Halictus rubicundus, the orange-legged furrow bee, is a species of sweat bee found throughout the Northern Hemisphere. H. rubicundus entered North America from the Old World during one of two main invasions of Halictus subgenera. These invasions likely occurred via the Bering land bridge at times of low sea level during the Pleistocene epoch.
The species exhibits different social behaviors depending on climate: it is a solitary species in high elevations or latitudes where the season is short, but eusocial in other areas. Often, solitary and eusocial colonies appear simultaneously in the same population. These sweat bees are extensively studied for their variability in social behavior, which has become a model for social plasticity. This variability has contributed to an understanding of evolution of social behavior.Yanega, D. (1997) Demography and sociality in halictine bees (Hymenoptera: Halictidae). pp. 293-315 in Choe, J.C. & Crespi, B.J. (eds.) Social Competition and Cooperation in Insects and Arachnids: Vol. II. Evolution of Sociality. Princeton Univ. Press, Princeton.
Taxonomy and phylogeny Halictus rubicundus is a species of Hymenoptera in the bee family Halictidae, more commonly known as sweat bees. This common name comes from their frequent attraction to perspiration. This species exhibits polymorphic social behavior that varies with environmental conditions, and other species of the family Halictidae are thought to have similar variability in sociality.
Description and identification
Many members of the family Halictidae are metallic in appearance but Halictus rubicundus are not metallic. Females are about 1 cm in body length and brown in color, with fine white bands across the apices of the abdominal segments, and rusty-orange legs. The males are more slender, with longer antennae and yellow markings on the face and legs; they can be distinguished from males of similar species by the absence of an apical hair band on the terminal abdominal segment. In social populations, females of the first brood, mostly workers, can be recognized because they are typically slightly smaller than the foundresses.
Distribution and habitat H. rubicundus has one of the widest natural distributions of any bee species, occurring throughout the temperate regions of the Holarctic region. It is believed that differences in climate across this vast range actually contribute to variation in their social behavior. Those living in more northern geographic locations or higher elevations are often more solitary in behavior than those in southern areas or lower elevations. This difference is widely studied, as it provides insight into the evolutionary transition from solitary to social behavior. Nests are haplometrotic, meaning that they are founded by single females. Social populations typically are found in warmer regions, such as Kansas, while solitary populations nest in cooler regions, such as Colorado, Scotland and Alaska. In intermediate regions, such as New York and southern Ontario, both social and solitary behavior can be found in different nests of the same population. The strictly solitary phenotype is expressed as a response to colder environments because the active season is not long enough to produce two broods.
Nesting
Both solitary and eusocial types of the species typically excavate nest burrows in southward facing slopes in isolated areas, consisting of sand or soil. This slope maximizes the heat absorption from the sun, making the nest warmer. The nests with a favorable slope were thought to increase foraging efficiency of adults and development of larvae with a stable thermal environment.
Stones or areas of vegetation are usually found near nest entrances, likely because of the heating properties of these objects. For each offspring, a female constructs a small underground chamber (a "brood cell"), into which she deposits several loads of pollen mixed with nectar, which is then formed into a ball; a single egg is laid upon this pollen mass, and then the brood cell is sealed. The female repeats the process with other brood cells, going progressively deeper into the soil, producing a tumulus around the nest entrance. A higher temperature increases the rate at which the larvae develop to maturity, as thermal regulation is important for the development of both the eggs and the larvae.
The nests can be up to 120 mm deep, and are constructed in a wide range of soil types. Because social nests produce more offspring than solitary nests, social nests will burrow further into the ground, as the second brood of the social population will extend the burrows downward. Females typically nest in dense aggregations, likely because the nesting females are relatives and demonstrate philopatric behavior.Yanega, D. (1990) Philopatry and nest founding in a primitively social bee, Halictus rubicundus. Behav. Ecol. Sociobiol. 27: 37-42.
Soil hardness The nests are typically burrowed into the ground in loam soil. Halictus rubicundus has a high tolerance for soil hardness. Soil hardness affects the density of nesting. Females prefer to nest in softer ground as they spend less energy and time excavating the nest. Unlike the similar species Lasioglossum zephyrus that build nests in very close proximity to each other, this species prefers less dense aggregations. Foundresses will choose to build their nests in patches of softer ground until they reach the critical nearest-neighbor distance of about 50 mm, at which point the close spacing poses a high risk of the nests collapsing. At this point, further foundresses would be forced to build their nests in harder soils where the nests could be built closer together without compromising nest architecture. Foundresses may test the hardness of the soil by biting into the surface or performing a short test dig.
Temperature Nest temperature determines egg development and foraging active time of females. As long as temperatures do not reach lethal thresholds, developmental rates of offspring will increase with temperature. Higher temperatures will also increase the thoracic temperature and allow females to fly more rapidly. With increased speed in flight, females are allowed more time for foraging, mating, and excavating nests.
Most nests of H. rubicundus are south facing and sloped because of this desire for increased temperature in nesting sites. Based on the distribution of this species, facing the south maximizes the period of time sunlight is shining directly on the nest. Sloping substrates increase the surface area of the nest and allow for higher absorption of sunlight. In order to test the temperature of the substrate, females often spend several seconds basking at various points on the ground while searching for nesting sites.
Soil water content The water content of the substrate in which foundresses build the nest is important, as waterlogging must be avoided by using well-drained soils, which provides another advantage to building in sloping ground. However, there must be an adequate moisture level to prevent desiccation of the brood cells. Soil samples taken near the nests of H. rubicundus demonstrate a relatively high humidity.
Parasites Halictus rubicundus nests are attacked by kleptoparasitic bees (e.g., Sphecodes), as well as flies (Bombyliidae, Anthomyiidae). Although it would seem as though nesting in dense groups would draw attention to aggregations and increase mortality by parasitism, such is not observed to be the case; because the sweat bees nest densely, it is assumed that there is a large dilution effect that proportionally decreases mortality rates by parasites.
Colony cycle and demographics H. rubicundus’s annual colony cycle is dependent both on hibernation and mating schedules. After hibernation during the winter, female foundresses who mated the previous cycle emerge in the spring. They each create their own nests in late spring, where they rear a single brood. Females are more likely to build nests where there is a warmer surface temperature, as this indicates a warmer interior of the nest for the offspring to develop more quickly. The foundress gynes will continue to forage for 3–5 weeks, after which they will stop constructing and provisioning brood cells. The brood cells are left inactive for 1–2 weeks before the emergence of the first brood. The first brood typically emerges in June, and most females from the first brood will stay with their natal nest and act as foraging workers, provisioning for a second brood, on which the foundresses will resume laying eggs.
Some females (the gynes) from the first brood, and all of the females from the second brood, will mate, disperse, and enter hibernation for the winter, and the colony cycle begins again the following spring.
Sociality effects on colony cycle The annual cycle differs slightly for eusocial and solitary populations, in terms of the number and sex ratios of offspring born in the broods. For example, in solitary populations, the first brood comprises 40% females, who all mate and enter hibernation. However, in eusocial populations, the emergence from hibernation occurs one or two months earlier, and results in a brood with mostly or exclusively worker females, with the female bias being greater the earlier nesting begins. Nesting for solitary populations begins in late May or June. The absence of a brood of female workers defines this nest type as solitary, so solitary populations produce only one reproductive brood that is provisioned by a gyne. The emergence of this brood is at approximately the same time as the emergence of the second or third brood in the social colony cycle. Upon emergence, the offspring mate and then females enter hibernation away from the nesting site. As in social colonies, the males and nest foundresses die at the end of the season.
Sex ratio variation In social populations where there are two or more broods in one season, there are different sex ratios for each brood., and even variation between years, depending upon temperature (warmer years lead to increased male bias). The first brood will typically contain 75–100% females, most of which become workers, to help the mother produce a second brood. The final brood is slightly male-biased, typically with a sex ratio of about 60% males, and the females of the second brood are all gynes. The peak of male production strongly corresponds with photoperiod, such that eggs laid on or near the summer solstice are almost all male. In solitary populations, nesting begins later and only one brood is produced. The brood has a sex ratio made up of 60% males, similar to that of the final brood in social populations. This brood is also produced at about the same time that the final brood is produced in social populations so that the colony cycle ends at about the same time regardless of where the population lives.
Body size Temperature affects not only the sex ratio, but also the size of offspring in the first brood of Halictus rubicundus, with warmer temperatures strongly correlated with larger offspring; there are various hypotheses that offer explanations as to why such is the case: At optimum temperatures, the maximum offspring size will be produced. Smaller sizes will be the result of both higher and lower temperatures based on differing stresses. So varying conditions around one optimum temperature will lead to different offspring sizes. This hypothesis is not supported by existing data. Low temperatures may reduce flowering, or reduce bees' ability to forage during the day, and therefore effect the rate of cell provisioning. If the rate of provisioning is lower, females may lay eggs on pollen masses before the mass reaches an optimal size. This hypothesis is supported by existing data.