How Are Aquatic Plants Pollinated?

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.

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 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!

Insects: Our Most Under-Appreciated Neighbors

Why should I want bugs, insects, and creepy crawlies in my yard or green space?

Insects are an incredibly diverse group of organisms, with 91,000 described species in the United States and likely an equal number yet to be described by scientists. Only an exceedingly small fraction of these species ever have negative impacts on humans as “pests” (<1% of species). Often the overabundance of pest species is due to human agricultural and landscape practice choices. The vast majority of insects in shared spaces with humans like yards and parks are going about their own lives. In addition to being fascinating creatures deserving of habitat in their own right, they also often contribute to unnoticed but very important tasks that help humans, termed “ecosystem services.” The next time you see one of these critters in your yard, consider thanking them rather than smashing them.

What are ecosystem services?

Ecosystem services are benefits that humans gain from the environment. Examples of ecosystem services include water filtration, raw material production, erosion control, and pollination. Some ecosystem services, like the maintenance of atmospheric gasses (e.g. plants remove carbon dioxide and produce oxygen that humans breathe), are noticeable and directly impact our everyday lives. On the other hand, services like decomposition may go unnoticed because they indirectly affect us.  

Insects (and their arthropod relatives like spiders and earthworms) play vital roles in many ecosystem services. This is often due to insects interacting with plants in some way, though insects also provide food for many other animals. Below are some examples of the ecosystem services that insects contribute to.

Water filtration

Filter-feeding insects positively affect water quality because they remove particles of dead organic material. Insects retain many of the nutrients they filter out of the water, thus reducing the likelihood of algal blooms, their associated toxins, and dissolved oxygen “dead zones.” This is crucial because clean water provides habitat for other plants and animals like fish and amphibians. It also means less effort is required to purify water for human use. 

Types of insects that improve water quality:

  • Blackflies, mayflies, stoneflies, and caddisflies (Note: the underlined insect groups are not “true” flies in the taxonomic Order Diptera; they are part of other orders.) 

Other types of organisms that improve water quality:

  • Mussels, crayfish, snails

More information: Why Care About Aquatic Insects

Biocontrol

Biocontrol is when natural enemies are used to suppress pests and reduce the amount of damage they cause. Natural enemies are insects that are antagonistic to pest insects. There are three types of natural enemies: predators, parasitoids, and pathogens. Preserving natural enemy populations is crucial to reducing our reliance on pesticides because when natural enemies are active, pest outbreaks are less likely to occur in the first place. Predators need food all year, so they also need alternate prey available in order to prevent pest outbreaks. Pesticides eliminate beneficial insects in addition to pests, so they should be used only as a last resort.

Fun fact: Fireflies spend much of the year as larval predators belowground, feeding on pests like grubs in turfgrass yards. If no prey is available in yards, then there will be no display of adult fireflies in the summer.

Types of insects used for biocontrol:

Other types of organisms used for biocontrol:

  • Fungi, birds, amphibians, reptiles, and mammals

More information: Approaches to the Biological Control of Insect Pests.

Seed dispersal

Seed dispersal is when seeds are moved away from the parent plant. Seeds are moved when insects knock them off while feeding or when insects collect and then move seeds to a new location. Seed dispersal is important because it reduces resource competition between the parent plant and offspring plants. It also makes germination and seedling survival more likely, especially in arid climates. 

Types of insect seed dispersers:

  • Ants (most effective), beetles, wasps, thrips, and some moths

Other types of seed dispersers:

  • Fruit-eating animals (frugivores), such as some monkeys, lizards, and bats
  • Unwitting animal dispersers of sticky seeds like this

More information:

Seed Dispersal – The Australian Museum

The Conservation Physiology of Seed Dispersal

Decomposition & nutrient cycling

Nutrient cycling and decomposition are two important processes that rely on one another. Nutrient cycling is when soil nutrients are taken up by plants, insects eat plants, and then those nutrients are reintroduced into the soil when dead insects and droppings are broken back down into nutrients via decomposition. Decomposer insects help clear dead animals and plants off the ground which would otherwise accumulate everywhere. They also help create soil texture and circulate nutrients back into the soil, which plant populations and productivity depend on.

Types of insect decomposers:

  • Many beetles, springtails, termites, wood cockroaches, and some fly larvae (maggots)

Other types of decomposers:

More information: Decomposers

Supporting food webs

Insects are a main source of protein and nutrition for many animals (and even some plants). They play a crucial role in transferring energy from plants to larger animals that eat insects like spiders, birds, frogs, fish, bats, foxes, opossums, and bears. This wide food base that they provide allows for functioning, stable ecosystems that are resilient to disruptions.

Fun fact: By weight, there are roughly 300 times more insects than humans on Earth.
There are so many animals that eat insects, but here are just a few examples:

  • Terrestrial bird species, in particular, feed their babies almost exclusively with insects, and if there are fewer insects, baby birds are less successful at fledging from nests.
  • Popular fish like salmon, bass, and trout eat insects, especially when they’re young.
  • Grizzly bears will eat tens of thousands of moths a day to prepare for hibernation.

Pollination

Pollination is the transfer of pollen between flowers, resulting in flower fertilization and seed/fruit production. It is an unintentional consequence of pollinators going from flower to flower to feed themselves. Pollination is crucial for human survival, as 80% of plant-based foods and products rely on animal pollination. According to the USDA, pollinated crops are worth $18 billion in the US alone. Foods requiring pollination include apples, blueberries, chocolate, coffee, grapefruit, peaches, peppermint, sugarcane, tequila, and vanilla. 

Fun fact: beetles were likely the first insect pollinators– starting 200 million years ago!
Types of insect pollinators:

  • Bees, wasps, beetles, flies, ants, butterflies, and moths

Other types of pollinators:

  • Birds and bats

More information:

Pollination Basics

What is Pollination?

Why is Pollination Important?

Pollinated Foods

By Yasmine Helbling, Kelsey McGurrin, and Karin Twardosz Burghardt, from the University of Maryland Department of Entomology, Burghardt Lab

Pollination: The Garden Thyme Podcast

The garden thyme podcast episode on pollination and pollinators

In this month’s episode of The Garden Thyme Podcast, we are excited to celebrate National Pollinator Week ( June 19-25, 2023)! A pollinator is any animal that visits flowering plants and moves pollen from flower to flower, which helps plants reproduce, making fruits and seeds. In North America pollinators include bees, butterflies, moths, flower flies, beetles, and wasps. Worldwide, approximately 1,000 plants grown for food, beverages, fibers, and spices need to be pollinated by animals.

We also have our: 

  • Native Plant of the Month – Beardtongues (Penstemon digitalis and P. hirsutus) ~16:40
  • Bug of the Month – Fig wasps (Agaonidae sp.) ~21:18
  • Garden Tips of the Month – ~30:00

If you have any garden-related questions, please email us at UMEGardenPodcast@gmail.com or look us up on Facebook. For more information about these topics, please check out the Home and Garden Information Center.

The Garden Thyme Podcast is brought to you by the University of Maryland Extension. Hosts are Mikaela Boley, Senior Agent Associate (Talbot County) for Horticulture; Rachel Rhodes, Agent Associate for Horticulture (Queen Anne’s County); and Emily Zobel, Senior Agent Associate for Agriculture (Dorchester County).

Theme Song: By Jason Inc

Trees & shrubs for pollinators

For some reason, I feel that every time I think about what to plant for pollinators, the list of plants that comes to me is one full of herbaceous ones… however, it is odd that this is the case, because it’s not like our region lacks larger plants (e.g., trees, large shrubs) that are both fully able to support pollinators while also supporting other biodiversity and even contributing to flood and rain management! And because if we’re interested in going the large(r)-plant path, we need a bit of planning, in today’s post I would like to present some native shrubs and trees that are great resources for our pollinators. This way, you can start planning where to get them for planting in late winter to early spring.

Why consider trees and shrubs for pollinators?

Large perennial plants such as trees and shrubs have many characteristics that make them very attractive to any pollinator-friendly person in our region. Indeed, while there are many of these plants that act as wonderful food resources for many pollinators (both adult and larval stages), these larger plants represent long(er)-term and abundant resources that can serve different aspects of our ecosystem: they provide shelter and food for birds, they can assist in managing stormwater runoff, retain soil, reduce surface temperatures by their shading abilities, and provide structural complexity to our landscapes. Trees especially are a key component of creating climate-resilient landscapes. In fact, one of Maryland’s climate change mitigation goals is to grow 5 million more trees by 2031!

Planting trees is not necessarily expensive

From a financial perspective, although these plants may be costlier to obtain than the smaller herbaceous ones, there is a multitude of incentives, state vouchers, and programs that strongly reduce or sometimes completely cover the costs of obtaining them. In Maryland, for example, the state provides incentives through the Maryland Department of Natural Resources (discounts to be used at nurseries; all details here), the PG County Rain Check program or the TreeMontgomery program, city incentives, and free tree plantings (e.g., see College Park’s here). In all of these programs, a lot of trees native to our region are covered. If you would like to participate in any of these programs, make sure to check the specific tree lists covered by each (also, see this list of recommended native trees for the state of Maryland). Note that these programs I mentioned here are just a few of the many that exist; if you’re interested in this, make sure to check your city, county, and state resources!

What to plant?

I hope by now I have at least made you curious about the idea of choosing trees and shrubs for pollinators. Below, I made a very small selection of a couple of plants that appear in the native lists, and that are great for pollinators. Let’s take a look at them.

Tulip tree (Liriodendron tulipifera)

Tulip trees are a great native plant that can serve as a great pollinator resource. This tree is in the same family as Magnolia trees. It can reach a large size and it displays stunning yellow and orange flowers. This tree grows fast and is large (considered the tallest native tree in the eastern USA, along with sweetgums), so it can be a good choice for large spaces where a canopy is wanted relatively quickly. The flowers produce a lot of nectar, which attracts a massive number of pollinators. This makes it kind of fun to stand under the tree on warmer days during the blooming time: the buzzing coming from the tree is pretty impressive. Here are some more details on the conditions preferred by this tree.

the yellow and orange flower of a tulip tree
Tulip trees are among the tallest trees in the eastern USA and have wonderful resources for pollinators that they carry in their stunning flowers. Photo: W. Cutler CC.

American linden or basswood (Tilia Americana)

This tree can reach relatively large sizes, and when it grows to full size it has a very rounded canopy. I personally love this tree, because of the fact that I feel it’s a “social” tree: one can sit with friends under its shade on hot summer days, and just enjoy the life it hosts and the cool breeze it forms under it. Once the season is coming to an end, this tree’s leaves turn a lovely yellow. The flowers of this tree are small and not very colorful, but they are extremely fragrant and full of nectar, which makes them a great magnet for pollinators. You can learn more about the requirements of this tree at Virginia Tech Dendrology.

white flowers blooming on American linden tree
The American linden has discrete flowers that are very attractive to pollinators. Photo: A. Zharkikh CC.

Hawthorns (Crataegus phaenopyrum and C. viridis)

These are mid-size trees that also sustain a variety of fauna through their flowers, fruits, and bird nest-friendly thorny branches. Their flowers are white, have a typical Rose-family structure (like those of cherry trees), and are attractive to bees, syrphids, butterflies, and hummingbirds. Besides being great for fauna, the two species do well in urban environments, because they both tolerate a wide variety of conditions. Here at pollenlibrary.com and on the Lady Bird Johnson Wildlife Center website you can learn more about each of these species.

White flowers of a hawthorn tree in bloom
Hawthorns have lovely white flower clusters. Photo: F. D. Richards CC.

Fringe tree (Chionanthus virginicus)

This small tree/tall shrub is a great addition to green spaces, and is ideal for hedgerows or just as a stand-alone plant. I am always surprised by the super cool shape of the flowers of this plant, which have very elongated petals that create long white fringes. These flowers attract bees and other pollinators, which come to collect some of the nectar that is produced. This plant is usually dioecious, meaning that one individual plant harbors either male or female flowers, but rarely both. This plant is ideal for areas that receive a lot of sun, because it is under those conditions that it will do best (although it can do fine in less-sunny areas as well). You can learn more about this plant by visiting this website.

white flowers of the fringe tree
The whimsical flowers of fringe trees are not just attractive to us, but also to many pollinators! Photo: 阿橋 HQ CC.

Serviceberry (Amelanchier laevis)

Although all of them can provide very good resources for pollinators, I picked this one to showcase because this species grows relatively fast and does not get too large. This is indeed a larger shrub that has beautiful white flowers, and later on, delicious small berries. Because of all this good stuff, the flowers are visited by many insects, and the fruits are favorites of birds (so you’ll have to win over them if you want to get at the fruits! 😉 ). Here and on the University of Maryland Extension website, you can learn a bit more about this cool plant.

Serviceberries make everybody happy: pollinators in the spring with their flowers, and birds and humans in the summer with their berries! Photo: Henna K. CC.

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!

Pollination of Vegetable Crops in a Warming Climate

Pollination is the movement of pollen from male to female flower parts of sexually reproducing plants. It is often accomplished by wind and insects and results in the development of some type of fruit containing seeds for the species’ continuation. Farmers and gardeners in the mid-Atlantic are finding that high day and evening temperatures can cause vegetable plants to drop flowers and small fruits or produce deformed and under-sized fruits. This  problem has been observed in crops like bean, tomato, and pepper (mostly self-fertile; individual flowers can pollinate themselves), and in crops like squash and pumpkin (require cross-pollination between flowers).

How do high temperatures affect pollination?

All fruiting plants have an optimal temperature range for the pollination/fertilization process. High temperatures can reduce pollen production, prevent anthers from releasing pollen, kill pollen outright, and interfere with the pollen tubes that serve as conduits for uniting sperm cells and eggs (fertilization) inside undeveloped seeds (ovules). High temperatures can even injure flowers before they open. Night temperatures are increasing at a faster rate than day temperatures as a result of climate change, and seem to be most responsible for these pollination problems.

pollination of a flower
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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.

Continue reading

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!