*Two-banded cellophane cuckoo bee (Epeolus bifasciatus) by Amanda Barth.*

The growing season is in full swing — warm temperatures and long, sunny days — and you can almost guarantee that your garden's flowers will be bustling with a variety of busy bee species gathering nectar and pollen for their offspring. Bees have structures and specialized hairs that allow them to collect and carry pollen, and depending on the family, these structures could be located on the hind legs, the abdomen, or even internally in a special compartment of the stomach. These bees also spend a lot of time and energy seeking out floral resources, nest building, and whatever else necessary to ensure the success of their next generation.

Cuckoo bees defy this conventional archetype — they are cleptoparasites!

Rather than to toil away building and provisioning nests for her young, a cuckoo bee will instead rely on the efforts of other bees, and lay her egg in another bee's nest. She does not have specialized hairs for gathering pollen, but instead may have unique structures that allow her to invade nests, avoid detection, or put up a fight if discovered by the owner of the nest. She spends her time flying low to the ground searching for nest entrances, hanging out on flowers to drink nectar and spot potential hosts, or waiting outside a nest entrance for the female to leave before she enters and lays her eggs. Her larvae may hatch earlier and eat the food stores gathered by the host bee, or they might have large mandibles that enable them to kill and even eat the host's larva. This is brood parasitism, an intriguing strategy more commonly associated with their wasp cousins — in fact, cuckoo bees are often mistaken for wasps. The cuckoo habit has evolved independently in many bee families, and cuckoo bees often belong to the same family as their host species (but not always).

*Nomad bee (Nomada sp.) by Amanda Barth*

Notable examples are bees in the subfamily Nomadinae (family Apidae) which are all cleptoparasites, and often have host bees in other families. The most familiar and species-rich genus is Nomada, with diverse host associations to many kinds of sweat bees (family Halictidae) and cellophane bees (family Colletidae). Nomad bees range in sizes but all are nearly hairless, with narrow bodies and vivid red, black, and yellow colors — very wasplike. The cuckoo leafcutter bees in genus Coelioxys (family Megachilidae) specialize on leafcutter bees in genus Megachile (also family Megachilidae), and look very similar to their hosts except that they also possess a sharply pointed abdomen and distinct spines on their tail end. In general, cuckoo bees are rarely seen because of their unique behaviors and their density depends on the availability of their hosts. This makes them extremely difficult to study or monitor, and consequently limited information is available to understand their diversity, distribution, and conservation needs.

*Sharptail bee (Coelioxys sp.) by* *Klaus Bielefeldt*

Cuckoo bumble bees are considered social parasites because the queen must invade the nests of host bumble bees, kill the queen, and integrate herself into the functioning colony to lay eggs that the workers will raise. A cuckoo queen needs to be equipped with strong armor, mandibles, and stinger, and she must be selective when choosing a nest to infiltrate — enough workers to raise her offspring but not so many that they can successfully defend their nest against her. Cuckoo bumble bees don't have workers of their own, so they are not as easily noticed foraging on flowers throughout the growing season. The likelihood of detecting a cuckoo bumble bee depends on the success and abundance of its host species in an area, effectively making cuckoo bumble bee species indicators of the health of the local bumble bee community.

For instance, the indiscriminate cuckoo bumble bee (Bombus insularis) is less restricted to host species than other cuckoo bumble bees. They have been recorded parasitizing 6 other Utah Bombus species: white-shouldered bumble bees, golden northern bumble bees, yellow-fronted bumble bees, Nevada bumble bees, red-belted bumble bees, and even western bumble bees.

*Indiscriminate cuckoo bumble bee (Bombus insularis) by Amanda Barth*

In just the last two and a half months, Utah community scientists have submitted more than 90 observations of this species around the state. For comparison, that's more than the total number of indiscriminate cuckoo bumble bee observations from 2020-2023 combined (67 in four years!). While it's a rough estimate of bumble bee community health, it can still be interpreted as a positive sign of increased resources like precipitation and flowering plants in their habitat.

*Longhorn cuckoo bee (Tripeolus sp.) by Amanda Barth*

Are cuckoo bees important to the ecosystem? Absolutely! They may not be the most effective pollinators because they aren't gathering pollen, but their need for energy-rich nectar means they visit flowers often and still move pollen grains that stick to their bodies — just like butterflies, moths, and wasps. They are potential prey for many insectivorous animals, and may also be parasitized by wasps, mites, and flies. And while public sentiment may not look as kindly on parasitic lifestyles, it's important to remember that half of the species on Earth are parasites of some sort. Parasites contribute to the complexity of the food web and the ecosystem by helping keep host populations in check and acting as selective pressures that contribute to overall genetic diversity of their hosts. Every living thing is a potential meal for something else. Remember, "Life, uhhhh...finds a way."

If you are lucky enough to spot a cuckoo bee, and are luckier still to get photos of that bee, take comfort that this bee represents wonderful things about the ecological relationships playing out all around you! And then submit those pics to iNaturalist!

The Unique Lives of Cuckoo Bees

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