Body snatchers and pollen thieves: bee nurseries are other insects’ cafeterias by Holly Holt, PhD

Bee nests are full of tempting food resources for parasites, parasitoids and predators. Nests house immature bees and stockpiles of pollen and nectar. Immature bees and their food provisions are both rich in nutrients, and parasitic species have learned how to exploit these resources for their own reproduction.1

Parasitoids are body snatchers

Parasitoid wasps and flies use other insect's bodies as incubators. Parasitoids lay their eggs inside or on other insects or arthropods (e.g., spiders). After hatching, the larval parasitoids feed on their host. These grisly feeding habits allow the parasitoids to mature, but also culminate in the host's death. Many parasitoid species help control important garden and agricultural pests. However, select parasitoid species attack native bees. For example, some wasps in the genus Monodentomerus attack mason and other native bees.

Mason bees (Osmia spp.) are solitary, but females may nest near each other. Members of this genus are excellent orchard pollinators2 and are commercially sold to fruit producers and gardeners. Mason bees nest in hollow twigs or tunnel-like rocky crevices. Their mud-lined nest cells are aligned end-to-end along the length of the cavity. Females begin cell construction at the back of the nest cavity. After stocking a cell with a pollen loaf held together with nectar, the female lays an egg on the loaf, seals the cell with mud and begins work on the next cell. She repeats this process until she seals the final cell at the nest entrance.1

Monodentomerus wasps are so small that they are easily be mistaken for gnats. After locating a mason bee nest, female wasps use their needlelike ovipositors to inject eggs into bee larva. Instead of producing an adult bee, each parasitized nest cell yields tiny adult wasps. Because of their short maturation period, wasp adults can parasitize other bees in the same nest or chew their way out in search of new nests.3

Social bees are not exempt from parasitoid attacks. For example, Apocephalus borealis is a phorid fly species native to North America. It has historically parasitized bumble bees and paper wasps. However, this fly has expanded its host range to include honey bees, posing yet another threat to colony health.4 Parasitized bees behave strangely; they take night-time flights and gather around lights, earning them the name “ZomBees" (and the parasitoid the nickname “zombie fly"). Female zombie flies lay their eggs inside a host bee's abdomen, and the hatched fly larvae feed internally. When the fly larvae are ready to pupate, they emerge from between the bee's head and thorax, leaving the dead bee behind. (Learn more about zombie flies, see pictures and watch a Ted Talk.5)

Cleptoparasites are thieves

Cleptoparasites lay their eggs inside bee nests, and the parasitic larvae hatch quickly. The larval parasites eat the pollen loaves intended for the immature bees and sometimes even eat the developing bees. If not killed outright by the parasite, the bee larvae starve because their food was stolen. Many types of insects and arthropods are cleptoparasites including mites, flies, wasps and even other species of bees.1

In the February of 2020, the Washington State Department of Agriculture announced6 that a well-known European cleptoparasite, dubbed the Houdini fly (Cacoxenus indagator), was introduced to the Pacific Northwest and is now likely established in Washington.6 The Houdini fly parasitizes mason bees and earned its name because of its remarkable method of escaping from bee nests.7 Female flies lay their eggs in nest cells before they are sealed. The fly larvae quickly hatch and eat the pollen stores intended for the bee larvae. Following pupation, newly emerged adult flies have a brief window of time to escape from the nest interior before their bodies harden. These flies use an anatomical adaptation called a “head blister" (ptilinum) to accomplish Houdini-worthy escapes. Flies find a small crevice in in the nest's mud partition, insert their deflated head blisters, and then pump the blisters full of blood. The rapid expansion of the blister exerts hydraulic pressure, causing pieces of the partition to crumble. Repeated inflation creates a small exit hole that flies can squeeze their soft bodies through. (Learn more about and see footage of Houdini flies8).

How can you protect bees from parasitoids and cleptoparasites?

In North America, diverse parasitoid and cleptoparasitic species naturally coevolved with their native bee hosts. The ongoing decline of managed and wild bee populations, however, is likely altering the dynamics between bee hosts and their parasites. Introduced parasitoids and cleptoparasites, or those that adapt to exploit new bee species as hosts, are particularly threating because newly parasitized bee species often lack natural defenses.

With the decline of honey bees, human efforts to support, manage and exploit wild bee populations have become increasingly popular. Human activities range from local nest box or 'bee hotel' installations to commercial enterprises that rear, harvest and sell native bee species. While most human activities are done with the best intentions, poor management of artificial bee nests can lead to parasite and disease build-up. Future studies are needed to determine which human practices are most beneficial as there is little data yet to prove that building artificial bee nests actually supports wild bee populations.3,9 Instead, artificial bee nests and hotels may be preferentially used by introduced bee species and native wasps, rather than native bees. Furthermore, artificial nests often sport a variety of cavity sizes with the intention of attracting many different bee species. This may lead to increased pest and disease transmission between bee species that would not normally nest near one another. Finally, some human practices, like the placement of “trap nests" on public lands to capture and sell wild bees, are ill-advised and damaging to bee populations.10

  1. Be a good bee host. Learn how to provide natural nesting habitat for bees. If you choose to build (or buy) a bee nest box or bee hotel, it is your responsibility to follow best management practices, including cleaning or replacing nest materials at appropriate times to remove parasites and microbial pathogens. Bee nests can also temporarily be relocated to uninsulated buildings during certain times of the year to protect bees from the elements, parasites or predators. Familiarize yourself with the bee parasites that attack bee species in your area; when appropriate screen nest contents and remove parasites.
  2. Do not move bee nests or sell bees outside of their native range. Long range nest movement can spread bees and their diseases and parasites to nonnative ranges, causing problems for local native pollinator populations. If you are purchasing bees, make sure that the business uses best rearing practices (including screening for parasites and disease) and that the business is only selling you species that are appropriate for your geographic location. If you have questions, contact your local extension service for more information.
  3. Participate in citizen science. Help track zombie flies in honey bees through ZomBee Watch. If you decide to install an artificial bee nest or hotel, determine if there are ongoing citizen science projects that are requesting data from artificial bee nests. The Minnesota Bee Atlas is one example. Many other citizen science programs request pollinator observations in the field. Check this list for more information.


  1. Wilson, J. S. & Messinger Carril, O. J. The Bees in Your Backyard. The Bees in Your Backyard (2016). doi:10.1515/9781400874156

  2. Biddinger, D. Orchard Pollination: Solitary (Mason) Bees. Penn State Extension (2018). Available at: (Accessed: 1st March 2019)

  3. Macivor, J. S. & Salehi, B. Bee Species-Specific Nesting Material Attracts a Generalist Parasitoid: Implications for Co-Occurring Bees in Nest Box Enhancements. Environ. Entomol. 43, 1027–1033 (2014).

  4. Core, A. et al. A New Threat to Honey Bees, the Parasitic Phorid Fly Apocephalus borealis. PLoS One 7, e29639 (2012).

  5. Casuso, N. A., Mortensen, A. N. & Ellis, J. D. Featured creatures: zombie fly. University of Florida (2014).

  6. Washington State Department of Agriculture Polliinator Health Program. Pest Alert: Houdini Fly, Cacoxenus indagator. (2020).

  7. Strohm, E. How can cleptoparasitic drosophilid flies emerge from the closed brood cells of the red Mason bee? Physiol. Entomol. 36, 77–83 (2011).

  8. WDSA Pollinator Health Program. Houdini fly found in Washington. Washington State Department of Agriculture (2020).

  9. MacIvor, J. S. & Packer, L. 'Bee Hotels' as Tools for Native Pollinator Conservation: A Premature Verdict? PLoS One 10, e0122126 (2015).

  10. Tepedino, V. J. & Nielson, D. Bee-Rustling on the Range: Trap-Nesting for Pollinators on Public Lands. Nat. Areas J. 37, 265–269 (2017).