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 females 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. 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. 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