Native Bee Spotlight: Cuckoo Bees ~ Coelioxys spp.

Cuckoo Bees ~ Coelioxys spp.
A female cuckoo bee, Coelioxys sp. nectars on
hairy false goldenaster, Heterotheca villosa in late fall
There are many types of cuckoo bees in North America. In the Coelioxys genus, there are approximately 46 speces. The common name "cuckoo bee" is typically used for any bee species that lays its eggs in the nests of other bees. These bees are known as cleptoparasites, where the cuckoo bee larvae kill the host larvae and feed on the provisions (pollen and nectar) provided by the host bee.

Coelioxys cuckoo bees are common in the summer months; in central Minnesota I typically see them from June until October. Both males and females can be observed visiting flowers for nectar and females looking for, or waiting to enter a host's nest. These cuckoo bees lay their eggs in the nests of leafcutter bees, Megachile spp. Less frequently, they have been documented laying eggs in the nests of Anthophora, Centris and Euglossa spp.

A female cuckoo bee watches for the
female leafcutter bee to exit the nest
in the rock cavity.
The cuckoo bee flies closer to the entrance
anticipating the exit of the host bee.

Females Coelioxys bees actively look for a host's nest. Once a nest is found, the female cuckoo bee waits until the host bee leaves the nest to collect provisions. With a short window of opportunity, the cuckoo bee slips in the empty nest and looks for a fully provisioned brood cell to lay its eggs in.

The host leafcutter bee, Megachile sp. enters the nest in the
rock cavity carrying a piece of leaf to line or cap the brood cell.
Females have a sharply tapered or triangular-shaped abdomen. This acute point on the end of the abdomen is used to pierce through the layers of leaf pieces that line the brood cells of their host, leafcutter bees. The egg(s) is laid within the layers of leaves, or underneath the pollen mass hidden from sight in case the host leafcutter bee is still in the process of provisioning the nest. The cuckoo bee eggs are often different in size or appearance which may be another reason why the cuckoo bee hides the eggs.

The large sickle-like mandibles that the cuckoo bee larvae use to kill its siblings and host larva.
Illustration from: Michener, C. D. (2000). The bees of the world (Vol. 1). JHU Press.
The menacing part of this cleptoparasitic life cycle occurs after the cuckoo bee larva has hatched and begins to develop. The larva develops large sickle-shaped mandibles that are directed forward (instead of downward) to prepare to kill the host egg or young host larva. By the third or fourth instar, the Coelioxys larva has killed any sibling larvae and the host. It now has an empty brood cell stocked with pollen and nectar provisions to feed on and develop.

A female cuckoo bee nectars on
purple prairie clover, Dalea purpurea
The timing of adult emergence of cuckoo bees is very critical; there is a short window of opportunity to overlap the timing of the host's adult emergence. Coelioxys spp. have been documented emerging slightly earlier or around the same time as their host. A larvae will often develop into an adult the same year as the nest is constructed and more female cuckoo bees are produced earlier in the season.

A female cuckoo bee perches on foliage low to the ground
watching for a leafcutter bee to emerge from a nest in the ground.
Female: Tapered abdomen ending in an acute point
Male: Pronged or multi-spined abdomen
Both Male & Female: Hairs on the bottom of the eyes
Relatively hairless, dark gray - black, often pocked appearance
Many species have red legs

Native plants I have observed Coelioxys spp. foraging on:
Butterfly milkweed, Asclepias tuberosa
Wild Bergamot, Monarda fistulosa
Prairie Coreopsis, Coreopsis palmata
Hoary Vervain, Verbena stricta
Joe Pye Weed, Eutrochium maculatum Smooth Oxeye, Heliopsis helianthoides Black-Eyed Susan, Rudbeckia hirta
New England Aster, Symphyotrichum novae-angliae
Hairy False Goldenaster, Heterotheca villosa
Purple Prairie Clover, Dalea purpurea 

Baker, J. R. (1971). Development and sexual dimorphism of larvae of the bee genus Coelioxys. Journal of the Kansas Entomological Society, 225-235.

Michener, C. D. (2000). The bees of the world (Vol. 1). JHU Press.

Rozen Jr, J. G., & Kamel, S. M. (2006). Interspecific variation in immature larvae of the cleptoparasitic bee genus Coelioxys (Hymenoptera: Megachilidae). Journal of the Kansas Entomological Society, 79(4), 348-358.

Scott, V. L., Kelley, S. T., & Strickler, K. (2000). Reproductive biology of two Coelioxys cleptoparasites in relation to their Megachile hosts (Hymenoptera: Megachilidae). Annals of the Entomological Society of America, 93(4), 941-948.