomass produced per worker. Inbred colonies produced fewer gynes (unmated reproductive females), whereas the numbers of males remained unchanged. As a result, sex ratios showed increased male bias, and the fraction of workers increased among the diploid brood. Males raised in inbred colonies were smaller, whereas the weight of gynes remained unchanged. The results probably reflect a trade-off between number and quality of offspring, which is expected if the reproductive success of gynes is more dependent on their weight or condition than it is for males. As males are haploid (with the exception of abnormal diploid males produced in very low frequencies in this population), and therefore cannot be inbred themselves, the effect on their size must be mediated through the workers of the colony. We suggest the effects are caused by the inbred workers being less proficient in feeding the growing larvae. This represents a new kind of social inbreeding depression that may affect sex ratios as well as caste fate in social insects.     

Complex sociogenetic organization and reproductive skew in a primitively eusocial sweat bee,Lasioglossum malachurum,as revealed by microsatellites

The sweat bees (Family Halictidae) are a socially diverse taxon in which eusociality has arisen independently numerous times. The obligate, primitively eusocial Lasioglossum malachurum, distributed widely throughout Europe, has been considered the zenith of sociality within halictids. A single queen heads a colony of smaller daughter workers which, by mid-summer, produce new sexuals (males and gynes), of which only the mated gynes overwinter to found new colonies the following spring. We excavated successfully 18 nests during the worker- and gyne-producing phases of the colony cycle and analysed each nest's queen and either all workers or all gynes using highly variable microsatellite loci developed specifically for this species. Three important points arise from our analyses. First, queens are facultatively polyandrous (queen effective mating frequency: range 1-3, harmonic mean 1.13). Second, queens may head colonies containing unrelated individuals (n = 6 of 18 nests), most probably a consequence of colony usurpation during the early phase of the colony cycle before worker emergence. Third, nonqueen's workers may, but the queen's own workers do not, lay fertilized eggs in the presence of the queen that successfully develop into gynes, in agreement with so-called 'concession' models of reproductive skew.     

Life history traits of a European Messor harvester ant

We present life history traits of a Central European harvester ant, Messor cf. structor, determined by gyne and male dissections, behavioural assays, standardised brood photography and laboratory rearing of brood. Messor cf. structor is polygynous and builds up unicolonial populations. Sexuals develop from hibernated larvae in a univoltine cycle and become adult from late summer to late autumn. Neither intra- nor extranidal mating was observed in autumn, even though gynes and males were mature. In spring, after hibernation, intranidal mating without swarming flight took place, even though the flight muscles of alate sexuals were still fully developed. Received 3 March 2005; revised 4 May 2005; accepted 9 May 2005.     

Multiple mating and offspring quality in <Emphasis Type="Italic">Lasius</Emphasis> ants

Genetic diversity benefits for social insect colonies headed by polyandrous queens have received intense attention, whereas sexual selection remains little explored. Yet mates of the same queen may engage in sperm competition over the siring of offspring, and this could confer benefits on queens if the most successful sire in each colony (the majority sire) produces gynes (daughter queens) of higher quality. These benefits could be increased if high-quality sires make queens increase the percentage of eggs that they fertilize (unfertilized eggs develop into sons in social hymenopterans), or if daughters of better genetic quality are over-represented in the gyne versus worker class. Such effects would lead to female-biased sex ratios in colonies with high-quality majority gynes. I tested these ideas in field colonies of Lasius niger black garden ants, using body mass of gynes as a fitness trait as it is known to correlate with future fecundity. Also, I established the paternity of gynes through microsatellite DNA offspring analyses. Majority sires did not always produce heavier gynes in L. niger, but whenever they did do so colonies produced more females, numerically and in terms of the energetic investment in female versus male production. Better quality sires may be able to induce queens to fertilize more eggs or so-called caste shunting may occur wherever the daughters of better males are preferentially shunted to into the gyne caste. My study supports that integrating sexual selection and social evolutionary studies may bring a deeper understanding of mating system evolution in social insects.     

Social Control over the Survival and Selection of Winged Virgin Queens in an Ant without Nuptial Flight: Iridomyrmex humilis

Laboratory and field experiments show that the number of winged virgin queens of the Argentine ant Iridomyrmex humilis allowed to remain alive in the nest is under social control. When reared in their natal nest, survival of virgin queens depends on the presence of males. In the presence of mature alate males or even male pupae only 20% of the gynes are executed before they are 4 days old. By contrast, if pupal and adult males are absent, about 55% of the gynes are executed. The workers tend to kill the lightest ones. Although the Argentine ant is a unicolonial species exhibiting complete acceptance of foreign conspecific workers and mated queens, regardless of whether the recipient nest is queenright or queenless, the acceptance of gynes is variable: they are executed by foreign queenless workers but widely accepted in queenright nests. These results are discussed with regard to possible existence of queen and gyne pheromones allowing recognition and to the lack of nuptial flight in this species.     

Field observations of Ostrinia nubilalis eclosion and post‐eclosion activity of females around their natal plants

Abstract The early part of the post‐eclosion, pre‐mating period were examined under field conditions for Ostrinia nubilalis. Post‐eclosion behavior of 25 and 21 females during the first and second flight periods were observed until they left their natal site. Summer generation larvae were reared under field conditions and the timing of adult eclosion was observed. Eclosion occurred at two times during the day, peaking before dawn and before dusk; 46% of females and 56% of males eclosed during the morning period and the rest eclosed during the evening period. After eclosion, females spent 30–60 min expanding their wings. Their typical behavior was to remain calmly on their natal site. None of the females exhibited calling behavior before leaving. All females left their natal sites sometime before dawn. The probability of leaving increased with time. Leaving rates were not significantly different between females of the first and second flight. These field observations indicate that females have several possibilities for pre‐mating movement, which might allow females to move out from their natal field before mating. In addition, we also discuss the influence of pre‐mating movements in relation to the rate of Bacillus thuringiensis (Bt) resistance evolution.     

Multiple functions of fire ant Solenopsis invicta mandibular gland products

Alarm pheromones of social insects are best known for their role in the defence and maintenance of colony integrity. Previous studies with the fire ant Solenopsis invicta Buren (Hymenoptera: Formicidae) demonstrate that the mandibular glands of workers (sterile females) and male and female sexuals produce an alarm pheromone, 2‐ethyl‐3,6‐dimethylpyrazine. The function of alarm pheromones in worker ants is well understood and divergent from the function of these compounds in the winged sexual forms. The present study quantifies the amount of pyrazine in the mandibular glands from male and female alate sexuals, as well as queens. Pyrazine production in female alates starts in the late pupal stage and increases until they reach mating flight‐ready maturity; however, after mating flight participation, the pyrazine level declines by >50%. Interestingly, mature male alates lose >85% of their mandibular gland pyrazine during mating flight activity. The results of the present study indicate that male and female sexuals use mandibular gland secretions for mating flight initiation and during mating flights. Furthermore, the ontogeny of mandibular gland products (pyrazine as the marker) from newly‐mated queens to mature colony queens shows a more than two‐fold increase in the amount of pyrazine by 6 months after mating. However, this is followed by a decline to trace amounts in mature colony queens (>2 years old), suggesting a function for mandibular gland products during colony development. Multifunctional use of social insect pheromones is well documented and data are reported in the present study suggesting new roles for mandibular gland products in fire ants.     

Female-dependent stenogamy in the mosquito, Aedes taeniorhynchus

A female-dependent stenogamous condition was found in the mosquito, Aedes taeniorhynchus. When confined in cages, males attempted with similar frequencies to mate with wild and colony females. Wild females remained uninseminated because they actively prevented firm genital union. During simulated sunsets, swarming flights of colony males began at higher light intensities than those of wild males, and throughout these sunsets many more colony males participated in swarming than did wild males. Nevertheless, wild and colony males did not differ in their mating effectiveness. Both kinds of males were highly successful in transferring sperm to colony females, yet most of their attempted matings with wild females ended in failure. Autogeny occurred in stenogamic colony females and in eurygamic wild females.     

Mating in a viscous universe: the race is to the agile, not to the swift

Charles Darwin's theory of natural selection has at its focal point the mating success of organisms. Among male animals, large body size is widely seen as the principal determinant of mating success. However, where mating takes place in a three-dimensional arena such as water, the arboreal habitat or air, small size with its concomitant aerobatic advantages might be advantageous. Despite considerable interest, the relationship between aerobatic ability and mating success has not yet been demonstrated in a single animal species. Here, we test the hypothesis that the known mating success of small male midges is due to their greater aerobatic ability. To do this, male midges collected from leks in the wild were flown and their flight paths in free flight were recorded on high-speed cameras in the laboratory. Four flight parameters that would seem relevant to male mate acquisition in flight, i.e. acceleration, maximum speed, tortuosity and turn-rate, were analysed with respect to body size. We show that, while in terms of maximum speed there was no detectable difference between small and large males, small males outperformed larger ones with respect to acceleration, tortuosity and turn-rate. We conclude that the hypothesis that small males gain their mating advantage through aerobatic superiority is consistent with the observations reported here.     

Competition and opportunity shape the reproductive tactics of males in the ant Cardiocondyla obscurior

Context-dependent adjustment of mating tactics can drastically increase the mating success of behaviourally flexible animals. We used the ant Cardiocondyla obscurior as a model system to study adaptive adjustment of male mating tactics. This species shows a male diphenism of wingless fighter males and peaceful winged males. Whereas the wingless males stay and exclusively mate in the maternal colony, the mating behaviour of winged males is plastic. They copulate with female sexuals in their natal nests early in life but later disperse in search for sexuals outside. In this study, we observed the nest-leaving behaviour of winged males under different conditions and found that they adaptively adjust the timing of their dispersal to the availability of mating partners, as well as the presence, and even the type of competitors in their natal nests. In colonies with virgin female queens winged males stayed longest when they were the only male in the nest. They left earlier when mating partners were not available or when other males were present. In the presence of wingless, locally mating fighter males, winged males dispersed earlier than in the presence of docile, winged competitors. This suggests that C. obscurior males are capable of estimating their local breeding chances and adaptively adjust their dispersal behaviour in both an opportunistic and a risk-sensitive way, thus showing hitherto unknown behavioural plasticity in social insect males.     

Relationships between phenotype, mating behavior, and fitness of queens in the ant Lasius niger

Considerable attention has focused on why females of many species mate with several males. For social hymenopteran insects, efforts have primarily concentrated on determining whether multiple mating increases colony performance due to the increased genetic diversity. Most of these studies are correlative because it is difficult or impossible to experimentally mate queens in most species. Thus, the positive associations found between multiple paternity and colony fitness in some cases may not be due to direct effects of genetic diversity but could, in theory, arise from high-quality queens having more mates. Here we show that in the ant Lasius niger variation in the number of matings covaries with queen phenotype. Young queens that were heavier at the time of the mating flight were significantly more likely to mate with several males. As a result, heavier queens stored more sperm. The initial weight of queens was significantly associated with the probability of surviving mating flights during the two years of the study, with queens of intermediate weight having the highest across-year survival. Queen initial weight was also significantly and positively associated with the quantity of brood at the time of the first worker eclosion as well as colony productivity at the time of hibernation. By contrast, there was little evidence for a positive effect of the number of matings on colony performance when the effect of mate number and queen initial weight were considered simultaneously.     

A nutritional profile of the social wasp Polistes metricus: Differences in nutrient levels between castes and changes within castes during the annual life cycle

In wasps, nutrition plays a vital role for colony cohesion and caste determination. However, there is no baseline data set for the nutritional levels of wasps during the different stages of the colony cycle. Here we examined the levels of carbohydrates, lipids, protein, Ca, Cu, Fe, K, Mg, Mn, Na, and Zn in the wasp Polistes metricus at different stages of the wasp's lifecycle. Individuals were collected at the following stages (1) spring gynes, (2) foundress colonies, (3) early worker colonies, (4) late worker colonies, (5) emerging reproductives (gynes and males), (6) early fall reproductives, and (7) late fall reproductives. All eggs, larvae, pupae and adults were analyzed for their nutritional content to determine if there were any differences between the nutrient levels in the different castes and how these nutrients changed within a caste during its lifetime. The results show there are differences in macro and micronutrient levels between the reproductive females and workers during development. Gynes showed changes in nutrient levels during their lifetime especially as they changed roles from a solitary individual to a nesting queen. Males also showed distinct nutritional changes during their lifetime. The implications for these nutritional differences are discussed.