Reproductive competition in queenless honey bee colonies (Apis mellifera L.)

Previously we reported that there are subfamily differences in drone production in queenless honey bee colonies, but these biases are not always explained by subfamily differences in oviposition behavior. Here we determine whether these puzzling results are best explained by either inadequate sampling of the laying worker population or reproductive conflict among workers resulting in differential treatment of eggs and larvae. Using colonies composed of workers from electrophoretically distinct subfamilies, we collected samples of adult bees engaged in the following behavior: true egg laying, false egg laying, indeterminate egg laying, egg cannibalism, or nursing (contact with larvae). We also collected samples of drone brood at four different ages: 0 to 2.5-h-old eggs, 0 to 24-h-old eggs, 3 to 8-day-old larvae, and 9 to 14-day-old larvae and pupae. Allozyme analyses revealed significant subfamily differences in the likelihood of exhibiting egg laying, egg cannibalism, and nursing behavior, as well as significant subfamily differences in drone production. There were no subfamily differences among the different types of laying workers collected from each colony, suggesting that discrepancies between subfamily biases in egg-laying behavior and drone production are not due to inadequate sampling of the laying worker population. Subfamily biases in drone brood production within a colony changed significantly with brood age. Laying workers had significantly more developed ovaries than either egg cannibals or nurses, establishing a physiological correlate for the observed behavioral genetic differences. These results suggest there is reproductive conflict among subfamilies and individuals within queenless colonies of honey bees. The implications of these results for the evolution of reproductive conflict, in both queenright and queenless contexts, are discussed.     

Brood pheromone stimulates pollen foraging in honey bees (Apis mellifera)

Foraging and the mechanisms that regulate the quantity of food collected are important evolutionary and ecological attributes for all organisms. The decision to collect pollen by honey bee foragers depends on the number of larvae (brood), amount of stored pollen in the colony, as well as forager genotype and available resources in the environment. Here we describe how brood pheromone (whole hexane extracts of larvae) influenced honey bee pollen foraging and test the predictions of two foraging-regulation hypotheses: the indirect or brood-food mechanism and the direct mechanism of pollen-foraging regulation. Hexane extracts of larvae containing brood pheromone stimulated pollen foraging. Colonies were provided with extracts of 1000 larvae (brood pheromone), 1000 larvae (brood), or no brood or pheromone. Colonies with brood pheromone and brood had similar numbers of pollen foragers, while those colonies without brood or pheromone had significantly fewer pollen foragers. The number of pollen foragers increased more than 2.5-fold when colonies were provided with extracts of 2000 larvae as a supplement to the 1000 larvae they already had. Within 1 h of presenting colonies with brood pheromone, pollen foragers responded to the stimulus. The results from this study demonstrate some important aspects of pollen foraging in honey bee colonies: (1) pollen foragers appear to be directly affected by brood pheromone, (2) pollen foraging can be stimulated with brood pheromone in colonies provided with pollen but no larvae, and (3) pollen forager numbers increase with brood pheromone as a supplement to brood without increasing the number of larvae in the colony. These results support the direct-stimulus hypothesis for pollen foraging and do not support the indirect-inhibitor, brood-food hypothesis for pollen-foraging regulation. Received: 5 March 1998 / Accepted after revision: 29 August 1998     

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     

Effects of foundress number on brood raids and queen survival in the fire ant Solenopsis invicta

In several ant species, colonies are founded by small groups of queens (pleometrosis), which coexist until the first workers eclose, after which all but one queen is killed. It has been hypothesized that, by producing a larger cohort of workers, cooperating queens may increase colony success during brood raids, a form of competition in which brood and workers from losing nests are absorbed into winning colonies. To test whether this benefit is sufficient to favor pleometrosis, newly mated queens of the fire ant Solenopsis invicta were assembled in groups of one, two, three, or four, reared in the laboratory until the first workers eclosed, then planted in the field in replicated assemblages. The proportion of colonies engaging in brood raids increased with average foundress number per nest and with colony density but was unaffected by variance in foundress number among interacting colonies. Within mixed assemblages of single-queen and multiple-queen colonies, queen number had no effect on the likelihood of engaging in raids or the probability of nest survival through the brood raiding period. However, following nearly 30% of raids, queens moved to new nests and displaced the resident queens. When queen relocation and subsequent mortality were accounted for, it was found that the survival of queens from four-queen groups was substantially higher than that of solitary queens. By contrast, the survival of queens from two-queen colonies was no greater than that of solitary queens. These results show that the competitive advantages of multiple-queen colonies are sufficient to counterbalance the increased mortality of queens within groups only when the number of foundresses is greater than two and when colonies are founded at high density. When colonies lose brood raids, the workers appear to abandon their mothers to join surviving colonies. However, in laboratory experiments, queens attempting to enter foreign nests were significantly more likely to displace the resident queen if their own daughters were present within the invaded nest. Thus, workers may be able to bias the probability that their mother rejoins them and displaces competing queens.     

Chick recognition and acceptance: a weakness in magpies exploited by the parasitic great spotted cuckoo

Hosts of brood parasites have evolved the ability to discriminate non-mimetic and even mimetic eggs, but not non-mimetic chicks. Here we demonstrate that the great spotted cuckoo Clamator glandarius does not provide its magpie Pica pica host with a super-normal stimulus that helps to avoid recognition, because single cuckoo chicks introduced into otherwise unparasitized magpie nests are not fed at a higher frequency than single magpie chicks introduced to parasitized magpie nests. Another series of experiments demonstrated that magpies have the ability to discriminate cuckoo chicks, mainly when these are introduced at the end of the nestling period, and especially when the cuckoo chick together with a magpie chick is presented to adult magpies outside the nest. This supports the idea that cuckoos exploit the obligatory reaction of magpies to feed all young that have been hatched in their nests and whose signatures they have learnt. Furthermore, the experimental cuckoo chicks in parasitized magpie nests were more likely to be accepted than they were in non-parasitized nests. This supports the hypothesis that magpies may learn to recognise their own nestlings as those present in the nest and may indicate that a comparison between cuckoo and magpie nestlings is the basis of discrimination.     

Larval hemolymph feeding in the ant Leptanilla japonica by use of a specialized duct organ,the “larval hemolymph tap” (Hymenoptera: Formicidae)

Summary The larvae of the migratory Japanese ant Leptanilla japonica Baroni Urbani have a specialized duct organ on each side of the 4th abdominal segment. Behavioral and histological studies have shown that the adult ants are able to imbibe hemolymph directly from the larval body cavity through these organs, each of which is referred to here as a larval hemolymph tap. Laboratory observations further confirm that larval hemolymph feeding (LHF) is the sole source of nutrient for the queens. L. japonica is cyclical in its brood production. All larvae in a colony develop in concert, and when they are mature, the queen performs much active LHF on them. Nourished in this way, she achieves full physogastry within a period of only a few days and then lays a batch of 100–200 eggs. LHF thus facilitates synchronization of brood maturation and concentrated production of eggs by the queen. The larval hemolymph tap has probably evolved de novo in Leptanilla, in relation to cyclical brood production by species with small colony populations. Obviously, the function of this larval organ is non-selfish, contributing to overall colony-level functioning. The larvae of Leptanilla are thus properly qualified as a distinct class of caste in ant societies.     

Intra-colony relatedness and sociality in the allodapine bee Exoneura bicolor

Summary Overwintered and newly co-founded nests of Exoneura bicolor exhibit different grades of sociality. Intra-colony relatedness was estimated for adults and female brood in both nest types using allozyme data and a multiallelic estimator. The higher relatedness among female brood from overwintered nests compared to newly-founded nests is consistent with the difference in sociality between these colony types (semisocial versus quasisocial). However, intra-colony relatedness among adults is higher in cofounded nests than in overwintered nests, suggesting that differences in sociality are determined by benefit/cost ratios associated with non-reproductive strategies, rather than relatedness between interactants. Rapid egg-production in newly founded nests allows eggs to be stockpiled. This reduceds the tasks available to non-reproductives in early phases of colony development and limits the payoffs available for reproductive altruism. It is suggested that the social flexibility characteristic of allodapines is a consequence of communal progressive rearing which allows many benefits from cooperative nesting to be gained without worker sterility.     

Electrophoretic variants of egg white transferrin indicate a low rate of intraspecific brood parasitism in colonial cliff swallows in the Sierra Nevada,California

Summary The frequency of intraspecific brood parasitism in two colonies of cliff swallows (Hirundo pyrrhonota) nesting in the eastern Sierra Nevada in California was assessed through an electrophoretic analysis of egg white (albumin) proteins. Albumin proteins are maternally derived and are presumed to directly reflect maternal genotype. Thus, a comparison of protein banding patterns produced by eggs collected from a single clutch allows any egg laid by a female other than the putative mother to be detected. Eggs were collected from 13 cliff swallow nests in 1984 and 41 nests in 1987, a total of 54 nests. Of the clutches collected in 1984, one showed evidence of intraspecific brood parasitism, giving a nest parasitism rate of 7.6%. In 1987, one of 41 nests contained a parasitic egg, for a parasitism rate of 2.4%. Over both years combined the rate of intraspecific brood parasitism was 2 of 54 nests, or 3.7%. These parasitism rates are much lower than those estimated for Nebraskan cliff swallows (22–43%) and 2nd-year purple martins (36%). Possible explanations for the discrepancy between parasitism rates in Sierran cliff swallows and other groups of swallows are discussed.Correspondence to: A.P. Smyth     

The function of polydomy: the ant <Emphasis Type="Italic">Crematogaster torosa</Emphasis> preferentially forms new nests near food sources and fortifies outstations

Many ant species are polydomous, forming multiple spatially segregated nests that exchange workers and brood. However, why polydomy occurs is still uncertain. We investigated whether colonies of Crematogaster torosa form new polydomous nests to better exploit temporally stable food resources. Specifically, we tested the effect of food presence or absence and distance on the likelihood that colonies would form a new nest. Because this species also forms little-known structures that house only workers without brood (outstations), we also compared the function of this structure with true nests. Laboratory-reared colonies were connected to a new foraging arena containing potential nest sites with or without food for 4 months. When food was present, most colonies formed polydomous nests nearby and the remainder formed outstations. When food was absent, the behavior of colonies differed significantly, frequently forming outstations but never polydomous nests. Distance had no effect on the type of structure formed, but when food was present, a larger proportion of the workforce moved shorter distances. Workers often fortified the entrances to both structures and used them for storage of dried insect tissue (“jerky”). In an investigation of spatial fidelity, we found that workers on the between-nest trail were associated with the original nest, whereas workers collecting food were more likely to be associated with the new nest or outstation. C. torosa appears to have a flexible colony structure, forming both outstations and polydomous nests. Polydomous nests in this species were associated with foraging and were only formed near food resources.     

Egg removal and intraspecific brood parasitism in the European starling (Sturnus vulgaris)

Summary From 1983 to 1986 we monitored 284 European starling (Sturnus vulgaris) nests in New Jersey for evidence of intraspecific brood parasitism and egg removal during the laying period. Egg removal occurred significantly more often at nests where intraspecific brood parasitism was detected (12 of 35 nests, 34%) than at unparasitized nests (23 of 249 nests, 9%). Brood parasitism (92% of parasitized nests) and egg removal (74% of nests with egg removal) were most common at nests where egg laying began in April of each year (i.e., early nests). Egg removal occurred at 26 (19%) and brood parasitism at 32 (23%) of 138 early nests. Both brood parasitism and egg removal were concentrated during the first four days in the laying period when brood parasitism is most likely to be successful and when host nests are most vulnerable to parasitism (Romagnano 1987). Both parasitism and removal usually involved a single egg at each nest. We detected brood parasitism and egg removal on the same day at five of 12 nests (42%) where both were observed. Because starlings do not remove foreign eggs from their nests once they begin laying (Stouffer et al. 1987) we hypothesize that parasite females sometimes removed host eggs while parasitizing nests.     

Reproductive effort of colonial and solitary breeding tree sparrows Passer montanus L.

A total of 250 nestboxes were arranged in five plots in a suburban area of Budapest, Hungary (19°04E, 47°41N). In each plot, 25 were placed at 50 m intervals to simulate solitary breeding and 25 3–5 m apart to simulate colonial breeding. Length of nest building period, feeding frequency, nestling mortality, nestlings' diet, productivity and parental condition were compared for colonial and solitary breeding tree sparrows Passer montanus. Parents with long nest-building periods, including the majority of first-year females, produced fewer young than parents which built over short periods. Parents fed nestlings morefrequently and nestlings had lower mortality in second than first broods; whether or not a third brood was reared was determined by the costs invested in first and second broods. Females that laid a third clutch had reared fewer young in first and second broods and were heavier than females that reared many young in two broods. Colonial birds had higher feeding frequencies, more similar diets and suffered lower nestling mortality than solitary parents for first broods, but they fed less frequently, diets were less similar, and nestling mortality was higher in second and third broods. It is suggested that colonial breeders benefited from the social stimulation of simultaneous feeding in first broods, but the advantage of synchronicity in feeding declined in second broods and the sparser breeding spacing of solitary parents was more advantageous for feeding in second and third broods. Birds that changed nest spacing between broods had fed nestlings less frequently and had higher nestling mortality before changing than birds which retained their spacing. Parents which changed from colonies to solitary nests fed more frequently with lower nestling mortality in the next brood than parents which retained colonial nests for their second (and third) brood. Solitary parents did not show such a relationship. The rearing of three broods caused higher weight loss in colonial than solitary parents.Correspondence to: L. Sasvári     

Differential reproductive success among subfamilies in queenless honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.) colonies

With very rare exceptions, queenright worker honeybees (Apis mellifera L.) forego personal reproduction and suppress reproduction by other workers, preferring to rear the queens sons. This is in stark contrast to colonies that have lost their queen and have failed to rear a replacement. Under these conditions workers activate their ovaries and lay many eggs that develop parthenogenetically into a final brood of males (drones) before the colony perishes. Interestingly, not all workers contribute equally to this final generation of drones in queenless colonies. Some subfamilies (workers that share the same father) contribute a disproportionately greater number of offspring than other subfamilies. Here we explore some of the mechanisms behind this reproductive competition among subfamilies. We determined the relative contribution of different subfamilies present in colonies to laying workers, eggs, larvae and pupae by genotyping samples of all life stages using a total of eight microsatellite loci. Our colonies were headed by free-mated queens and comprised 8–17 subfamilies and therefore differed significantly from colonies used in an earlier study investigating the same phenomena where colonies comprised an artificially low number of subfamilies. We show that, first, subfamilies vary in the speed with which they activate their ovaries after queen-loss and, second, that the survival of eggs to the larval stage is unequal among subfamilies suggesting that some subfamilies lay eggs that are more acceptable than others. However, there is no statistically significant difference among subfamilies in the survival of larvae to pupae, indicating that ovary activation and egg survival are the critical components to reproductive competition among subfamilies of queenless honeybee workers.Communicated by R. Page     

A South East Asian ponerine ant of the genus Leptogenys (Hym., Form.) with army ant life habits

Summary The emigration and raiding behavior of the SE Asian ponerine ant Leptogenys sp. 1, which resembles L. mutabilis, were observed in the field (Ulu Gombak, Malaysia). The ants formed monogynous colonies that consisted of up to 52 100 workers. The bivouac sites of this species were found in leaf litter, rotten logs, ground cavities, etc., and were rarely modified by the ants. The colonies stayed in these temporary nests for several hours to 10 days; afterwards, they moved to a new nest site. The emigration distances ranged from 5–58 m. Since nest changing takes place at irregular intervals, and pupae and larvae are always present in the nest relocations of Leptogenys sp. 1, the emigration behavior is not linked to a synchronized brood development. Leptogenys sp. 1 is a nocturnal forager; in our study, up to 42 600 workers participated in each raid. The ants move forward on a broad front; behind the swarm a fan-shaped network of foraging columns converges to form a main trunk trail. A new system of foraging trails is developed in each raid. The workers search for their prey collectively; they attack and retrieve the booty together. The diet of Leptogenys sp. 1 consists mainly of arthropods. Army ant behavior is characterized by (1) formation of large monogynous colonies, (2) frequent emigrations, and (3) mass raids in which all foraging activities are carried out collectively. Since Leptogenys sp. 1 performs these typical army ant behavior patterns, this species represents the army ant ecotype. However, this species differs considerably from army ant species that have synchronized broods and huge colonies with dichthadiiform queens.Dedicated to Professor Dr. M. Lindauer on the occasion of his 70th birthday     

Fire ant polymorphism: the ergonomics of brood production

Summary Social organization is generally assumed to increase colony efficiency and survival; however, little quantitative information is available to support this assumption. Polymorphism is an important aspect of labor division in colonies of the fire ant, Solenopsis invicta. Our objective was to investigate the effect of fire ant polymorphism on brood production efficiency. We set up standardized polymorphic colonies with a full range of worker sizes and artificial monomorphic colonies that contained only small, medium or large workers respectively. Polymorphic colonies produced brood at about the same rate as colonies composed of only small workers (Fig. 2A). Colonies composed of only medium workers produced about 30% less brood, and colonies composed of only large workers produced little or no brood at all. This pattern was independent of colony size; however, smaller colonies (0.75 g, live weight) produced almost twice as much brood per gram of workers as larger colonies (3.0g). Additional experiments revealed that the size of workers in the artificial monomorphic colonies affected all stages of brood rearing. Large workers not only inhibited the development of early and late instar larvae (Fig 4), but also reduced the queen's oviposition rate (Fig. 3). Brood production efficiency on an energetic basis was determined by dividing the grams of brood produced per unit time by the energetic costs expended for the maintenance and production of each worker size class. Worker maintenance costs were estimated from respiration while production costs were determined from the caloric content of worker tissue divided by their average longevity. Worker respiration per milligram body weight decreased about 40% as body size increased (Fig. 5). Large workers lived about 50% longer than small workers (Fig. 6) and contained 9% more energy per milligram of tissue (Fig. 7). Energetic efficiency in polymorphic colonies was approximately 10% higher than in colonies composed of only small workers (Fig. 9). In other words, when food supplies are limiting, polymorphism may offer a slight advantage in brood production.     

Parental care of nestlings by male red-winged blackbirds

Summary Nestling feeding by males is less common among birds with polygynous mating systems than in monogamous species, because of the pronounced trade-off between parental behavior and the attraction of additional mates. In this study, however, we found that male red-winged blackbirds (Agelaius phoeniceus) commonly assisted females in feeding nestlings in several Ontario marshes. Male parental care was additional to that provided by females, and it significantly enhanced the fledging success of nests (Table 2). Male redwings did not help to feed all nests on their territories: primary and secondary nests were much more likely to receive male parental care than tertiary and later nests. Contrary to expectation, male parental care was not restricted to the nests of primary females: a greater proportion of secondary than primary nests were assisted (Tables 4a and b). The presence of new females settling on the territory at the same time that a resident female had nestlings, resulted in males deferring parental care until a later brood. This suggests that males trade off the recruitment of females against parental care to an existing brood. Although the number of nestlings fledging from a male's territory was strongly influenced by the number of females in the harem, males could additionally increase their reproductive success by feeding nestlings in one or more nests on their territories (Fig. 2). The reproductive success of females was significantly enhanced by male assistance in feeding nestlings (Table 3). However, those females not receiving male assistance on territories of feeding males did not suffer a significantly reduced reproductive success in comparison to females on territories of non-feeding males (Table 2). Males varied considerably in the quality of brood care given. We therefore suggest that the quality of male parental care may be a factor considered by females in choosing a breeding situation.     

Behavioral ecology of the neotropical termite-hunting ant Pachycondyla (= Termitopone) marginata: colony founding,group-raiding and migratory patterns

This study provides the first detailed field account of colony founding, group-raiding and migratory habits in the neotropical termite-hunting ant rPachycondyla marginata, in a semi-deciduous forest in south-east Brazil. New colonies can originate by haplometrosis, pleometrosis, or colony fission. Incipient colonies with multiple foundresses persisted longer in the field, and most excavated nests contained more than one dealated female. A total of 202 group raids by P. marginata were registered, and in all cases the raided termite species was Neocapritermes opacus. Nearly 20% of the workers within a colony engage with raiding activity. Colonies of P. marginata hunt for termites approximately every 2–3 weeks, and group-raids may last for more than 24 h. Target termite nests are up to 38 m from the ant colony, and occasionally two nests are simultaneously raided by one ant colony. Raiding ants carry 1 or 2 paralysed prey, and nearly 1600 termites can be captured during a 9-h raid. Migration by P. marginata colonies lasted over 2 days and covered distances of 2-97 m (n = 48). Average residence time at a given location was 150 days. Three basic migratory patterns were noted: colony fission (only part of the colony moves), long-distance migrations, and short-distance migrations. Both raiding and migratory activities appeared to be strongly affected by seasonal factors. The group raiding and migratory patterns of P. marginata are compared with other ant taxa with similar habits. It is concluded that P. marginata presents a rudimentary form of the so-called army ant behavior, which is highly developed in the subfamilies Dorylinae and Ecitoninae. The extremely specialized diet of P. marginata and the associated high costs of migration are features likely to prevent it from evolving a full army ant life pattern.     

Increased grooming after repeated brood care provides sanitary benefits in a clonal ant

Repeated pathogen exposure is a common threat in colonies of social insects, posing selection pressures on colony members to respond with improved disease-defense performance. We here tested whether experience gained by repeated tending of low-level fungus-exposed (Metarhizium robertsii) larvae may alter the performance of sanitary brood care in the clonal ant, Platythyrea punctata. We trained ants individually over nine consecutive trials to either sham-treated or fungus-exposed larvae. We then compared the larval grooming behavior of naive and trained ants and measured how effectively they removed infectious fungal conidiospores from the fungus-exposed larvae. We found that the ants changed the duration of larval grooming in response to both, larval treatment and their level of experience: (1) sham-treated larvae received longer grooming than the fungus-exposed larvae and (2) trained ants performed less self-grooming but longer larval grooming than naive ants, which was true for both, ants trained to fungus-exposed and also to sham-treated larvae. Ants that groomed the fungus-exposed larvae for longer periods removed a higher number of fungal conidiospores from the surface of the fungus-exposed larvae. As experienced ants performed longer larval grooming, they were more effective in fungal removal, thus making them better caretakers under pathogen attack of the colony. By studying this clonal ant, we can thus conclude that even in the absence of genetic variation between colony members, differences in experience levels of brood care may affect performance of sanitary brood care in social insects.     

Fire ant thermal preferences: behavioral control of growth and metabolism

Summary Thermal preferences of well-fed and food-limited fire ant colonies (Solenopsis invicta) were studied in relation to colony growth and metabolic costs. The growth curve for well-fed colonies was strongly skewed toward warmer temperatures with maximal growth occurring near 32° C (Fig. 2A). The growth curve for food-limited colonies was skewed toward cooler temperatures with maximal colony size occurring around 25° C (Fig. 2B). Food-limited colonies apparently grew larger at cooler temperatures because metabolic costs of workers were reduced. A series of binary choice tests confirmed three predictions concerning fire ant thermal preferences (Figs. 3–4). First, well-fed colonies preferred brood temperatures very near the optimum for colony growth (31° C versus 32° C). Colonies were also able to select appropriate suboptimal growth temperatures when the optimal range was unavailable. Secondly, as predicted, a large percentage of colony workers ( 30% in well-fed colonies) consistently chose cooler temperatures than those selected for the brood. This strategy probably increases longevity of workers not directly associated with brood care. Thirdly, food-limited colonies preferred cooler temperatures than well-fed colonies. Metabolic costs of food-limited colonies were reduced by approximately 7% because of (1) slightly cooler brood temperatures (30° C versus 31° C) and because (2) an additional 20–30% of the workers selected cooler temperatures. The addition of excess food reversed food-limited thermal preferences within 12 h for the brood (Fig. 5) and several days for the workers. Contrary to expectations, thermal preferences for brood in food-limited colonies did not match the food-limited growth curve, perhaps because fire ant colonies can choose to rear brood at warm temperatures while maintaining accumulated colony biomass at cooler temperatures. Correspondence to: S.D. Porter     

Opportunistic discrimination of alien eggs by social wasps (Polistes biglumis, Hymenoptera Vespidae): a defense against social parasitism?

Foundresses of the social wasp Polistes biglumis were tested to see whether they were able to recognize alien eggs experimentally introduced into their own nests. Foundresses removed alien conspecific reproductive-destined eggs while they accepted worker-destined eggs. The results indicate that social wasps discriminate among eggs and that they discriminate against alien eggs destined to produce unrelated reproductives. P. biglumis is a strictly solitary founding species, with no reproductive competition within colonies; thus, brood discrimination abilities could have evolved as a counteradaptation against intra- and inter-specific brood parasitism. Received: 12 May 2000 / Revised: 15 July 2000 / Accepted 20 July 2000     

Regulation of ovary activation in worker honey-bees (Apis mellifera): larval signal production and adult response thresholds differ between anarchistic and wild-type bees

One-day-old anarchistic (selected for successful worker reproduction) and wild-type honey-bee workers were introduced into queenright colonies of honey-bees of two treatments. In treatment 1, all eggs and larvae were offspring of queens from an anarchistic line. In treatment 2, all eggs and larvae were offspring of wild-type queens. In both treatments, adult workers were wild type. This experimental arrangement was used to test the importance of larval genotype on ovary activation in young adult workers. After 12 days, the introduced bees were dissected to determine the frequency of ovary activation. In those colonies provided with wild-type brood, 0% of introduced wild-type bees and 16% of anarchistic bees had activated ovaries. In those colonies provided with anarchistic brood, 13% of introduced wild-type bees and 41% of anarchistic bees had activated ovaries. These results strongly support the hypothesis that selection for high levels of worker reproduction in anarchistic stocks has reduced the amount or composition of brood pheromones produced by larvae that normally signal workers to refrain from reproduction. They also suggest that anarchistic workers have a higher threshold for these signals than wild-type bees.