Pollinators

Stem-Nesting Bees in Maryland

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With their large diversity, bees display a huge variety of nesting preferences. Some bees that are relatively commonly encountered in our region are those that nest in stems. In today’s post, I want to tell you about who they are, their biology, and their preferences when the time comes for them to pick their nesting sites.

Who are the stem-nesting bees in our region?

When we consider bees that nest in stems, we can think about two main groups. In the first group, we have bees that actively dig into pithy stems or wood to build their galleries. In the second group, we have those that use pre-existing cavities in stems and slightly adjust them to accommodate their brood. This difference may seem trivial; at the end of the day, they will all end up in stems, right? Well, that is true to some extent; however, whether we talk about one group or the other will define what actual taxonomic group of bees we’ll be referring to, and a different set of morphological adaptations that allow them to build their nests properly.

In our region both types of bees are present and many of them are relatively common and easily observed in our green spaces.

Stem nesters that dig their nests

In addition to large carpenter bees (Xylocopa), this group includes Halictid bees such as Augochlora and the Apidae Ceratina. Because they all have to dig actively into stems to build their nests inside, all these bees are equipped with very strong mandibles, which have modifications that reinforce them, and strong muscles that allow them to increase the force they can exert on the stems.

a metallic green bee going into a stem nest cavity
Metallic bees are very common in our region. They often can be found emerging or looking for wooden resources to build their nests. Photo: K. Shultz (CC).

Depending on the species, we can find them building nests on different substrates. While Augochlora can often be found building galleries in rotten logs, Ceratina is mostly associated with stems that tend to be a bit hollower, such as those of raspberries and blackberries (you can check out this other post on how to trim those plants to protect their nests), or those of plants of the genus Verbena. In all these cases, the nests have the shape of a gallery, with small cells built consecutively. Each of these cells is carefully built, provisioned with nectar and pollen, populated with one egg, and finally sealed with sawdust or compacted pith.

Stem nesters that use stem cavities that already exist

Several families of bees belong to this group in the Mid-Atlantic. Species of the genus Hylaeus in the family Colletidae are present in our region, and readily nest in hollow stems. Another group that is very commonly seen in our region is that of leaf-cutter bees (Megachilidae). Although this group tends to be more flexible in the types of cavities they will use for building their nests, many species will readily use natural or artificial “stem-like” cavities (these bees are very common in bee hotels).

several bees nesting inside of a plant stem
A small bee commonly seen in bee hotels is that of genus Hylaeus, also known as the yellow-faced bees. These bees will readily use small hollow stems and cavities. Photo: R. Cruickshank (CC).
Stem-nesting bees (Hylaeus) were observed in a raspberry cane on May 10, 2023, in Montgomery County, MD. Video: Christa Carignan, University of Maryland Extension

Because these groups do not need to actively dig into wood to build their nests, their mandibles are not as developed as those of the other group of bees I presented above. A common characteristic of all these bees is that they have relatively long and slender bodies, which is believed to allow them to move with more ease in relatively small cavities. It is also for this reason that specialists think that all these species transport pollen either internally or on the lower part of their abdomens (instead of on their legs, for example); this reduces their “width” and allows them to fit into cavities that may otherwise be too narrow. Another characteristic of this group of bees is that they often line their brood cells with special materials, such as leaves, petals, resins, or mud. This means that building these nests is a lot of work!

leaf-cutter bee carrying a leaf piece to a nest
Leaf-cutter bees of genus Megachile are also very commonly seen in bee hotels and nesting in cavities around our homes. Here, M. centuncularis, brings leaf cuttings to line the nest in a cavity offered in a bee hotel. Photo: B. Plank (CC).

Although bees (and most insects) are generally negatively affected by urbanization, this group of bees appears to benefit from their interactions with humans. Unlike ground-nesting bees which are very negatively affected by land development and urbanization, it seems that stem-nesting bees can easily use many cavities created by humans, such as those appearing in buildings, walls, fences, and gardens.

Read more: This Year, Host Bees in Your Garden

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

Anahí also writes an award-winning 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!

Help us improve the Maryland Grows Blog! Please complete our brief survey to give us your feedback and let us know about your interests.

A Little-Known Group of Pollinators: Beetles!

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As we mentioned in previous posts, when we think about pollinators, we tend to think of butterflies and bees, but rarely about the super important hoverflies or other groups of organisms. In today’s post, I want to tell you about another of those little-known pollinator groups; let’s talk about beetles that act as pollinators!

Beetles: “hard-shelled” pollinators

From a taxonomic perspective, beetles are a group of insects that belong to the Order Coleoptera. Among other important characteristics, they are recognized by their extremely well-protected body, in particular by structures called “elytra”, which provide a very hard cover to their wings (the wings are placed under the elytra). Generally, beetles have mouth parts that are adapted to chewing, which means that they have large mandibles that allow them to break their food. You may be wondering why I am talking about these structures in a pollination post… well, as for all pollinators, the shape and function of a pollinator define what they do (and don’t do). Let’s see how this affects our pollinating beetles.

a black and yellow beetle on a purple flower
Beetles are common flower visitors, with some of them being very effective pollinators. Because of their very well-protected bodies (see elytra covering the “back” of this beetle), they often spend a lot of time on flowers, where they feed on pollen, nectar, and floral parts. In this picture, we can see a cetoin scarab beetle with prominent elytra and a very hairy body that helps them transport pollen grains between flowers. Photo: C. J. Sharp (CC).

In the case of beetles, the fact that they are well protected by those elytra makes them more “confident.” That “hard shell” provides a great deal of protection against predation by other arthropods, which in turn makes them generally more “chill” in their visits to flowers. Unlike butterflies, bees, or hoverflies, beetles tend to move little within and between flowers, taking all their time to get the resources they need from them. For this reason, they are often considered as more generalist and inconsistent pollinators than their less-protected counterparts.

The shape of their mouths and their feeding habits also affect their efficiency as pollinators. In fact, beetle visits to flowers tend to be relatively destructive because they are attracted to them by their floral tissues, including in some cases pollen and the ovaries! In this respect, beetles tend to visit flowers to feed on them, which in some cases can lead to floral destruction. So, given this, are beetles good pollinators or just flower herbivores?

Beetles as pollinators

Among all the many different groups of beetles, some of them are considered to be particularly good pollinators. Specifically, these belong to the beetle families known as soldier (family Cantharidae) and longhorn (family Cerambycidae) beetles, families that depend on floral resources for their survival at least at one stage of their development. Other families such as scarabs can also be strongly associated with flowers for their survival. In all these groups, the beetles in question have clear adaptations that make them good agents of pollen transfer. For example, we observe different parts of their bodies covered with abundant hairs. This improves pollen transport and thus increases their ability to effectively cross-pollinate the flowers they visit.

a beetle with pollen grains on its head
Beetles that are good at transporting pollen have hairier bodies that can carry pollen grains, like this cantharid beetle that got its face covered in pollen while visiting these flowers. Note the yellow “powder” – pollen – that covers part of the beetle body and take a look at the large mandibles the beetle uses to feed on floral parts. Photo: J. Tann (CC).

Further, although these beetles tend to still feed on floral parts, they are usually much less destructive than their more generalist cousins. They often feed on specific flower parts (instead of on all the floral sections), leaving the central reproductive structures (e.g., the ovaries) intact, and thus allowing plant reproduction. These beetles are also often more specialized in their floral choices, preferring a small range of plant groups (usually one or a few species within the same plant genus), and moving more readily between flowers. Combined, all of this means these beetles can not only carry a lot of pollen (they are hairy) but also move it from flower to flower more effectively and do not destroy completely the flowers they visit.

A cool beetle pollination example

Beetle pollination is a very understudied topic in temperate regions such as Maryland, and we are still learning a lot about it. It is thus not surprising that the number of examples from our region is not super high. Let’s talk about one of them that happens to be relatively well-studied elsewhere but that also occurs in North America.

yellow and black soldier beetle on a magnolia flower
Beetles are considered some of the most important pollinators of species of the genus Magnolia. Here, a long-horned (Cerambycid) beetle on a Magnolia grandiflora flower. Photo: D. Hill (CC).

An example of beetle pollination of native plants that are also present in our region is that of Magnolia flowers. Although most of our knowledge on the pollination of this plant genus comes from studies done on species outside of North America, there are a couple of reports of floral visitors in several of our local species. From elsewhere, we know that these trees display flowers that appear to be particularly attractive to beetles: they are white, easily accessible, offer a lot of pollen, are fragrant, and in some cases even produce heat! In many of these species, the flowers appear to attract scarab beetles, which, once landed on the flowers, feed on the petals, mate, and then actively move between flowers and cross-pollinate. In North America, some studies like this one and this other one (in PDF) have found a wide variety of beetles attracted to our native species. Many of these beetles are very small and visit Magnolia flowers to feed on them, mate, and spend the night protected within the flowers.

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!

Help us improve the Maryland Grows Blog! Please complete our brief survey to give us your feedback and let us know about your interests.

Floral Supplements Support Ecosystem Services in Our Green Spaces

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Winter is a good season to plan for spring. In today’s post, we want to tell you a bit about a type of planting that one can consider. This planting will boost the stability of our green spaces, turning them into havens of biodiversity. Let’s talk about floral supplementation and floral supplements.

a row of yellow flowers
Example of a floral supplementation in the form of a flower strip (e.g., “pollinator strip”) in a Mid-Atlantic agroecosystem. Photo: K. Evans.

What are floral supplements?

Floral supplements are semi-natural habitats that are added to green spaces and are designed to concentrate a large amount of plant resources into a small area. By including a mixture of herbaceous or woody flowering plant species, floral supplements provide food and habitat to local wildlife. This increase in floral resources correlates to local increases in the number of insect and other animal species and their abundance, which results in the improvement of ecosystem services in the area. Common examples include pollination, pest suppression, nutrient (re)cycling, soil conservation, and improvements in water provision and quality.

skipper on a flower
inside of a stem looking at a leaf-cutter bee nest
ground nesting bee
Hairstreak butterfly collecting nectar (left); a leafcutter bee using yellow bits of partridge pea flowers to line the interior of their nest (center); early spring andrenid bee returning to her underground nest in May. Photos: K. Evans.

While supplements formed of mainly herbaceous flowering plants offer great nectar, pollen, and nesting resources to insects that act as pollinators and pest control agents, other plants can also be used to create floral supplements. In particular, woody plants and shrubs can provide valuable food and cover for various songbirds and small mammals. The benefits of floral supplementation go beyond supporting insects, birds, and mammals. In fact, these plant additions also have been shown to assist with weed suppression and to reduce soil erosion, while permanent woody plantings (e.g., hedgerows) can also serve as windbreaks.

hedgerow along a road
Hedgerows are coming into leaf along a road in the spring. Photo: McEnnerney/Alamy.

Nowadays, floral supplements can be found nearly everywhere, from home gardens to roadsides, and croplands to city greenspaces, and are commonly referred to as “pollinator strips” or “conservation strips”. Generally, floral supplements are primarily implemented as a conservation initiative to support wildlife and arthropod biodiversity, particularly in floristically impoverished areas such as agricultural monocultures and urban environments. 

The origin of floral supplementation

The concept of floral supplementation has roots in agriculture. For example, the use of companion plantings and flowering margins in and around crops has been long known to enhance biological control by boosting natural enemies of crop pests, such as predatory bugs and beetles, and parasitic wasps. In this respect, the addition of these plants to the agroecosystem has been recognized as a strategy to improve pest control with lower pesticide inputs.

spider on a flower
samurai wasp parasitizing brown marmorated stink bug eggs
Spiders (left) are important natural enemies of pests, such as this crab spider lurking on a black-eyed Susan. Floral supplementation also supports the presence of other pest control agents, such as this samurai wasp, Trissolcus japonicus, parasitizing brown marmorated stink bug eggs. Photos: K. Evans; S. Athanas.

Today, the concept has expanded in conjunction with aesthetic and conservation initiatives. For example, the Beautification Act of 1965 encouraged roadside wildflower plantings, and actions such as establishing pollinator gardens and defining no-mow months increase floral resources for pollinators. The USDA also offers conservation programs for monarchs and other pollinators by encouraging farmers and ranchers to plant pollinator-friendly wildflowers, shrubs, and trees (https://www.nrcs.usda.gov/).

No Mow April
No Mow initiatives promote not mowing the lawn to allow early spring flowers to grow and bloom, thus providing food for early spring insects.

How to start with floral supplements?

Although there is not a hard definition for what constitutes a floral supplement, guidelines have been proposed by government and conservation societies including the Xerces Society and the USDA. What species to plant ultimately depends on the location and purpose of the supplement. For example, recommended species to plant for upland or grassland habitats will be different than those for wetter habitats. Generally, however, regionally native or naturalized species are often recommended to sustain more native animal diversity than exotic species. In fact, because most ornamental plants used for aesthetic purposes are often non-native, they tend to offer unsuitable or no resources to the local animal communities. Further, even among native species, regional varieties (i.e., local ecotypes) are often preferred because they are better adapted to the local climates, growing conditions, and living communities.

milkweed in a field
Monarch City signs
Flower plantings can sometimes support specific pollinators, such as monarch butterflies. Examples of this are roadside and field margin milkweed plantings (left) and the participation of cities in the Monarch City Certification Program. Photo: Bill Hubick, Maryland Biodiversity Project. 

Another aspect to consider when starting or creating floral supplements is the diversity of plants used. In fact, diversity is a common theme for floral supplements: one seeks to obtain a high diversity in flower color, flower shape, and bloom times. For example, the best supplements contain a mixture of annuals, perennials, and woody plants that bloom at different times throughout the growing season to ensure that floral resources will be sustained over time. Further, including flowers with diverse shapes, colors, and sizes facilitates access to nectar and pollen for arthropods with different mouth and body shapes.

Although one can try to do it by oneself, designing a floral supplement can be tedious and sometimes overwhelming. Luckily, federal agencies, seed companies, and local nurseries have mixes available for certain habitat types and regions, that one can order directly.

To learn more visit: 

USDA, Natural Resources Conservation Service

Xerces Society in the Northeast

Pollinator Partnership

Site preparation guidelines for pollinator gardens (PDF)

By Katy Ciola Evans, Ph.D. student in the Espíndola Lab, University of Maryland Department of Entomology. View her website to learn more about her research.

How Do We Study Pollination?

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Pollination is such a buzzword right now, and a lot is said about pollination and pollinators. However, how do people learn about pollination and pollinators? How do people know who the pollinators are and how pollination works? Because I happen to do research on pollination, I wanted to use this opportunity to share with you some insights into how pollination is studied, what it teaches us about plants and their reproduction, and how this connects with the things we hear about pollination.

What are we studying when we study pollination?

As we mentioned in previous posts, pollination is basically how plants reproduce. Generally, pollination involves the deposition of pollen grains on the female organs of a plant, which in many cases leads to ovule fertilization and often the production of seeds and sometimes fruits. So, when we study pollination, what we are studying is the reproductive strategies of plants. If we think about plants and the many different ways they have to reproduce, it may be simple to imagine all the different aspects one can try to understand about pollination. Here I will present a couple.

The reproductive strategy of plants – selfing, crossing, or both?

Unlike us humans, many plants can self-pollinate. This means that the species that can do this can technically accept pollen from their own flower and use it to fertilize their own ovules. The consequence of this is that plants that can self-pollinate do not necessarily need pollen from another individual to produce offspring. This may seem like a nerdy technicality of mine, but the ability or not of a plant to self can have a lot of wide-ranging consequences. From an evolutionary perspective, this can affect the genetic diversity in a species or group of species, which can define whether a species can adapt or not to certain conditions, among other things. This can also have consequences for food production and plant breeding; if a plant cannot self, several stocks need to be present in a field for it to be able to produce fruit. This for example happens to some varieties of cherries, where more than one tree needs to be planted in an orchard for the plants to produce fruit.

reproductive parts of flowers
To test for plant reproductive strategies, researchers can perform controlled crossing experiments, which follow generally what is presented in this figure. Image: University of Waikato.

To understand whether a plant species can self, a basic experiment can be done. In this experiment, one creates groups of plants of the same species that will be pollinated following different methods. By comparing the number of seeds produced by each method, one can infer how the plant reproduces. In its simplest forms, one of these experimental groups of plants is manually selfed, which usually involves removing all anthers from a flower and then manually depositing pollen from the same plant on its stigma. In another group, plants are crossed using pollen from another plant. The flowers and potentially fruits of both groups of plants are then left to develop, and once fruit/seed maturity is reached, one counts the number of fruits and seeds per group of plants. If the two groups present significantly different numbers of fruits or seeds, then we can infer whether the plant is able or not to self.

Who pollinates a plant?

Imagine that we figured that a plant requires cross-pollination. Now, a question we may want to ask is how the pollen of one flower can get to the stigma of another flower. Again, this is not just a biologist niche question; this has practical and evolutionary consequences. For example, if a plant is wind or water-pollinated, it will be able to produce offspring in the absence of animal pollinators. Alternatively, if a plant needs animal pollinators, then their absence can lead to the plant’s inability to sustain its population over time. In a food production world, plants that need animal pollinators will benefit from the presence of those pollinators, leading to higher fruit or crop production when more pollinators are present (this is the case of, for example, almonds and strawberries).

To study this, scientists have a large palette of methods. Here are some. One of them involves observation of the flowers in question. For example, one can assume that a flower that produces nectar or that has special color markings aimed at directing pollinators when they visit a flower will be more likely pollinated by animal pollinators than one that does not offer any floral rewards in exchange for flower visitation. Similarly, the general floral shape gives clues about how it may be pollinated. Flowers pollinated by animals tend to have specific shapes that improve pollen deposition by animal visits, while those pollinated by abiotic factors are usually droopy or displayed in very humid areas.

markings on flowers only visible with UV light
Flowers that can look “plain” to us may have special colorations that only pollinators can see, like this black-eyed Susan, which shows its floral markings that can be seen only under UV light (a wavelength many insects can see). Observing markings or “signs” on flowers indicates that the plant may use animals for pollen dispersal and transfer. Photo: A. Davidhazy.

As said before, observations are a huge part of studying pollination and the identity of pollinators of a plant. This generally also entails spending hours upon hours over several seasons carefully observing and sometimes capturing any floral visitor and potential pollinator of a plant population. This requires patience and focus, and careful recording of the abundance, frequency, and identity of any flower visitor. This also often requires hours of identification of captured floral visitors, often under the microscope (given that most floral visitors are often insects), and through the use of taxonomic keys or the consultation of experts of specific taxonomic groups.

a man in a field using an insect sweep net
One way to understand what pollinates a plant involves hours of focused observation of flowers and capture of the animals seen visiting them. Photo: A. Espíndola.
fluroescent dyes are used to track movement of pollen by pollinators
Fluorescent dyes can be used to track the movement of pollinators and the potential for pollen transfer. Photo: Huais et al., 2022.

Along with these hours of observing and describing what is being seen, other more “manipulative” approaches exist. For example, many scientists try to understand who visits a flower by marking flowers with powdery dyes and then seeing if any animal seen visiting the flowers becomes colored with the dyes. More “technological” approaches use high-resolution cameras and artificial intelligence (AI) methods, as well as DNA sequencing to identify the presence and species identity of pollen grains on animals seen or suspected of visiting flowers.

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!

Want Flowers Next Year? Here Are Some Pollinator-friendly Plants to Plant This Fall

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Although the end of the summer/early fall may seem like an odd time to think about planting, don’t be fooled! This is actually prime time to allow plants to establish and grow strong for next spring. In fact, planting in the early fall gives time for plants to establish their root system, acclimate to the new conditions, and be ready to grow as soon as the spring conditions become ideal for them to develop. In today’s post, I want to present a couple of very neat plants that can be planted now to bloom and provide resources for next spring’s pollinators. And because these are some plants that are just close to my heart, let me try to convince you to add some (or all! 😊) of these to your green spaces, so you can enjoy them next year. Let’s talk about mountain mints, beardtongues, and Culver’s roots.

Narrow-leaved Mountain Mint – Pycnanthemum tenuifolium

As its name may let you infer, this is a plant that belongs to the mint family (Lamiaceae) and, as a mint, it is very aromatic. The genus is native and restricted only to northern North America, and we are lucky to count several species within Maryland’s native flora. As is the case for most Lamiaceae, mountain mints do not only present beautiful flowers; they have been used traditionally as a food seasoning and in medicinal teas to treat colds, coughs, and fever by many Native American tribes. Although some species are currently protected in the state, some are common, one of which being the narrow-leaved mountain mint (P. tenuifolium) I want to introduce you to.

white flowers of mountain mint with a butterfly visiting for nectar
(Left) The plant establishes very well in sunny areas where it displays its clusters of white flowers. Photo: SB_Johnny (CC). (Right) The delicate flowers of the mountain mint attract a very large diversity of pollinators, such as this Pearl Crescent butterfly. Photo: S. Cadwell (CC)

This plant is a favorite of mine because it is relatively tall (~ 3-4ft), makes a lot of flowers, attracts a bunch of insects, and tolerates conditions that many other plants don’t like. As is the case for all mountain mints, the flowers of this plant are clustered, and in this species, the flowers are white and bloom in the summer. The plants attract a very large variety of insects and for that reason are one of the recommended plants by the Xerces Society for supporting pollinators in our area. Bees of all sizes, beetles, butterflies, wasps, flies, and hoverflies… nobody can resist this beauty! And to top it all, this plant grows great in full sun and even in relatively dry conditions, which makes it a great one to plant close to roads or in those areas of our green spaces where other more water-needy plants may not do so great.

Hairy Beardtongue – Penstemon hirsutus

I have to say that I have a weakness for Penstemons specifically and plants of the whole family they belong to (the figwort family; Scrophulariaceae) generally. Their complex flowers always get to me, and plants of the genus Penstemon are to me one of those that I can look at and marvel at forever. So, this is one of the first ones I want to grow every time I can… maybe I’ll convince you to plant it too?

The genus Penstemon is almost restricted to North America, where they represent one of the largest groups of native plants on the continent. They are characterized by having tubular flowers, and their coloration varies by species, going from white, to pink, purple, red, and blue. Although there are a few species native to Maryland, and several can be grown, I want to talk a bit about the hairy beardtongue, P. hirsutus (but also check out the foxglove beartongue, P. digitalis!).

pink-lavender flowers of hairy beardtongue
lavender flower with a bee
(Left) Hairy beardtongue plants display a large number of tubular flowers. Photo: K. Woods (CC). (Right) The tubular flowers of hairy beardtongue provide nectar and pollen to a variety of pollinators, such as bumblebees, other bees, hoverflies, and butterflies. Photo: A. Espíndola

The flowers of this species are multicolored, with purple tubes tipped with yellow and white. The flowers are visited by bees (including bumblebees), hummingbirds, and butterflies, and have been described to support the adults of the Baltimore checkerspot, our state insect! The plant itself is not overly tall (~2-3ft) and makes a lot of flowers. They prefer drier conditions and full sun to some shade and will bloom in the late spring/early summer.

Culver’s Root – Veronicastrum virginicum

I feel that plants with small white flowers (like this one) are often kind of forgotten, to the benefit of showier and more colorful flowers. However, Culver’s root is a little gem native to our region that any local interested in supporting pollinators should consider having around.

Belonging to the Plantain family (Plantaginaceae), the genus counts only a couple of species, one of which is the only North American native: Culver’s root (V. virginicum). Like all members of the genus, this species presents its white flowers arranged in long spikes. This species will become taller over the years, reaching 4-5 feet at full maturity. They prefer sunny to shadier spots, where sufficient moisture is present (e.g., wood edges).

The flowers mature sequentially, and because there are so many flowers in their long spikes, a single plant is likely to flower for weeks. Besides its sustained floral display, this plant is super interesting and important for pollinators because it happens to flower at a time when few other plants flower in our region (July-August). Their white flowers attract and provide food for bees, butterflies, wasps, and (hover)flies.

spiky white flowers of culver's root
Once established, Culver’s root can reach 4-5 feet in height, displaying their long spikes of white flowers. Photo: E. Enking (CC)
close up view of culver's root flower spike
A tiny bee collects pollen on Culver’s root (can you spot it?). Note the multitude of flowers present on the long spikes. Photo: A. Espíndola

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!

How Are Aquatic Plants Pollinated?

Maryland Grows1 Comment

When we think about pollination, we tend to only think about terrestrial plants. However, a large number of plants are not and actually live fully or partially in the water. These plants also need to reproduce, and thus need to have their flowers pollinated to produce seed. How do they do it? In today’s post, I will try to give a (short) answer to that question, using some native plants as examples.

You may recall from previous posts, that flowering plants require pollination to be able to produce seeds and thus reproduce. Since we are terrestrial organisms ourselves, we tend to be more aware of other organisms and processes that share that trait with us, and pollination is no exception. However, there are lots of flowering plants that are completely or partially aquatic, and these plants also require pollination to produce seeds. Depending on the specific requirements of the plants in question, some of them may use different strategies for pollination.

Wind pollination

Many aquatic or semi-aquatic plants depend on wind to transfer pollen to the female reproductive structures. Especially under conditions distant from land, using wind as a means of pollen dispersal can be extremely advantageous. In fact, being distant from land tends to reduce the types and number of animals that can visit the flowers of aquatic plants. By depending more heavily on wind, these plants usually display light and abundant pollen that can be readily blown away and potentially deposited on the stigma of the female counterparts. A global evaluation of this indicated that about a third of all aquatic plants in the world are wind-pollinated.

small leaves on the surface of water - watershield plants
a red flower protruding from the surface of the water
Watershields are very common wind-pollinated aquatics from our region. These plants display their small flowers well above water, first exposing their stigmas and the next day releasing pollen into the air. Photos: S. Elliott, S. Osborne (CC)

In Maryland, an aquatic plant known to be wind-pollinated are watershields (Brasenia schreberi). This plant has non-showy flowers that display both anthers and stigmas. In order for the plant to promote cross-pollination (i.e., avoid receiving pollen from its own flowers), the flowers of these plants go through a complex blooming process that spans two days. This process involves on the first day the receptivity of the stigma (the female part that receives the pollen) and on the second day the maturation and release of the pollen grains. When the grains mature, they are swept by the wind and can reach stigmas from other flowers that are at that point going through their first flowering maturation step.

Animal pollination

It has been shown that a large number of aquatic plants are at least partially pollinated by insects or other animals. In fact, as is also the case in terrestrial plants, aquatic plants can sometimes use both wind and animals to transfer pollen, increasing the chances of some pollen eventually reaching the stigma. Animal-pollinated aquatic plants are pollinated by a large variety of organisms, but their identity will depend on the specific place where the plant is growing and the ability of the pollinator to reach the plant and even survive in that environment. For example, while large bees may be able to fly further away from land, smaller insects may mostly visit plants that are close to land.

a closeup of the flower of arrow arum - its yellow inside
the leaves of arrow arum plants are pointed like arrows
a tiny fly with red eyes is the pollinator of arrow arum
The arrow arum or tuckahoe is an aquatic native that displays a nursery pollination system in association with small flies. Photos: Sheri, N. DeBarros (CC), K. Shulz (CC)

A special case of insect pollination of a Maryland native is that of the arrow arum or tuckahoe (Peltandra virginica). The species belongs to the Araceae family and displays a stunning pollination system. As is often the case in this family of plants (see also the skunk cabbage example we talked about in a previous post), the maturation of the female and male flowers is linked to the production of specific aromas. In the case of the arrow arum, these smells attract small flies, and in particular individuals of Elachiptera formosa. These flies seek the flowers to mate, feed on pollen, and eventually lay eggs on the plant, making this an example of what is called nursery pollination (the plant receives a pollination service in exchange for providing a brood site for the pollinator). By moving along the flower, these tiny flies move pollen from the anthers to the stigmas. Some of this pollen may come from the same plant, but other pollen may come from a different flower already visited by the flies.

Water pollination

Finally, many aquatic plants display flowers that are either completely submerged or floating on the surface of water. These plants usually use water currents to disperse their pollen. As with wind, this dispersal is very inaccurate, which usually leads to the release of a large amount of pollen. These plants have either pollen that floats on water or remains attached to the anthers which float to the stigma.

aquatic plants with tiny white flowers on the surface of water
The American pondweed is one of our native species that uses water as their means of pollen dispersal. Note the very small white flowers that are placed on the surface of water. Photo: C. Fisher

A very common native from Maryland that displays this type of pollination is the pond- or waterweed (Elodea canadensis). This species native to North America displays flowers that have either anthers or pistils, but not both. The flowers with anthers are often displayed over the water, from where they release the pollen, which lands and then travels on its surface. By moving on the surface of the water, the pollen can reach the slightly submerged stigmas of the pistilate (female) flowers, which are held on flowers that float at the very surface of the water. Because such a dispersal can lead to large pollen loss, pollen release in this species is only done when the wind is light and the water current is low. This promotes a more “controlled” dispersal and increases the chances of the pollen effectively reaching the stigmas.

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!

Cicada Killer Wasps Are Scary But Good

Maryland Grows1 Comment
a close-up view of a cicada killer wasp showing its light yellow and black striped abdomen
Cicada killer wasps are good pollinators who keep cicadas in check.
Photo: Dawn Dailey O’Brien, Cornell University

It’s big. It’s creepy. It’s the cicada killer wasp and it has some local folks worried. But it’s a good guy. Honest. 

Looking like yellowjackets on steroids, 2-inch-long cicada killer wasps are yellow and black and a bit intimidating. But it’s all a show. 

Unless you’re a cicada, you have no worries. These wasps help control the annual cicadas buzzing in our trees.

In fact, male cicada killer wasps don’t have stingers at all and females aren’t likely to sting unless you step or sit on one.

In addition to their ginormous size, cicada killer wasps worry folks because they do figure eights over lawns, looking like they are Up To No Good.

Nope. Those are just males establishing or defending territory. Boys will be boys. 

The dudes have been hanging out since July, scoping out territory while waiting for the ladies to arrive. Their manly posturing results in often spectacular wing-whirling combat, all bluster and bluff.  

Check out the video of a close encounter with University of Maryland entomologist Mike Raupp’s Bug of the Week feature.

Following a brief romantic interlude, the female cicada killer wasp digs a finger-sized nesting chamber in the ground, leaving telltale piles of excavated soil.

Then she climbs trees in search of the cicadas which she uses to feed her young. 

When she finds a cicada, she stings it to paralyze it, then flies the cicada down to the ground, dragging it to her nest. This is no mean feat since cicadas are much larger than she is. That’s one determined mama.

She stuffs the cicada into her nest, lays an egg on it, and seals the opening. When the egg hatches, the larva will chow down on the cicada which is, unfortunately, still quite alive. Ah, the circle of life.

Well fed, the larva will wrap itself in a case, pupate and stay underground before emerging as an adult next summer. 

Interestingly enough, female cicada killer wasps can choose the sex of their babies. If they give them one cicada as food, they turn out to be boys. Given two cicadas, they become larger females.

A female cicada killer wasp’s work is never done. As soon as she seals one nest, she makes a new one and goes cicada hunting again, helping to keep their population in check.  

See female cicada killer wasps in action in this Bug of the Week profile.

Cicada killer wasps also are good pollinators, moving pollen from plant to plant as they feed on nectar as adults.  

Cicada killer wasps congregate around some petunias.
Photo: John Lefebure

What should you do if you find cicada killer wasps in your yard? Not a thing. Tolerating them is best since they’re only around for a few weeks and are beneficial. Chemical controls are not necessary.

But if you’re bothered by the holes they make in your lawn, wet down the area with a sprinkler.  Cicada killer wasps don’t like to build nests in moist soil.

They also avoid nesting in dense lawns. So their nests are a clue that your lawn may need some beefing up.  

Cicada killer wasps may be big and a little scary looking. But I hope you’ve gained some appreciation for these fascinating insects and enjoy watching them dance over your lawn.  

By Annette Cormany, Principal Agent Associate and Master Gardener Coordinator, Washington County, University of Maryland Extension.

This article was previously published by Herald-Mail Media. Read more by Annette.