Halictid bees, commonly known as sweat bees, constitute a large and diverse family of bees found across nearly all continents, excluding Antarctica. This family, Halictidae, ranks as the second-largest among bees, encompassing almost 4,500 described species. Their widespread presence and varied behaviors highlight their role in ecosystems worldwide. They contribute to the reproduction of numerous wild plants and agricultural crops, benefiting both natural environments and human food production.
Distinctive Features and Identification
Halictid bees exhibit a wide range of appearances, though many share characteristics that aid in their identification. They are small to medium-sized, measuring between 3 to 10 millimeters in length. Many species are recognized by their metallic coloration, which can include shades of green, blue, copper, or gold. Other species appear darker, in brown or black, sometimes with yellow markings, particularly on the faces of males.
A distinguishing feature for specialists identifying halictid bees is the strongly curved basal vein found on their wings. They also possess short tongues, a characteristic that sets them apart from some other bee families. Most species prefer to nest in the ground, in areas with bare soil like fields, gardens, grasslands, or river banks. A smaller number of species also nest in decaying wood.
Diverse Social Structures and Nesting Habits
The social organization within the Halictidae family is remarkably diverse, ranging from solitary lifestyles to complex eusocial systems. Most halictid bees are solitary, meaning each female constructs and provisions her own nest independently. In these solitary species, the female lays her eggs on a mass of pollen and nectar within a waterproof cell, sealing it so the larva has all its food at once.
Some species exhibit communal nesting, where multiple females share a single nest entrance but each female provisions her own individual cells. This arrangement provides benefits, such as shared defense of the nest, without a strict division of labor. For example, some sweat bee species form joint nests where certain individuals function as workers without laying eggs.
Halictid sociality includes primitively eusocial species. In these species, a queen bee lays eggs, and a worker caste assists with nest construction, foraging, and brood care. Unlike highly eusocial insects like honey bees, the division of labor in primitively eusocial halictids is less rigid. The level of sociality varies based on factors like climate, season, or geographic location. Halictus rubicundus can switch between solitary and social behaviors, with sociality increasing in cooler climates or under certain environmental conditions.
Halictid bees construct their nests by digging burrows in the ground. These nests extend several inches underground and feature multiple entrances and chambers, forming tunnel networks. Some species, such as Caenohalictus, build nests in vertical earth banks of firm soil, sometimes in dense aggregations. The architecture of these nests varies, with some species creating lateral burrows leading to single cells, while others organize cells into clusters within a cavity.
Pollination and Interaction with Humans
Halictid bees are important pollinators for a broad spectrum of plants, including many agricultural crops and wildflowers. They contribute to the pollination of crops such as alfalfa, apples, watermelon, stone fruits, sunflowers, and various fruits and vegetables. Approximately 30% of crops in the United States depend on native bees, including halictids, for pollination.
Many halictid species are generalist foragers, visiting a wide array of flowers for nectar and pollen, making them valuable pollinators. Some smaller sweat bee species are also capable of buzz pollination, a method where they vibrate their flight muscles to release pollen from certain flowers that other bees, like honey bees, cannot pollinate. This specialized pollination method supports crops such as tomatoes, eggplants, kiwi, and blueberries.
The common name “sweat bee” originates from their attraction to human perspiration. These bees seek the salts and moisture found in sweat to supplement their diet. While they may land on exposed skin, halictid bees are not aggressive. A sting occurs only if the bee is agitated or trapped against the skin, and the venom causes only mild irritation.