These invasions likely occurred via the Bering land bridge at times of low sea level during the Pleistocene epoch.

[10] 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.

This difference is widely studied, as it provides insight into the evolutionary transition from solitary to social behavior.

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.

[3][12] Both solitary and eusocial types of the species typically excavate nest burrows in southward facing slopes in isolated areas, consisting of sand or soil.

The nests with a favorable slope were thought to increase foraging efficiency of adults and development of larvae with a stable thermal environment.

With increased speed in flight, females are allowed more time for foraging, mating, and excavating nests.

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.

[13] 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.

[13] The foundress gynes will continue to forage for 3–5 weeks, after which they will stop constructing and provisioning brood cells.

[5][12] The annual cycle differs slightly for eusocial and solitary populations, in terms of the number and sex ratios of offspring born in the broods.

[17] Halictus rubicundus is widely studied for their variability in behavior depending on geographic location, and with changes in temperature.

[5] The nest-founding females recruit their early-emerging daughters as workers, but later-emerging offspring mate prior to dispersal and hibernation.

[3][5][9] The cooperative breeding behavior of worker bees in eusocial colonies benefits the fitness of the foundress queen.

[3] Warmer temperatures during the first brood provisioning phase in the spring leads to a higher ratio of male to female offspring, and a significantly lower proportion of females are recruited as workers, reducing the overall level of sociality expressed in that population, ranging from above 75% in some years to below 45% in others, in the same location.

[3][7] Nest-site fidelity may be due to one of three reasons, or a combination: In H. rubicundus, females fly back, after 10 months in diapause at remote locations, to within centimeters of the nest they emerged from, even though the nesting area used by the population is many square meters, suggesting the first of these mechanisms is of primary importance (i.e., within the nesting area as a whole, only a model of strict philopatry predicts a non-random distribution of females within that larger area, and the observed distribution is extremely non-random).

[20] In H. rubicundus, the Dufour’s gland produces pheromones that may aid females in recognizing brood cells as well as other individuals in the nest.

[13] Following standard models, it is assumed that worker bees who help their mother raise the following brood are in a position to benefit from the effects of kin selection.

[9] This does not necessarily mean that social behavior is governed by certain genes, but it could be linked to certain genetic lineages that are more suited for certain environments.

[9] Empirical data from within a single population, however, indicates that females who do not remain as workers in their mother's nest are not more likely to have daughters that similarly depart.

[5] Although there is only limited research to date on the correlation between genetics, the environment, and social behavior, there is ample evidence that there are links between these three factors.

[5] Furthermore, the one study providing empirical data for H. rubicundus shows that a typical worker contributes to the production of 0.9 sisters and 0.8 brothers, far below the threshold for inclusive fitness effects to favor helping, but above the threshold at which their mother benefits directly; as such, this species provides evidence that kin selection effects would not apply to this species, and instead suggests that mothers manipulate some of their daughters into acting as workers because of direct gains to maternal fitness, even though these manipulated daughters have lower fitness than they could have had if they had been gynes.