Are leafcutters solitary or social insects?

Context: Solitary bees, such as leafcutters, do not form colonies. Unlike social insects, leafcutters work alone building isolated nests. Similar to honeybees, female bees perform nearly all essential tasks of brood rearing. These native insects perform essential tasks, pollinating wild plants. The alfalfa leaf cutter bee, native to Europe, has been semi-domesticated for crop pollination. In North America, the species was deliberately imported to assist in the pollination of food crops, but has now become feral and widespread.

Answer: solitary