Bees to look out for: leaf-cutter bees!

Spring is well-established and many flowers have already started to bloom. In my garden, I have seen several sizes and shapes of insects visiting flowers, going from small flower flies to butterflies, to very tiny and shiny, fuzzy large, and very large bees. And with all these flying organisms starting to come around us, I thought today would be a really good time to introduce you to some really cool bees that are very common in our area: the leaf-cutter bees!

What are leaf-cutter bees?

As its name suggests, these are solitary bees known to cut leaves (now you may be thinking, “duh, I could have guessed that without a blog post”, but bear with me!). These bees belong to a very large family of bees called Megachilidae, which is present on all continents except Antarctica and well-represented in our region.

Like most solitary bees, the female of leaf-cutter bees builds nests with small brood cells, in each of which a food provision is left and one egg laid. And this is where the “leaf-cutter” name comes from. When building their nests, many of these females line their brood cells with specific materials, in particular plant tissues. In fact, many of the species are known to cut leaves and/or petals to line their nests, using them to stabilize the brood cells, and likely to provide protection to the larva and the food provisions. In a fascinating way, it is suspected that these bees are able to exploit the antimicrobial effects of certain compounds present in these flowers and leaves, indirectly using them to protect their offspring until they finish their development in the nests.

If you ever saw neat and relatively large holes that seem to suddenly pop up on certain plants in your green spaces, it is very likely that they were made by some of these bees that may be nesting close to you! If you keep an eye out on those plants, it is very likely that you will end up seeing these busy bees carefully cutting, then rolling, and finally flying away with the neat plant circles!

Female leaf-cutter bee cutting a leaf
Leafcutter bee nest and brood cells made with leaf pieces
Megachilids are known for cutting leaves that they use to line the brood cells of their nest. Note that here the nest is in a soil mound and each brood cell is completely covered with leaves but capped and separate from the neighboring cells. In this nest, each cell contains one egg. Photo: E. Soh.

Although this group of bees receives its name from cutting leaves, not all of them do this. In fact, this large family of bees is extremely flexible in their use of resources for nesting, and some of them are even parasites of the nests of other bees! Species in this group are able to use many materials to build their nests (e.g., mud, plant materials, flowers, resins), and they tend to utilize many different types of cavities (e.g., twigs, crevices, holes in walls or in rocks, or even empty snail shells!) to establish their nests. This huge flexibility in terms of nesting requirements may explain why they are so abundant and present virtually globally. This flexibility is also probably why this is a group of bees that we can see regularly in the green spaces we humans create. Interestingly, because many of the species seek cavities, this is a group of bees that usually nests in bee hotels, where holes are provided for them to build their brood cells.

Megachilids nest in a variety of places: bee hotels, mud nests, or empty snail shells
Megachilids nest in a variety of places and use many different materials. Many nest in cavities such as those offered by bee hotels (top, left; J. and F. Hort; note the characteristic abdomen hairs), build mud nests (top, right; C. Leel), or use natural cavities such as those offered by empty snail shells! (bottom, Müller et al., 2018).

These bees are so neat! How do I recognize them?

Species from our area are medium in size, about the size of a honeybee with some species smaller and some larger. Although each species has slightly different coloration and size, a very easy way to recognize bees of this family is a specific structure that is only present in this group. The majority of the bees in this group display a characteristic series of hairs on the ventral part of their abdomen, a structure called a scopa. This structure is central to their ability to carry pollen to their nests in an efficient way because this “hairy belly” allows for A LOT of pollen to stick. Any yellow-bellied bee that one would see is certain to be a member of this family, with all the yellow being a load of pollen that will be carried to the nest to eventually feed the larvae.

A Megachilid bee with its abdomen full of pollen
It is common to see Megachilids with their characteristic abdomen hair full of bright yellow pollen. Image: I. Boyd.

And because they can carry so much pollen, they are usually really good pollinators. To release a maximum amount of pollen from the flowers, these bees move a lot when visiting them, making clear contact with the anthers and actively releasing pollen. Because of this high level of activity, these bees are often very good at not only releasing but also depositing pollen on the flowers, which makes them very good pollinators. Although many of these bees visit many wildflower species and even crops, some are more selective in their flower visitation, visiting specifically different Asteraceae, beardtongues (Penstemon), or blue- and huckleberries.

Our local leaf-cutter – The flat-tailed leaf-cutter bee Megachile mendica

two photos of flat-tailed leaf-cutter bees
The flat-tailed leaf-cutter bee is very common in our region, with it visiting a variety of flowers, and using its sharp mandibles to cut leaf circles. Images: snanz; H. Holbrook.

There are several species of this family present in our area, with the flat-tailed being one of the most common. This species is very widespread in the USA and is usually seen in the mid-Atlantic during the summer and early fall. It is a relatively small species, measuring about half an inch and often nesting in the soil. The species is known to be pretty generalist in terms of its floral choices, being attracted to many different types of plants. These leaf-cutters do cut leaves, so if you see circular holes in plants in your garden during the summer, it is very likely that these bees are responsible for that! Keep checking your plants and maybe you’ll get a sneak peek of them at work! 😊

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!

Leave a Reply

%d bloggers like this: