What a wonderful season for a walk in the woods. Wildflowers like trout lily, blood root, and spring beauties abound on the forest floor. While photographing some spring beauties, I was lucky to spy a small bee loaded with pollen. In previous episodes we visited honey bees, mason bees, and plasterer bees, but this bee is another member of the solitary bee clan commonly known as a mining bee. The little gal I photographed was pretty in pink after spending the day collecting lavender pollen from spring beauties. The dense hairs on the legs of this bee are called scopa and they bulged with pollen. Batches of pollen were destined to become food for the brood of this ground nesting bee.
Nesting sites of mining bees such as Andrena erigeniae are usually found in well drained loamy soils with sparse vegetation. Often times these ground nesting bees build their subterranean galleries beneath thin layers of fallen leaves. Where soil conditions are favorable, as many as 20 females may build burrows in a square meter of soil. Using jaws and legs the female bees excavates a gallery in the soil, leaving a small pile of dirt near the entrance hole. This gallery can be as long as 15 centimeters and contain numerous lateral brood chambers. During the daytime she forages for pollen on flowers of spring beauties, which apparently are the sole source of food for her brood. Pollen from these blossoms is formed into balls and placed into brood chambers.
This bee is not what you call a crack of dawn forager, as scientists have observed the first load of pollen arriving at the nest about 10 am. Nor are these ladies into working overtime, as the last load of pollen usually arrives back at the nest between noon and 2:30 pm. The work schedule is likely determined by favorability of spring temperatures for flight, the hour of the day when blossoms open, and how rapidly the pollen produced each day by spring beauties is depleted by pollinators. Andrena erigeniae also do not forage much on chilly and rainy days when flight is difficult due to low ambient temperatures and when blossoms may be closed for business. Pollen sorties are rather lengthy affairs and one study reported transit times ranging from 10 to 92 minutes from departing the nest to returning with a full load of pollen. After the last pollen–run of the day, the bee enters the gallery and seals herself in with her brood by plugging the entrance with soil. This likely excludes nocturnal predators and also provides some high quality private time with her brood at the end of a hectic work day.
As brood chambers are built and provisioned with pollen, the bee deposits a single egg on a pollen cake. During spring and early summer developing larvae consume the pollen, and later in summer they will form pupa. By late autumn development of the adult is complete and winter is spent in the adult stage within the brood chambers. Newly minted adults emerge each spring coincident with the appearance of spring beauty’s’ blossoms. Male and female bees emerge more or less synchronously in spring and their romantic interludes are reported to take place on the flowers of spring beauties. Work days from 10 ‘til 2, rainy days off, dining on nectar and pollen, and ardor among spring flowers - a good life for the mining bees!
Bug of the Week thanks ever helpful Sam Droege for providing inspiration and assistance in identifying bees for this episode. The wonderful story of mining bee biology “The nest biology of the bee Andrena (Ptilandrena) erigeniae Robertson (Hymenoptera: Andrenidae)” by L. R. Davis and W. E. LaBerge was used to prepare this episode.