Spooky Behaviors of Pollinators: The Curious Lives of Parasitic Bees

Fall is here and along with the pumpkins and falling leaves, there is one thing that pops up everywhere: Halloween! And because I can’t add Halloween decorations to a blog post, this blog will have a “conceptual” Halloween twist. Today, I want to talk about something that may seem spooky to many, but that to me showcases the stunning diversity of (pollinators’) life. In today’s post we will talk about bees that are a bit “special”: parasitic bees! Come along and marvel with me about these incredible creatures that coexist with us right here in Maryland!

Parasitic bees? What!?

Yes, you read it right. Although most of the bees we know are solitary and build and provision their nests, there are several groups that have taken an evolutionary path a bit different from their relatives. These bees have evolved parasitic behaviors, exploiting the nests and food from other bee species, and in the process actively killing the host’s brood. Because they display behaviors similar to cuckoos, birds who lay eggs in other birds’ nests and have their chicks reared by the host parents (check out this video), these bees are known as cuckoo bees.

Parasitic (right) and non-parasitic (left) bees look very different. One of the main differences is the fact that parasitic bees do not have structures to collect pollen (like hairy legs with pockets), as we can see in these photos. Photos: J. Gallagher.

Cuckoo bees look different from non-parasitic bees

Because these bees have evolved to not collect nectar or pollen for provisioning (adults do eat nectar and pollen, though), and do not build nests, they have also lost the morphological structures that allow bees to do so. Cuckoo bees thus lack all the structures commonly present in bees that collect pollen (e.g., little pockets on their legs, hairs), and all the structures that allow bees to collect materials and build nests. Unlike non-parasitic bees, who often can lay only one egg per day, cuckoo bees can lay many eggs on the same day. This adaptation allows them to take full advantage of a suddenly-available nest they can parasitize. Finally, as one can imagine, host bees are not super happy about having other bees come and exploit their nests… and they defend them! For this reason, cuckoo bees are strongly “armored”, with thick and bulky structures that can protect the parasitic females against the likely attacks from the host bees. And, last but not least, some cuckoo bees can camouflage using body odors that are similar to the host, which allows them to enter the nests without being “smelled”. Cool, heh?

But how do they do it?

A trait common to all these bees is that they have high levels of specialization on what other species they parasitize, meaning that one parasitic species will often parasitize a relatively small group of closely-related non-parasitic bees. For this reason, there are different methods cuckoo bees use to parasitize their hosts.

Larvae in many cuckoo bees are equipped with impressive mandibles that they use to attack and kill other larvae developing in the parasitized brood cell. Look at these weapons! Image: Rozen et al., 2019; American Museum Novitates.

The first main way is parasitizing brood cells that have been already closed. In this group, the females enter a foreign nest where closed cells are present, open the cell(s) where they want to lay the egg, kill the host’s egg with their sting or mandibles, then lay an egg in the (now empty) cell and close it. Other species that also parasitize closed cells are those in which the females open the cells, but instead of killing the host egg, just lay theirs in the cell before sealing it back. In this case, it is not the female but the larvae that will kill the host egg/larva. These parasitic larvae have strong mandibles that allow them to attack the resident larvae and kill them, keeping all the resources for themselves. Finally, other cuckoo species do not wait until the host cells are closed. The females of these species enter nests where cells are still open, and lay their very small and hard-to-see eggs in the open cells. The host female often oversees them and closes the cells with the parasitic egg in it. The parasitic larva develops in the closed cell and also uses its strong mandibles to attack and kill the host larva while in the cell.

Do parasitic bees exist in Maryland?

Yes! Although these life histories may seem like they are coming from another planet, we do not need to travel to exotic places to be able to encounter these species! They also occur right here!

The small Macropis cuckoo bees Epeoloides pilosula are very rare in Maryland and protected in most of their North American range. Photo: M. Veit.

A very cool species that is very likely present here in Maryland is the Macropis cuckoo bee Epeoloides pilosula, which parasitizes nests of the oil-bee of genus Macropis. Because of the level of specialization of both the oil-bee (on their host plant; see here to learn more) and its parasite, E. pilosula is very rarely encountered and is currently protected at different levels in Eastern North-America.

There are several species of Nomada or Nomad cuckoo bees in Maryland. These species often parasitize nests
of ground nesting bees. Photo: M. Lucas.

Another very neat example of local cuckoo bees are the parasites of Andrena and other mining bees: the parasitic bees of genus Nomada. The rule of lack of hairs and structures to collect provisions for the nest is very much true for this species! There are about 30 species of this genus known to be present in our state, and many of them are rare. While the spotted cuckoo bee Nomada maculata is somewhat regularly found in the state, Nomada bethunei is known only from a couple localities. Most of these Nomada species are, however, rare and often under conservation threats.

Will parasitic bees drive other bees to extinction?

Parasitic and non-parasitic bees have been co-evolving for millions of years, and it is very unlikely that this type of interaction would drive species to extinction. Indeed, the parasitized species also have evolved ways to protect their brood (something for another post). Interestingly, however, because parasitic bees are so specialized on their hosts, it is they who may be even more at risk of extinction than their hosts! Indeed, cuckoo bees are rare, hard to find, and are likely to have populations die out as soon as their host species disappears from a locality. From this respect, and if we want to protect the diversity of this super cool group of pollinators, providing resources for them and their hosts (see this and this post to learn some ways to do this) is key to maintaining the populations of these rare and fascinating parasitic bees!

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here, extensionesp.umd.edu, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

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