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.
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What are local ecotype plants and why do they matter to pollinators?

With the planting season upon us, many of us are starting to think about what flowers may be the best for our gardens and pollinators. We may have started to look into floral mixes or even flower starts, but probably there are too many choices and now we’re overwhelmed and don’t know what to do. In previous posts, we talked about the importance of diverse floral choices and how appropriate native species are when choosing plants for pollinators. There is, however, an extra twist that is becoming more mainstream in this story and today I want to talk about it. Let’s chat about local ecotypes, what they are, what they contribute, and how to get them (and how to not get them).

What are local ecotypes?

In a few words, local ecotypes are native plant species that have a genetic background typical for the local region and adapted to it. I know, there were a lot of technical words in that sentence, so let me break it down to make it easier to understand.

Like all organisms, plants have lineages that reflect their ancestry. In the same way that we as humans are genetically more closely related to members of our own family than to those of other families, plant populations are also more closely related to other plants of the same species that live close to them. From a genetic point of view, this means that plants that come from regions close to each other will tend to have more similar genetic characteristics than those from regions far apart from each other. This genetic makeup specific to a given region is what we call broadly a local genotype.

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Helping pollinators in small green spaces

Spring is almost almost aaaaaalmost here, and if you’re like me, you have already started visualizing what flowers will grow where and what pollinators you’ll need to keep an eye out for. Unlike in other posts, where we talked about how to help pollinators in large spaces, today we’ll talk about how to help them in very small yards, balconies, porches, or other small spaces.

small garden in front of a town home
Having a small yard is no reason to not help pollinators. Small yards can be great spaces to support them! Photo: G. Cripezzi.

Small yards

If you have access to a small yard, plenty of opportunities are available! Of course, you will not be able to plant lots of large plants, but that doesn’t mean you cannot plant anything. When offered little space, you can use not just the horizontal, but also the vertical space. While it is possible to cover the ground with a mix of perennials and annuals, there are also possibilities of installing trellises on which flowering vines can grow.

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Planning your garden to support specialized pollinators

Looking out my window, as the ground is covered with snow and I am getting ready for another snowstorm coming tonight, it seems ironic that I have been spending many hours these days ordering seeds and planning my garden. While I am thankful that the winter brings some rest to the soil in my garden, planning this season brings me happy memories of the scents and buzzes in my yard during the growing days… which reminds me that I should also plan for my little buzzing pollinator friends when I plan what to grow this season. In today’s blog, I want to chat about how we can plan for many types of pollinators, with a special focus on planning for specialists and not just for generalist pollinators.

Specialist pollinators – never heard of them?

As we mentioned in a previous post, pollinators visit plants to feed on nectar and/or to collect pollen to feed themselves or their offspring. However, pollen is not just there for pollinators to feed on; pollen is central to plant reproduction, so plants tend to make it both attractive to pollinators but hard to digest. For this reason, and in order to be able to properly digest the pollen, pollinators are often specialized in their pollen choices. This is because being able to digest the compounds that plants add to their pollen to make them hard to eat requires some level of adaptation, which often involves a trade-off with the ability to eat anything. There are, of course, many levels of specialization, and, while many pollinators feed on many plant families, others are more specialized than that, and feed on only specific plant genera or even species! For us gardeners, this means that if we want to support many different pollinators, we need to make sure that we are also providing for those very specialized pollinators as well!

Luckily for us, the floral choices and pollen specialization is known to some extent for Maryland and Eastern USA bees (see this site to learn more). For this reason, we know that many specialized bees in our region are also rare or uncommon… another reason to try to provide resources for them!

Who are pollen specialists in our region?

Many known pollen specialist bees in our region belong to bee genera Andrena, Colletes, Osmia, and Melissodes, which have many species considered rare or uncommon in Maryland and Mid-Atlantic.

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How do pollinators find plants and flowers?

As we know, pollinators help plants spread their pollen among flowers, and many plants do indeed need them to be able to reproduce and set seeds. We also know that by planting flowers and providing nesting habitats, we can help pollinators’ populations and thus assist with plant pollination. However, how do pollinators find plants? In this post, we will talk about that topic, which can help us become even better at helping pollinators and the plants they pollinate.

The big picture – pollinators need to be in the area

In order for pollinators to find plants, pollinators need to be present in the general region. In fact, although the vast majority of pollinators can move and travel from place to place, all of them have limitations on the distance they are able to travel. For example, hummingbirds can travel for miles (in Maryland, they are migratory), while large bees are able to travel relatively large distances for an insect (~500m-1km), and smaller insects will not be able to travel that far. This means that if, say, we lived in the middle of a very developed area with very few pollinator-friendly resources (few flowers, lots of cement, no green areas, etc.), planting a pollinator garden will attract few pollinators at first. This is due to the fact that it is likely that few pollinators are present in that area, and thus it will take a while for certain groups to arrive and establish in our garden.

It is for this reason that many communities tend to try to establish joint pollinator-friendly actions, and encourage many people in the region to participate (e.g., becoming Bee City USA-certified, creating “pollinator highways or corridors”). By increasing the regional number of pollinator-friendly resources, the whole region becomes more pollinator-diverse, and any supplementary action is more likely to improve pollinator support. As we talked about in a previous post, if you are interested in promoting pollinator-friendly habitat on your property, it may be a great idea to talk to your neighbors or your City, and see if others may also want to participate. In terms of pollinator-friendly activities, the saying “the more, the merrier” is very much true!

pollinator habitat sign in a garden
Pollinator-friendly actions are very effective when they are coordinated across regions. Photo: A. Kokai.

The local picture – different pollinators prefer different plants

As we mentioned in other posts, not all pollinators are made equal, and this is also true in terms of what plants will be found by what pollinators. For example, hummingbirds tend to visit tubular and reddish flowers, while syrphids prefer open flowers, and bees tend to visit flowers that they can access with their mouth parts (see this post to learn more). 

These floral preferences are due to the different pollinators’ abilities to see different colors, the presence of specific attractive floral scents in different plant species, and the ability of different pollinators with different body and mouth part shapes to handle and feed on flowers, and the matching of pollinator presence and flowering time. The practical consequence of this is that if we want to help many different pollinators find their preferred plants, it is necessary to grow different types of plants in our green spaces. By doing this, we would always provide resources that will be preferred to at least one pollinator, and by providing different types of resources, we can make sure that many different types of pollinators are supported by our plants. In order to do this, there are different floral mixes that exist that allow us to plant diverse floral resources appropriate for our region, which lets us build a diverse and welcoming floral bed for many pollinators.

Planting diverse floral resources will attract many different types of pollinators. Photo: C. Celley/USFWS.

The super-local picture – pollinators need to see the plant to access it

This will sound silly, but pollinators need to be able to have access to the plant to find it. For example, if a plant is not clearly displayed or hidden by many other plants or structures, it will be hard for pollinators to find it… even if the pollinator is present in the area and the plant in question is a preferred plant. This means that for us to help pollinators, we need to make sure that our plants are findable by the pollinators. Picking appropriate parts of our green spaces to plant our pollinator-friendly plants is thus key! For example, plants that require full sun to grow should be planted in those conditions and not under the shade of other plants or behind structures.

To know what these specific conditions are, there exist several resources (for example, see this useful and simple resource (PDF) published by the City of College Park, MD). These resources allow us to pick the best growing spot for our plants, making them easily findable by their pollinator friends.

Finally, pollinators are more likely to find plants if there are several of them! This is particularly true for smaller herbs, which may not display many flowers. By increasing the number of plants planted in an area we are also making the plant species more easily findable to the pollinators.

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!

Introducing the king of fall fruits: persimmons!

It’s fall, the air is starting to get crisp, you are walking and you see these strange trees. You are sure that they are trees, yet they have these fruits that look like tomatoes… but they are on a tree. What are those fruits? They look so attractive with that wonderful orange… should you harvest them? Should you eat them? Are they any good? What IS this? In today’s post, I want to (re)introduce you to these plants and their fruits, and hopefully the next time you see them you will have answers to all those questions and will know what to do. 😉

persimmon tree in a yard
In the fall, persimmon trees are recognized for their many orange fruits that look like tomatoes! Photo: P. Tain.

What are these orange fruits?

These fruits are what here in the USA we call persimmons (from the Powhatan word “pichamin”). Persimmons are the fruits of a group of trees that belong to the same family as ebony, and that can be found on a number of continents, including North America. Among all the persimmon species that exist, a number of them are edible, producing fruits in late fall. In the USA, there are two persimmon species that produce edible fruits, and one of them is native to right here in Maryland: the American persimmon (Diospyros virginiana).

Although the wild American persimmon still grows in our forests and was well-known by native Americans, who used its hardwood, consumed the fruits, and introduced them to the European colonists (see some Native American legends involving persimmons here), the American persimmons we see cultivated in orchards come from selected lines. Indeed, varieties of American persimmons have been selected, and many cultivars of American persimmons can be purchased and grown in gardens and orchards to produce fruit. Besides the American persimmon, there are also other species available for purchase, in particular the Oriental persimmon (D. kaki), which is very well-known in Europe.

close-up of an orange persimmon fruit in a tree
The American persimmon tree harbors fruits that turn orange, soft, and tend to fall when they are ripe. Photo: W. Pollard.

Although American and Oriental persimmons are edible both raw and cooked (see here for some recipes), it is important to note that the fruits are very astringent prior to ripening, meaning that they have to be ripe for them to be palatable. The level of ripening is usually shown by the coloration of the fruit (ripe fruits are orange), its softness (ripe fruits become soft to the touch), and their voluntary falling from the tree while not rotten. It is often said that persimmons need to go through a frost in order to ripen. This is in fact not accurate: unripe persimmons will simply rot after a frost; ripe persimmons will not rot after a frost and will in fact start slowly drying out, which will make them become sweeter. This may have led people to assume that frosts actually lead to ripening, while the frost will not help in the ripening of any fruit that was not already ripe at the moment of the frost.

Is it true that I need to plant more than one persimmon tree to have fruits?

The short answer is mostly no. American persimmon trees are what we call dioecious plants. This means that each plant will either harbor female flowers (which will become fruits) or male flowers (which will provide pollen for pollination and will not produce fruits). The first consequence of this is that if one plants or encounters a male plant, it will be impossible to ever harvest fruits from it. The second consequence of this is that a female persimmon flower needs to receive pollen from a male plant in order to produce seeds (and reproduce). In most wild forms of American persimmons, pollination is also required for fruit production.

small yellow flowers on a persimmon tree
American persimmon trees display small and delicate white flowers, which are either female or male. Photo: M. Beziat.

That being said, female plants of most selected cultivars of American persimmons can actually produce fruit without pollination. If they do not receive any pollen, these female flowers will still develop into fruits, which will not harbor any seeds and which will be fully edible. If one were to plant these cultivars in their garden or orchard, fruit production would not be restricted by female flower pollination.

But does that mean persimmons do not need pollinators?

Not really. Wild persimmons still need pollinators to transfer pollen from the male to the female plants. So who are these pollinators? In fact, we know relatively little about wild persimmon pollination. In terms of flowering time, American persimmons flower between May and June, and their flowers are small and white (and cute!). Floral visitors have not been extensively studied, but there is at least one study describing a large variety of wild bees (e.g., sweat bees, bumblebees, leaf cutter bees) visiting their flowers. From this respect, persimmons play a role in sustaining this group of pollinators and will benefit from their pollination services.

immature and adult moths that use persimmon trees for food
Persimmons also support other insects including many lovely moths from our region, such as Luna moths – top – and Regal moths – bottom). Photo: Askalotl, C. McClarren and A. Reagol, M. Clock-Rust.

Although not pollinated by them, persimmons also support other types of insects (and sometimes pollinators): moths! In fact, persimmon leaves are the favorite food of caterpillars of many native moths. In particular, Luna moth and regal moth (besides many others) caterpillars prefer persimmon leaves. It appears then that persimmons do not just feed us with their delicious fruits, but also feed many of these beautiful native moths, allowing for them to maintain their populations in our area!

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!

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!