After the male and female andrenas have mated, the cycle is repeated. Although Andrena is gregarious, so that we may find hundreds and hundreds of burrows together, we must still call it a solitary bee. Its life history is much simpler than that of the truly colonial bumblebees and can serve as an example of the life cycle of many other species. After all, social life in the group of the bees is by no means general, although it certainly is a striking feature. On the basis of its life history, we like to think that Andrena is more primitive than the bumblebees. The way in which it transports its pollen is not so perfect, either. It lacks pollen baskets and possesses only a large number of long, branched hairs on its legs, on which the pollen grains will collect. Still Andrena will do a reasonably good job, so that an animal with a full pollen load looks like a gay little piece of yellow down floating in the wind.

Closely related to the andrenas are the nomias or alkali bees. Nomia melanderi can be found in tremendous numbers in certain parts of the United States west of the Great Plains, for example, in Utah and central Washington. In the United States Department of Agriculture's Yearbook of Agriculture, 1952, which is devoted entirely to insects, George E. Bohart mentions a site in Utah which was estimated to contain 200000 nesting females. Often the burrows are only an inch or two apart, and the bee cities cover several acres. The life history of the alkali bee is similar to that of Andrena, but the first activity of the adults does not take place until summer, and the individuals hibernate in the prepupal stage. In most places, there are two generations a year, a second brood of adults appearing late in the summer.