The middle of the winter may seem like an odd moment to think about flowers. In fact, if I look out my window, I can barely see the ground, with most of it covered in snow or ice. So, flowers and all the greenery of spring and summer seem like something that happened in another world which today seems sooooo far away! There are some flowers, however, that defy some of these rules of low temperatures and do a revolution in nature (OK, maybe not a revolution, but still…). In today’s post, and although surrounded by ice and snow, I invite you to join me and warm up using the heat that some plants produce, since we’ll be talking about heat-producing plants!
Really?! Can plants produce heat?
The short answer is yes, which is super cool in itself. The long answer is yes, but let me tell you how, and what are the advantages of producing heat (and why not all plants do it).
The production of heat in plants is relatively rare, and this feature is found mostly in plants considered to have evolved a long time ago from an ancestor to most living plants. Today, heat production in plants is restricted to groups associated mostly with tropical and subtropical environments, such as water lilies, aroids, palms, and birthworts.
Unlike what we may expect, heat production in these groups has not evolved to keep the plant alive, but rather to help the plant reproduce. In these groups, heat production is associated with floral maturation and in particular to pollinator attraction. In these cases, different parts of the flowers increase their metabolism at a specific point in their development, leading to an increase of temperature that in some cases can be extremely noticeable.
In some cases, the heat has been described as a reward for pollination. (Check out this other post for other “special” rewards pollinators get from plants.) This is because it may occur at times of the year (or the day) when the environmental temperatures drop a lot, and when pollinators that visit the flower could benefit from an extra source of heat.
In other cases, heat is known to promote the release and spread of floral odors, which attracts the preferred pollinators. In many cases where heat production is present, it has been observed that plants also display a way to retain the pollinators, such as floral chambers, and in many of those cases the interaction between the plant and its pollinators involves luring and temporarily trapping the pollinators!
Some arum family plants (here, the European Arum maculatum) temporarily trap their pollinators. The species shown here displays a chamber in the lower part of its “flower” that is closed by hairs (shown on the right), which let the insects in but makes it hard for them to leave. While the flies are trapped in the chamber, the flowers mature, dropping pollen on the flies. These flies will be released after a while, and some of them will be caught again by another plant, leading to the cross-pollination of the species. Sneaky, sneaky! Photos: A. Espíndola.
While it takes a lot of energy from the plants to produce heat, this usually allows them to flower early in the season while other flowers are not around. This reduces inter-plant competition for pollinators, makes the plants easy to find by pollinators, and increases the chances of having pollen be transferred from one flower to another — all of which increases the number of seeds plants are able to produce.
Our very own heat-producing plant, the Eastern skunk cabbage
Many of you may be familiar with Eastern skunk cabbage (Symplocarpusfoetidus), which is among the first plants we see peek out their leaves in the snow or the ice in the early spring. Besides being among the first plants to appear in our region, they are actually also among the first to flower, and that flowering and their pollination are intrinsically related to heat production.
Skunk cabbages belong to a mostly tropical family of plants, the arum family (Araceae). Fortunately to us, some of them do occur also in more temperate regions like ours (another native Araceae from our region is the beautiful Jack-in-the-Pulpit, Arisaema triphyllum). As with many other Araceae (for example, the titan arum), skunk cabbage attracts their pollinators through the production of a very pungent odor, which gives the plant its common name. And like all Araceae, their flowers have a very special shape that allows them to not only produce but also retain heat.
The skunk cabbage has a typical arum family flower with a bract that covers the central part of the flower. The central part displays all the reproductive parts. In the picture shown here, it is possible to see the yellow pollen exposed on the flowers of this plant. During flower maturation, the reproductive part heats up, reaching temperatures of over 20˚C (about 70 ˚F)! Center and right images show pictures of the flowers taken using a camera able to measure surface temperature, with a color scale that relates shown colors with temperatures. Photos: left: Janet and Phil; center and right: Onda et al. 2008.
When their flowers start to mature, the “head” of the flowers heats up reaching temperatures of about 70˚F (!!!), even if the surrounding temperatures are below freezing. This heating leads to the release of aromatic odor bouquets, formed mostly by compounds rich in sulfides (thus, the stinky odor). Even though this odor may not be the most attractive to us, it is very much so to the skunk cabbage’s favorite pollinators: small flies and beetles that may be lured by the flower “thinking” it is their favorite food or egg-laying site.
In defense of the plant, though, even though the attraction may be slightly dishonest, the lured pollinators may actually benefit a bit from the visit. While some of them may indeed find laying sites on the plant material, most of them will benefit from the heat received, which especially that early in the season is a much-welcome reward! This heat also allows the pollinators to become more active and sometimes even mate within the plant’s flower, which also benefits their own reproduction. Finally, and most importantly for the plant, through this heat release and all the insect movement associated with it, insects passively pollinate the flowers, getting covered in pollen and later transferring it to other equally stinky and warm skunk cabbage flowers. Isn’t this super cool?… I mean, warm?
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.New! Anahí is starting 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!
Last week my neighborhood hosted the traditional Christmas tree lighting event. Usually this event involves lighting the large Christmas tree across my street, having Santa come visit the kids on the firefighter truck, and sharing a cup of warm chocolate while chatting with the neighbors. This year, things were a bit different, with the lighting being live broadcasted, Santa parading the neighborhood on a truck, and chocolate being picked-up at one of our neighbor’s yard and enjoyed at home.
I have been since thinking a lot about this event, and how important it is to maintain the social ties in our neighborhood. However, I also have been thinking a lot about how the food at this event is almost as important as the event itself; how the chocolate was not left out of this year’s modified event. And this made me realize yet again how foods are central to our social ties, and how losing them would also make us a bit lonelier. So today’s post, the third in our comfort foods series, will be about that food that was so important to my neighborhood this past weekend: chocolate. Join me today in exploring how cacao comes to be, and how partnering with nature helps its (re)production.
Chocolate – the ultimate winter comfort food. Photo: Kathy Smail
What is cocoa?
The cocoa we find in the chocolate we eat and drink comes from beans of the cacao tree, a small tree in the mallow family. As for the other comfort foods we talked about in my last two posts (spices and vanilla), cacao is also not grown in the USA, and thus has to be imported. (Interestingly, it also has to be 100% imported into the countries we usually associate with chocolate, like Switzerland and Belgium.) Cacao, indeed, can only grow in very humid rainforests and can only be cultivated close to the Equator. Today, the major producers of cacao are in West Africa (e.g., Ivory Coast, Ghana) and the Americas (e.g., Ecuador, Brazil).
The plant of cacao, Theobroma cacao, is a small tree naturally occurring in South and Central America. The fruits of cacao plants grow directly attached to the trunk. Photo: F. and K. Starr
Even though cultivated in Africa, the cacao plant originates in South and Central America, where the species grows in the wild. Studies demonstrated that the wild plant was domesticated one or two times, first about 5,000 years ago in the Amazon, and about 3,500 years ago in Central America.
Although, as I said before, cacao beans are the central ingredient of chocolate, it is suspected that the first uses of cacao were not based on the consumption of their beans, but rather of their pulp, which is sweet and readily ferments to produce alcoholic beverages. Researchers believe that the use of beans for making the chocolate drinks the first Spaniards saw Aztec emperors drink was indeed a secondary use of the fruit.
How is cacao produced?
Unlike many of the crops we eat, most of cacao production is done by small-scale farmers. Being small trees, cacao fruits are produced in cacao orchards, usually established in areas previously occupied by rain forests. The fruits grow directly on the trunk of the tree, and need to be harvested regularly, since all fruits do not ripen at the same time. Once harvested, the fruits are cut open, and the pulp and beans are separated from the husks. While the husks are discarded, the beans are left to dry out, at which point they become dark and start looking like the little pictures we sometimes see on our chocolate bars.
The fruits of cacao are large husks that contain the beans and a sweet pulp. Note the violet/whitish color of the fresh beans, which will eventually turn brown after drying. Photo: Presidencia República Dominicana
As we see, a central part of cocoa production (and us getting the yummy chocolate we like) is the production of fruits, which seems to be defined by many aspects of the production. On the one hand, poor soils lead to yield reductions. Interestingly, cacao trees are adapted to growing in the understory of the rain forest and for this reason had been initially grown under other trees. However, once it was observed that their productivity increased if exposed to full sun, the accompanying trees started to get cut off, further contributing to the deforestation of the rain forests where they are usually grown, and increasing the monoculture of cacao plants.
After some years of higher yield, farmers realized that their trees became less and less productive, and came to understand that the presence of other trees in the orchards maintained the nutrients in the orchard’s soil, what eventually benefited fruit production. Today, in order to maintain yield and sustain the soils, cacao is recommended to be grown in what is called agroforestry systems, meaning that orchards are interplanted with other trees, which enrich the soil with nutrients, and provide a more natural shady environment in which the cacao trees can grow. The little label with a frog that we see on some certified chocolate packages indicates indeed that the farms where the cocoa used in that chocolate was produced following such environmentally friendly practices. Interestingly, as for many environmental practices, it was shown later that using agroforestry methods for cocoa production was not only beneficial to soil fertility; it also indirectly improved fruit pollination, thus improving yield through different paths!
Agroforestry practices allow cacao plants to grow under the canopy of larger trees. This improves the quality of the soil, promotes the presence of pollinators, and leads to higher yield. Photo: J. Rocha, from Rocha et al., 2019
How is cacao pollinated?
Why am I talking about pollination if I was just talking about planting trees? There’s a relationship, I promise! Let’s back up a bit. Unlike other crops (e.g., pecans) most cacao plants need to be cross-pollinated to produce pods and beans. This means that most cacao varieties need to receive pollen from another plant to produce fruit. In the case of cacao, the pollen cannot be transferred by wind, which makes animal pollinators central to cocoa production. In a surprising turn of events, even though we tend to think about pollinators as bees or butterflies, this wonderful fruit is mainly pollinated by a very unexpected organism: a biting midge! 🤯
Midges of the genus Forcipomyia are the main pollinators of cocoa flowers. These tiny flies visit cocoa flowers and get covered in pollen, as seen in the picture on the left. Photos; left: S. Forbes; right: C. Quintin
Males and females of a group of midges (genus Forcipomyia) act as the main pollinators of the small cacao flowers. These midges visit the flowers to feed on nectar and pollen, which provides energy to the insects and helps females in egg production. While moving from flower to flower to feed, they transfer pollen between flowers from different trees, and increase fruit production. From this perspective, we need to thank these midges for the delicious chocolate we eat and drink!
And this is where planting trees relates to pollination. These midges prefer to develop on humid and shady environments, using leaf litter as a laying site. Making the soils shadier and increasing their leaf residues, agroforestry practices in cacao plantations directly benefit midges’ populations… and cacao production! Thus, through increasing the diversity of trees in these plantations, farmers can both make the soils provide nutrients for the plants to grow, and maintain large midge populations that ensure the effective pollination of cacao flowers. Isn’t it impressive what we can accomplish when we work with nature? And I mean, isn’t chocolate worth it?
Happy Holidays, everybody!
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
You decide to bake some cookies. You have your butter stick ready to go, you open your pantry to look for the ingredients. There is flour, oats, sugar, chocolate chips; things look good. You then realize that you’re missing that one ingredient, the one that makes it all come together: vanilla! Luckily, you can quickly buy some fresh vanilla pods or vanilla extract. In a couple hours you are there, enjoying your cookies and the pretty fall landscape.
This is all good, but have you ever thought how that spice – vanilla – gets to your pantry? And who is allowing for that to happen? In today’s blog, the second in our comfort food series (part 1 is here), we will talk about this spice that is so present in our lives that we may not even think about it. Let’s talk about vanilla and how appreciating it is tightly linked to understanding pollination and the key role of pollinators in our food system.
What is vanilla?
What we consume as vanilla is the fruit and the seeds of an orchid, the vanilla plant. This fruit comes in the form of a pod, and the tiny “dust” that comes off it is the hundreds of tiny seeds that this plant produces in each fruit. Vanilla orchids have a vine habit and in the wild are found clinging to trees in the forests of Central and South America. Considering this natural habit, all vanilla cultivation is done vertically, using different types of support.
Vanilla orchids have a vine habit, and the pollination of their flowers leads to the development of the pods and the tiny seeds we consume. Photos: M. Paredes, M. Manners, Joy.
Although vanilla is now cultivated in several parts of the world, it is accepted that all cultivated varieties/species are Meso- and South American. Indeed, the plant species had been known to be selected and used by Natives of those regions prior to the arrival of Europeans in the New World, but it is only following that arrival that Europeans created a strong demand for the spice. From this respect, if we can today enjoy our yummy cookies and cakes (and more!), recognition is due to the ancient selection done by Aztecs, Totonac, and Mayas in the current Mexican territories.
Each plant produces several pods that are harvested and dried before commercialization. Historical descriptions (here, from 1651) indicate that the plant we know today was cultivated by Natives in current Mexico, who called it “Tlilxochitl” or “black flower”. Images: Hernández (1651), Foam.
Today, vanilla is produced mostly in Madagascar, Indonesia, and Mexico, and is the second most valuable spice in the world (after saffron). Its production, however, experienced a bumpy road and still today goes through regular difficulties, which leads to extreme annual fluctuations in vanilla prices. In fact, vanilla plantations occur in regions regularly affected by extreme weather events, such as cyclones, which can destroy a whole year of production. These events lead to large variations in yield from year to year, leading to crazy changes in vanilla prices, going for example from $20/kg in 2010 to the current $350/kg.
How is vanilla produced?
Although vanilla became a European favorite quickly after it was first introduced to the continent, the production of vanilla pods remained elusive for a long time. Indeed, people realized very quickly that without active transfer of pollen to the stigma of the flower, the flowers would not develop into fruits (see how that works), and thus the much-searched-for vanilla beans would not develop at all!
In fact, after much observation of the plants in their natural habitat, people realized that their pollination required especially the visit of a group of bees restricted to the New World, the euglossines, or orchid bees. Restricted to South and Central America, these bees have strong associations with orchids, from which the males are known to collect floral scents they use for courting females (this is super fascinating, and worth a future blog post). Some species of this group of bees are currently suspected to act as pollinators of vanilla flowers in the wild. During their visits, they passively deposit pollen on the stigma of the flower, which leads to the vanilla bean development. Although these bees do pollinate, flower visits by these bees are not common, so even in regions with bee populations, fruiting rates remain relatively low.
In their natural habitats, vanilla flowers are thought to be pollinated by beautifully metallic euglossine bees. Photo: Gil Wizen, www.gilwizen.com.
Adding to this, once vanilla was “discovered” by Europeans, it was introduced into a variety of colonial lands, especially to Indian Ocean islands (e.g., Madagascar, the Comoros, la Réunion) and to French Polynesia. However, and because as I said before, the pollinators of this plant are restricted to the Americas, vanilla production was not successful in those regions. Plants would flower, but the lack of pollinators would lead to virtually no pod production. This changed when a solution was found. Indeed, there had been some early attempts to develop human-based pollination methods, which were as complex as impossible to use. It was finally a slave from the Réunion Islands, Edmond Albius, who developed a simple method to pollinate the flowers by hand, helped with a stick and his own fingers. It was only after this method development that vanilla production could bloom (actually, fruit 😉) to reach its current extent.
Edmond Albius was the Réunion slave who revolutionized vanilla production, developing the hand-pollination method still currently used today across the globe. Photos: Antoine Roussin (1863), F. and K. Starr.
Although one may expect the techniques to have changed since the first development of this method, the vast majority of today’s global vanilla production is still hand-pollinated following Albius’ technique! In other words, the production of the second most valuable spice in the world is currently based on pollination done by hand. And this is what I wanted to stress today. We hear a lot about the importance of pollinators, but I feel that the case of vanilla is such a clear example of how important pollinators are to maintaining not just food supplies, but also global economies: take the pollinators away and you lose basically the whole vanilla bean production chain and market. Doesn’t that make you feel especially thankful for pollination and pollinators for that great flavor in your cookies?
Happy Thanksgiving to all!
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
Wow, fall is here. When did that happen?! And because this fall comes after a tortuous year, I want to spend time doing some soul pampering. It is for this reason that, until the end of the year, I will be talking about many of the yummy foods we love and that many times help us through rough times. And to start the series, and matching the fall season, let’s talk about how the spices that create “pumpkin spice” – cinnamon, nutmeg, ginger, cloves – go around getting pollinated and reproduced.
How do you know it’s the fall in the US? We’re surrounded by pumpkin-spice everything! Photos: PatentingPatch, M. Mozart, J. Kramer, theimpulsiveguy.
As a general introduction to these spices, we have to realize that they all originate outside of the US, and that most of them are even today not produced in the US. This is important to mention because it is humbling to realize how much our food habits (especially those related to comfort foods) are based on foods that are imported by the US. Further, even if that may seem futile, markets for these spices have been historically and are still currently huge, with power over these markets driving major geopolitical clashes, setting the foundations of the current global distribution of wealth, and sustaining (and sometimes undermining) societies around the world.
The cinnamon bark is collected by “peeling” the tree (Photo: P. Nijenhuis). Cinnamon flowers are pollinated by many insects, but several Apis species are particularly important (Photo: D. Valke). Shown are Apis cerana and Apis dorsata visiting other flowers (Photos: Peterwchen, R. Thumboor).
Cinnamon
The cinnamon we eat comes from the bark of cinnamon trees. This bark is either ground or consumed in strips, which are added to savory and sweet foods. Cinnamon trees originate in South Asia and are adapted to growing in wet tropical forests. Today, the most important cinnamon producer is Sri Lanka, and most of the exports go to the USA and Western Europe. Even though we do not eat the fruits of this plant, pollinators play a key role in their reproduction. Cinnamon flowers are poor “selfers,” meaning that they produce the most seeds if receiving pollen from a different flower. These flowers thus rely on insect pollinators for their reproduction (see this post for more details about how flowers function) and among the most abundant species are three Asian “cousin” species of the managed honeybee, as well as some flies.
Nutmegs are the seeds found in the fleshy fruits of the nutmeg tree (Photo: B. Vauchelle). Nutmeg flowers are very small, pollinated mostly by thrips, and through a deceit-based pollination called “mistake pollination.” (Photos from Sharma and Armstrong, 2013). White bars indicate 1mm.
Nutmeg
The nutmeg we consume is the seed of the fragrant nutmeg tree, which originates in Indonesia. Even though currently it is cultivated heavily in Indonesia and Malaysia, it is also produced in the Caribbean. Because the food we consume is part of the fruit of this tree (there is no nut if there is no fruit), pollination of this crop is central for food production — and this is a super-fun crop to learn about!
Indeed, only some nutmeg trees bear fruit, because half of the seeds of this plant produces male trees (which produce pollen) and the other half produces female trees (which will make fruits and nutmegs). The fun pollination story doesn’t end there, though. The flowers of these plants are tiny, bloom in the night, and need pollinators to transport the pollen from the male trees to the female flowers. So who does this job? A lot of insects! Studies in the species have demonstrated that most pollination is done by tiny thrips, and probably also some beetles, flies, and maybe some bees (here you can read about pollinators other than bees).
But let’s spice up (pun intended) this story! This plant is not only pollinated by uncommon types of pollinators; it also tricks them into pollinating! In fact, the insects are interested only in male flowers, where they can collect pollen they can feed on, and they do not care about visiting female flowers, which do not offer any pollen or nectar. Thus, the strategy used by female flowers to attract pollinators is to trick them by making them assume they are actually male flowers, a strategy known as “mistake pollination”. It’s only after they entered the female flower and deposited pollen on the stigma that the insects realize their mistake.
Ginger plants grow from the rhizomes we consume and production is based on clonal reproduction (Photo: S. Podhuvan). Flowers are showy and small, but their pollinators are not well known (Photo: Ogniw).
Ginger
The part of the plant we consume from ginger is its rhizome, meaning that one can plant the piece of ginger one buys in the store and one would grow a ginger plant! This plant species also originated from the Southeast Asian archipelago. The plant is easy to grow, and thrives in warm climates, but most of the world production is currently from India. Under production conditions, ginger is multiplied through the planting of rhizomes, meaning that most of the production is not based on seeds. For this reason, the pollination of this species was not of high production interest until only recently. Indeed, while ginger propagation is based on rhizomes, this does not allow for the use of sexual reproduction for the development of better new varieties that may be resistant to diseases or pests. Recent studies indicated that ginger is extremely hard to pollinate because pollen has a low rate of successful pollination, leading to very low seed success. Several researchers are now focusing on identifying its pollinators, so stay tuned to know more!
Cloves are the dried immature flowers of the Clove tree. Flowers are harvested right before they open and are dried to reach the product we find in our markets (Photos: Midori, A. Heijne, Peripitus).
Cloves
As with most other spice plants treated here, cloves also come from a tree, which originates from the Moluccas, in Indonesia. While the plant originates in those islands, most of clove production is currently from Indonesia and Madagascar. The part of the plant consumed is the flower buds, which are harvested and then dried to produce the spice we buy. Although we consume the flowers, these plants still require seed production to reproduce, and this is central to maintaining clove production. Because the plant can self-pollinate but its genetics are improved by cross-pollination, pollinators are very important for its reproduction. Here, again, pollination is not very well known, but flies, bees, and some butterflies are suspected to play an important role in transferring pollen, as it has been observed in a closely related (but not cultivated) species.
Note: this blog post is dedicated with love to Luke Harmon, who despises Pumpkin Spice. <3
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
The summer is slowly leaving us, with many of us starting to harvest the last wave of vegetables and enjoying the beautiful late summer flowers. At this time of the year, I like to review what happened in the peak of summer and consider what I may want to plant next year, based on what did and did not work this season. I enjoy thinking about how to help pollinators with their favorite plants. I invite you to join me today in exploring how our plant choices are important, using the famous butterfly bush as an example.
Butterfly bushes grow and flower extremely quickly, displaying beautiful and strongly- scented flowers. Photos: Ptelea
Butterfly bushes, a double-edge sword
You may have seen these plants with their blue-purple flower clusters poking out of the bush, and a ton of butterflies and bees visiting the flowers as soon as it gets warm and sunny. The butterfly bush (Buddleja davidii) is a plant species that originates in China. Given its beauty and ease of growth, it was popularized as an ornamental plant first in Europe and later on in other parts of the world.
Its popularity has real reasons: the plant grows fast, flowers very early on in its life cycle, and produces flowers throughout its life span of up to 30 years. The flowers smell good, are very showy and pretty, with large clusters that bloom for several days. Because their flowers need cross-pollination to produce seeds (see how this works in this post), these plants depend on pollinators for reproduction. In their natural habitat, pollination is done mostly by butterflies.
After reading this you may be wondering where I’m trying to go with this post. It seems that these plants are great, so there is not much more to say. If I am considering planting something that will be attractive to pollinators and that is easy to grow, this is a no-brainer, right? Well… maybe not; let me explain.
The problem with plants that are too good to be true is that they usually have a down side. The down side of the butterfly bush in our region is that they are so good that they can “take over” other native plants, which has a number of negative consequences.
Taking over native plants
First, let’s talk a little bit about what I mean by these plants “taking over.” Like all organisms, plants have specific needs to survive in a given place. These needs and abilities evolved over millions of years and are often an evolutionary response to the climatic and soil conditions and the identity of the other organisms living in that community (for example, pathogens, herbivores, pollinators, etc.).
When a foreign species arrives in a new region (like the butterfly bush being introduced in the USA), the fact that they have not evolved in that region can play against them, since they may not tolerate the climate or soils, or may be attacked by parasites or herbivores that they can’t properly defend against. This is why it is often hard to grow species that come from other regions.
Other times, however, not having evolved in the new area can help, especially when most other organisms are not adapted to the species. This means that the new species now does not have to deal with any parasites or herbivores, clearly giving them an advantage over the local species. These advantaged foreign species end up becoming invasive and problematic. Unfortunately, the beautiful butterfly bush is one of these invasive species.
Invasive plants such as the butterfly bush establish and spread quickly across the landscape. Photo: Forest and Kim Starr, Starr Environmental, Bugwood.org
After being introduced into the USA, the butterfly bush is today present and spreading in many regions, including Maryland. This species can reproduce so well, grows so fast, and importantly, has so few herbivores and diseases, that it is able to not only survive, but also spread into new areas at very high speed. Today, the species is listed by the USDA as a weed or noxious weed. It is displacing native flowers as well as agricultural and forest species.
Butterfly bushes get their name for the very large number of butterflies they attract. Here a Pipevine Swallowtail is feeding on the attractive flowers. Photo: Quadell
Monopolizing pollinators
The butterfly bush not only displaces natives by physically occupying the space that native plants would need to survive. The displacement seems to also come in more indirect ways. Because butterfly bushes offer copious amounts of nectar, they become extremely attractive to pollinators, distracting them from other native co-flowering species, and reducing the native’s reproductive success which eventually also harms the native’s populations. This is something that seems to be happening at least in some parts of the USA, suggesting that by favoring this plant in our gardens, we may be indirectly harming the survival and successful reproduction of many of our dear native plants.
You may remember from my previous posts (What should I plant to help pollinators? and Why do pollinators visit flowers?) that plants support pollinators with resources like nectar and pollen. Although the butterfly bush offers abundant nectar to local pollinators, it has been argued that its nectar is too concentrated and could serve as “junk food.” This is likely inaccurate, since studies of nectar concentration in this species indicate that it falls within the usual concentrations seen in other plants preferred by butterflies (watch out for a future post on the super fun topic of nectar concentrations).
Along with this, many people worry that the pollen may not be as nutritious as that of native plants, and from that respect the butterfly bush may be doing more harm than good to the local pollinators. This is currently very much investigated, and even though it seems that the nutrients in its pollen are not equal to those of native plants, they do contain the essential elements the tested pollinators need.
As we covered in other posts, pollinators do not only consume pollen and nectar. In fact, caterpillars of moths and butterflies feed on other plant parts (e.g., the leaves). In our region, the butterfly bush has little to offer in that respect. Because it is not native to our area, few species of caterpillars can feed and develop on this plant. From this perspective, this plant species is not only physically displacing other native plants that are good hosts of local butterflies, it is also unable to provide the food the local caterpillars need… thus finally harming the butterfly population!
But I want a lot of butterflies… what should I plant?
Despite its name, if we want to benefit butterflies, opting out of the butterfly bush seems like the right thing to do. Instead, plant some of our many beautiful native species. A good place to start for that is my other post: What should I plant to help pollinators?
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
In seasonal regions such as Maryland, nature goes through cycles, with some seasons reserved for growing and reproducing, and others for resting and waiting for conditions to get better. One of the groups of organisms that in my opinion represent clearly those changes in seasons are butterflies, which go through extreme modifications in their bodies and ecologies to closely match the changing seasons. Today’s post is going to explore two beautiful butterfly species that we can find right here in Maryland: the common Black Swallowtail and the beautiful but imperiled Baltimore Checkerspot.
First Things First: The Life Cycle of Butterflies
The life cycle of butterflies is fascinating and complex, and in our region is usually tightly linked with changes in season. Image: Kids Press Magazine
Before going into details about these butterflies, I think it is important to explain how these organisms develop because their life cycles are usually tightly related to our seasons. Caterpillars have indeed pretty special and fascinating life cycles. In these insects, a female lays eggs on the preferred host plant of the species. This way, the first larva (a tiny caterpillar) that emerges from that egg will not need to move far to feed on its favorite and most nutritious plant.
Once larvae hatch they start feeding on plant material, becoming bigger as they eat. Because insects such as butterflies are covered with a special hard ‘skin’ called an exoskeleton (this is really an external skeleton!) that gives them support and structure, every time the caterpillar gets too big, the exoskeleton becomes too tight (imagine a kid outgrowing a T-shirt). At that point, the caterpillar breaks the old exoskeleton and grows a new larger one in which it can fit.
While going through these ‘changes of skeleton’ (called molts), the caterpillar is able to grow until it is large enough to make their last change: pupation. At this stage, the very large caterpillar is ready to become an adult. For this, the caterpillar will molt a last time and become a pupa, which is the form that builds the cocoon in which the last body changes happen before the adult butterfly emerges.
As one can see, because the life cycle of butterflies has so many stages, there are many chances for things to go wrong during their development, which can also explain some annual fluctuations in butterfly populations. For example, if a wave of particularly cold or hot weather happened during one of the stages at which the caterpillars are sensitive (e.g., pupa, first instars), we may not see many butterflies later in the season. The same is true if there are important disease outbreaks, if predation was particularly high earlier in the season, if the host plants were not as abundant as other years, or if insecticides were applied close to some of the preferred host plants.
Now that we have a better idea of how the life cycle goes, let’s take a look at what our two species do and how they differ in their food preferences, life cycles, and how that affects how we can promote their presence in our surroundings.
Black Swallowtails
Even though it may seem obvious to some, let me start by saying that this species gets its name from the shape of its hind wings, which look like the pointy tails of swallows. Black Swallowtails (Papilio polyxenes)are common butterflies in our region and are present across the whole eastern USA.
In Maryland, this species has between two and three generations per year, with the first generation(s) of a season reaching adulthood within the season, and the last one spending the winter in pupal phase and emerging as an adult the following spring. (Check this other blog to learn more about butterflies in the winter: Where are all the pollinators?)
Like all butterflies, Black Swallowtails are specialized on what they feed. What defines what makes a plant yummy or not to the caterpillars are the chemical compounds the plant carries. In fact, plants have evolved to produce different chemical compounds that protect them against the multitude of herbivores that exist. Black Swallowtails in particular have evolved to tolerate the chemical compounds present in plants of the parsley family (Apiaceae). It is for this reason that these caterpillars can be found in your garden feeding on carrot, parsley, or dill leaves. Adults (butterflies) are usually seen collecting nectar from flowers such as clover, milkweed, and thistles.
Black Swallowtails can have several generations per year, feeding on plants of the carrot family, and finally emerging as beautiful adults either in the same or the following season. Photos: eggs (wikiCommons), larva (PINKE), pupa (Woodleywonderworld), adult (J. Flanery).
Black Swallowtails are currently considered to be a species that is not particularly at extinction risk. However, to maintain their populations it is recommended that open grasslands with plants that serve as caterpillar hosts are present. Interestingly, because they are so common in our area, they can be easily reared indoors, something that is really fun and can be a great summer project for kids (and adults!).
Baltimore Checkerspot
If Maryland has a state dessert, I feel it’s only fair that it also has a state insect! The Maryland state insect is the exquisite Baltimore Checkerspot (Euphydryas phaeton), which was chosen because its colors remind us of those of the Maryland flag. In our area, Baltimore Checkerspots are not as common as Black Swallowtails, and, unlike Swallowtails, have only one generation per year.
Their diets are also significantly more specialized than those of the Swallowtails: young caterpillars feed exclusively on white turtlehead, on which their eggs are laid. While later on in their development they are able to feed at least partially on alternative plants, white turtleheads are required for them to survive the early caterpillar stages.
Finally, unlike most butterflies in our region which spend the winter as pupae, this species spends its winter as a caterpillar and pupates only in the spring. The caterpillars of this species are super cute and they always remind me of the Dust Bunnies of the movie “My friend Totoro”. Adults of this species feed on milkweeds, dogbanes, and wild blackberries.
Baltimore Checkerspots go through different developmental stages, hatching on their preferred host, and feeding on them as caterpillars, before entering pupal phase and finally emerging as adults. Photos: eggs (NABA.org), larva (wikiCommons), pupa (4.bp.blogspot.com), adult (S. Snyder).
Unfortunately, even though these beauties are our state insect, they are currently imperiled in our area. The reasons for this have to do with changes in land use, which led to less white turtleheads being available to the caterpillars both because less natural habitats are present and because the deer populations are so large that they eat most of the host plants!
If you would like to try to contribute to these butterflies’ populations, you can plant white turtleheads in your yard, but in particular support conservation actions already happening in Maryland, such as the Baltimore Checkerspot Recovery Team.
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
Bumblebees often can be seen feeding at the flowers of foxglove beardtongue (Penstemon digitalis), which blooms from late May through June. Photo: C. Carignan
We all want to protect pollinators and it seems that the best way to do that is to have a lot of flowers so they can feed on them. But if you’ve ever checked a seed catalog or visited a plant nursery, you may be overwhelmed by all the options. How do you choose what to plant? In today’s post we will chat a bit about the why’s of these choices and we’ll share some resources that may be useful next time you’re trying to make those decisions.
Each pollinator species is unique
As all species in the world, each pollinator species has unique reproductive, nutritional, and habitat requirements to survive. For example, a bee that nests in the early spring needs food and habitat that will be different from those of another bee that nests in the summer, or of a butterfly that emerges from its metamorphosis in late spring. For an early-spring bee it will be key that flowers are available early in the season. Those will be of no help to a summer bee. Likewise, a late spring butterfly will be able to enjoy the nectar from flowers that were not available to the early-spring bee.
Along with the timing of emergence, each pollinator is unique in its anatomy and sensorial abilities. For example, long-tongued bees can reach the nectar of flowers that may be too deep for short-tongued bees. Similarly, because of their extremely long mouth parts, hummingbirds and butterflies usually can access very tubular flowers that are just out of reach for other pollinators.
It’s not only the shape of the mouth parts of the pollinators that will play a role in what flowers they can feed on. Their general body shape and physical abilities will also define this. For instance, butterflies can’t regulate their flight as well as hoverflies or bees do, and because of this, when they visit flowers they need to have large surfaces on which to land, while bees and hoverflies may not really need them.
Swallowtail butterfly on butterfly weed (Asclepias tuberosa). Photo: UME Home and Garden Information Center
Finally, different pollinators have different sensorial abilities, with some being able to see some parts of the light spectrum that others may not. On this, butterflies and hummingbirds can see many different colors including UV light, bats that pollinate are blind, and bees have a broad spectrum of light vision but can’t differentiate many of the colors we can.
So now you may be asking yourself why I am talking about all of this. How does this relate to the topic of this post: how to help pollinators with flowers? Bear with me, I’m getting there!
How should I choose what to plant to help pollinators?
As you may be guessing by now, because each pollinator has slightly different life requirements, if you want to help as many pollinators as possible, your best shot is trying to diversify your garden or flower bed. I like to think of this as if I were holding a dinner party at my house and I want to have as many of my friends enjoy the food.
If I know that some of my friends are vegan, lactose intolerant, or allergic to nuts, I will make sure that they find something to eat at my table. If I don’t have anything for them, they will be hungry and sad, and they will also probably decline any future dinner invitations from me (how sad is that!?). So, I like to think about these pollinator plantings as a party I am hosting for a whole season, and where I will make sure that all my little friends always have something to eat so they come back next time I invite them over!
A diverse flower garden includes flowers of many different colors and shapes and will have different plants in flower throughout the season. Photo: Carol Norquist
The key to attract the most pollinators is diversifying our gardens! Ideally, the choice of plants should include different flower colors, shapes, and sizes available throughout the season. This means that there will be always several different types of flowers blooming at the same time, even though no one plant may be flowering throughout the season. Along with this, if one is trying to attract specific pollinators that have very specific food requirements (for example, oil-bees, monarchs), one would also have to make sure that the pollinators’ required food is also present (take a look at this recent post to learn more: Why do pollinators visit flowers?)
Another aspect to consider when deciding what to plant is the fact that native pollinators usually get appropriate nutrition at the right time of their life cycle if they feed on plant species that are also native to the area. For this reason, if one wants to help pollinators, native plant species are usually recommended, and in particular, avoiding invasive exotic species is key. In fact, invasive species, in addition to not providing ideal food for native pollinators may also displace native plant species, reducing even more the diversity of your flower bed and the pollinators who will visit it. Finally, this also means that a “good” flower mix for pollinators from Europe is probably not going to be ideal for Maryland pollinators.
This perennial flower garden includes a variety of species that bloom at different times throughout the growing season. Photo: C. Carignan
But then, what should I do?
There are so many things to think about! This is truly a brain twister, right? Luckily for you (and me) many biologists, ecologists, and conservation specialists have been thinking about this for a while. Today, floral mixes have been created that are appropriate to different regions of the United States. In the state of Maryland, the Department of Natural Resources has created a neat list of species you can plant depending on the conditions on your land. The Xerces Society has also put together a list for plants appropriate for different states. Alternatively, if you would like to just favor specific pollinators, you can target their preferred plants. For finding seeds and starts for these plants, take a look at this great resource the Maryland Native Plant Society has put together!
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.
Eastern skunk cabbage at pine hole bog by dreamexplorer is licensed under CC BY-NC-SA 2.0
