Genus: Stelis Panzer, 1806
Subgenera: Dolichostelis, Heterostelis, Malanthidium, Protostelis, Pseudostelis, Stelidomorpha, Stelis
Common name: cuckoo bees
Stelis are cleptoparasitic bees, ranging from metallic blue or green to black in color (Michener 2007). They may sometimes have contrasting orange, yellow, cream, or white-colored maculations to the abdomen and thorax (Michener 2007). They are generally stout-bodied and range in body length from 3–14 mm. Females do not gather pollen and lack scopae (Michener 2007).
Stelis contains 100+ species in 7 subgenera worldwide; 51 species in 3 subgenera are known to occur in the U.S. and Canada (Wilson and Carril 2016).
(modified from Michener 2007)
Stelis can often look very similar to their hosts. Some have yellow bands like many anthidiines, while others are solid blue and look very similar to Osmia. Others may be quite small and black like Ashmeadiella (Michener 2007). They can be differentiated by the diagnostic characteristics listed above.
There are no known invasives.
Females do not gather pollen from flowers, since the larvae develop cleptoparasitically on their host’s pollen provisions (Michener 2007). A variety of flowers are used for nectar and nesting resin (Parker et al. 1987).
These are one of the various groups of solitary brood parasites collectively referred to as “cuckoo bees.” Stelis are cleptoparasitic on a wide range of other megachilid bees (Parker et al. 1987; Wilson and Carril 2016). Some groups may specialize on a limited range of hosts, such as the subgenus Heterostelis, which parasitizes Trachusa spp. (Michener 2007). Eggs are laid inside the host’s nest cell as the female is provisioning the nest (Michener 2007). The pharate 1st instar larva undergoes its first molt while still inside the egg. Sometime after eclosion, the larvae will use their robust pointed mandibles to destroy the host egg or larva and feed on the pollen stores of its host (Michener 1955). In some Stelis, like subgenus Dolichostelis, the female parasite destroys the Megachile sp. host egg before oviposition (Michener 2007).
Stelis is a large genus of bees that occur primarily in the Holarctic, with a few members found in warmer climates such as Costa Rica, Malaysia, Kenya, and South Africa (Michener 2007). They are transcontinentally distributed in the U.S. and Canada and are even found at latitudes above the Arctic Circle (Michener 2007; Wilson and Carril 2016).