June 23, 2022

Last week I found a surprising diversity of pollinators feeding on leafy spurge, a flower that everyone hates. But I also noticed a very different community of pollinators on the nearby two-groove milkvetch (Astragalus bisulcatus). This is a beautiful native plant, a flower that’s easy to like.

Today, in celebration of Pollinator Week, I’ve returned to pay close attention to the two-groove milkvetch and the insects it is supporting. Which insects are visiting the milkvetch flowers? What are they doing? And together with last week’s investigation of leafy spurge, what can this teach us about supporting pollinator diversity?

Symphony on the milkvetch

On this warm, sunny morning, the milkvetch patch sounds like a symphony of chainsaws. Dozens of honeybees (Apis mellifera) are working the long purple flower clusters. Every short flight they make is accompanied by a whining buzz. There are leafcutter bees here, too, about the same size as the honeybees and making similar-pitched buzzes. They’re easy to pick out, though. The leafcutters (family Megachilidae) have the undersides of their abdomens covered in bright golden-orange pollen. Leafcutter bees are the only group to carry pollen like this. All other bees gather pollen on their legs (except for a few that carry pollen internally, in their crop).

A few bumblebees (Bombus spp.) are stopping here this morning, too. They’re the basses of this symphony. One is lifting off right now, its wings giving a low, deep-throated roar.

Pollen dance

One of the leafcutter bees lands on a milkvetch flower. I inch closer until she’s just inches from my face. She pokes her head adeptly between the lower, canoe-like keel petal and the upper, purple-striped banner, forcing them apart. The keel contains the milkvetch anthers, with their bright red-orange packets of pollen. Between the anthers is the female part of the flower, the pistil.

Now the leafcutter bee does a dance that looks complicated to me, though it must be routine for her. She combs backwards with her middle legs. At just the right moment, she engages her hind legs, rowing them backwards as well. With the leg dance, she transfers pollen to the underside of her abdomen. It all happens so fast, it’s hard to see how she’s doing it. Her abdomen is already densely covered with pollen, held there by special, branched hairs. Mission accomplished, the leafcutter revs her wings and helicopters over to the next flower.

What about the honeybees?

The honeybees are acting differently. First, they aren’t flying as much. Instead, they’re spending a lot of time crawling from one flower to the next. A honeybee lands on a flower in front of me and I watch her closely. Like the leafcutter bee, she butts her head between the keel and banner petals, opening the flower. But she doesn’t bother to comb pollen onto her legs. In fact, the pollen baskets on her hind legs are completely empty. She doesn’t seem interested in the milkvetch anthers at all. Instead, her attention is directed farther inside the flower, where the keel and banner petals meet. Is she getting nectar instead of pollen?

I remove a flower for closer investigation. Unlike these bees, I’m not a milkvetch expert. Instead of using their well-practiced petal-shove, I open up the flower by indelicately ripping off the banner petal. At the base of the keel petal, I notice two lobes where the lateral petals, the wings, join the keel. Is there nectar there? I don’t see any obvious glands, like the nectaries of the leafy spurge from last week. But clearly there’s something here that is attracting the honeybees’ attention. It isn’t pollen, so likely it’s nectar, even though I can’t spot it with ten-times magnification.

Ghosts among the milkvetch

Something that I love about spending a day watching flower pollination is how much I only glimpse. Just like the massive trout that gets away, it’s a sure sign that there’s more going on here than we can grasp. Right now, I spot a hawkmoth (family Sphingidae) visiting the milkvetch flowers six feet away from me. It’s the size of a bumblebee queen. Its flight is silent and its wings are partially transparent. I reach for my insect net. The hawkmoth spots the distant motion and shoots away, a silent and wary ghost. 

What species is this hawkmoth, and what’s its story? Unless I’m able to catch one for a closer look, I’ll never know. So I keep my eyes peeled, hoping to get another glimpse of “the one that got away.”

Some of these milkvetches have already been in bloom here for three weeks. Most of the flowers are a deep, royal purple, but some plants are a pale lavender and I even spot one with all-white flowers. There are buds still at the tips of the plants. Below them are the open flowers the bees are visiting. Still lower, green fruits are swelling like miniature, drooping peapods. It’s easy to see the two deep grooves running along each fruit, the field mark that gives this plant its common name. The grooves are light green, outlined by red ridges.

Of peas and selenium

For those of you who are gardeners, you’ll recognize many family resemblances between these milkvetches and garden peas. They have unusual two-lipped flowers, pod-like fruits, and compound leaves made up of many leaflets. And like garden peas, the milkvetches are legumes (family Fabaceae). Their root nodules harbor nitrogen-fixing bacteria, allowing these plants to obtain nitrogen from the atmosphere.

Two-groove milkvetch is chemically interesting for reasons besides nitrogen fixation. It often grows on soils rich in selenium. Humans and many other animals need this element at small concentrations, but at high concentrations it becomes toxic. Two-groove milkvetch is rather unusual when it comes to selenium: it can accumulate high levels of the element in its tissues. By using particular chemical pathways to store selenium in forms the plant can recognize and handle with care, it avoids the potential toxicity of this element. Why is this advantageous? There are a number of reasons: 

• Two-groove milkvetch can grow in selenium-rich soils, areas inhospitable to many species. 
• By storing selenium, these plants become much less palatable to herbivores such as grasshoppers and prairie dogs. (However, some specialized insects have co-evolved to tolerate high selenium levels and feed on these plants.)
• These plants can act as selenium pumps, boosting levels of this nutrient in the soil around them. This makes conditions even less hospitable for intolerant plants, giving milkvetch a competitive advantage.

What about pollinators? So far, studies have shown that bees don’t seem to discriminate between flowers of selenium-accumulators and non-accumulators. In two-groove milkvetch flowers, selenium concentrations are high, but there aren’t any studies on selenium concentrations in this plant’s nectar or pollen. And whether selenium from flowers may affect bees (either negatively or positively) remains unknown. (However, honey from bees in selenium-rich areas seems to contain amounts that are beneficial for human health.)

Aphid ranching

Moving on from the puzzle of selenium ecology, I spot a few ants crawling around the milkvetch flowers. They’re much less abundant than the ants I saw on the leafy spurge last week. And what are these ants doing, anyhow? They aren’t actually entering the flowers, just clambering past them and around them. Then I see why: between the flowers, several stems are covered with colonies of black aphids. The ants are associating with the aphids, presumably protecting them from predators.

The relationship between aphids and ants is well-known: while the aphids feed on their host plant, they excrete a sugary honeydew for ants. In exchange, the ants guard the aphid colonies. So while the ants on the leafy spurge were feeding on nectar that the plant offered freely, and probably transferring some pollen in the process, these ants seem to be stealing sugars from the milkvetch by way of aphid ranching. And it seems they aren’t contributing to pollination.

Aphid ranching has its risks, though – and here comes one of them now. It’s a transverse ladybug (Coccinella transversoguttata), one of our native aphid predators. The ladybug crawls methodically along a milkvetch leaf, then flies nimbly to a raceme of flowers. This flower cluster doesn’t have aphids, but it’s just a matter of time until this ladybug will find its juicy lunch.

Ladybug ecology

All of this crawling and flying is typical ladybug behavior. Aphid colonies are short-lived. It’s hard to predict where they may show up, so these ladybugs have become adept at finding them. This is one of the reasons why buying ladybugs for your garden is usually a waste of time: they’ll probably fly away. Buying these frequent fliers can be problematic for other reasons, too. One of the most commonly sold species is the convergent ladybug (Hippodamia convergens). Like the transverse ladybug, this species is another of our native aphid-eaters. But ladybug suppliers don’t rear these in captivity – they collect them by the billions from places where these ladybugs gather to overwinter. How does this mass-removal of ladybugs impact our wild populations? No one seems to know yet. 

We do know, though, that these releases are rarely effective. What’s more, shipping these ladybugs around the country can spread diseases and parasitoids. For aphid control, a better bet might be to create good habitat around your garden for ladybugs and other predators. What makes good ladybug habitat? Consider planting two-groove milkvetch and other native plants. These plants host their own species of aphids throughout the season and also provide pollen and nectar.

I’ve already spotted two more ladybugs on the milkvetch: another transverse ladybug and a native Hippodamia (H. quinquesignata or one of its close relatives). With good habitat – aphids and a diversity of native plants – ladybugs will probably fly to your garden on their own. In fact, you may attract not just ladybugs, but also other aphid predators such as syrphid flies.

Bees beware

What’s that reddish wasp doing over there, lurking on the grasses near the milkvetch flowers? It’s actually a fly, not a wasp! It’s a pretty good mimic, though, with its slender, wasplike orange abdomen. Bees beware: this is a thick-headed fly (family Conopidae), a sneaky parasitoid. This one seems to be a member of the genus Physocephala, a group I’ve collected here before. How do these flies make their living? A female will lay in wait where bees and wasps are active, attacking them on flowers or in flight. She will rapidly insert an egg into the hapless host’s body. If she succeeds, her larvae will feed inside the bee or wasp, eating it from the inside until it dies. It’s gruesome, but it’s also just part of the complex world of this milkvetch patch. Insects are stranger than science fiction!

A tiny, iridescent greenish-black bee is flying from flower to flower now. Its gentle buzzing is impossible to hear over the chainsaw symphony of the honeybees. I catch this one and see that its abdomen is covered in orange pollen. It’s another species of leafcutter bee! Then I spot a third leafcutter species, this one with a rusty-haired thorax and shiny black abdomen. The underside of its abdomen is just lightly dusted with pollen.

Windy afternoon

Throughout the morning, Helena’s usually-ferocious wind has been almost still. Now, as if it has realized its lapse, the wind has become a force to be reckoned with. Its gusts whoosh through the grasses, overpowering the buzzing of the bees. The bumblebees are nowhere in sight now. The milkvetch is bobbing so much that I can barely spot pollinators, let alone identify them. But still, the leafcutters and honeybees are holding on, climbing tenaciously into one flower after another. With such dedicated pollinators, it’s no wonder so many fruits are developing successfully.

Undoubtedly what I’ve seen this morning isn’t everything: there must be other pollinator species that visit the milkvetch. But for now, the wind is making further observations impossible. 

Yellow and purple

With the exception of the honeybees, none of today’s insects have overlapped with the pollinators I found visiting leafy spurge last week. And no wonder: these two flowers are as different as night and day. Yellow versus purple. Easy-to-reach nectar versus valuable pollen; slender wasps versus fuzzy bees. Both are flowering together here. And together, they’re supporting a much more diverse pollinator community than either could on its own. What if we could add more flower diversity here? If we added lots of other native plants – the flowers with which our pollinators have coevolved – how many more insects could we support?

The wind isn’t letting up. If anything, it’s getting stronger. With afternoon insect observations thwarted by the wind, I’m going to range farther afield and look for flower diversity. What other native plants can we find around here, flowering now or soon, that might add to the pollinator habitat in this grass-dominated patch?

Willow and beeplant

Along the edge of the stream is a slender, silvery-green stand of sandbar willow (Salix exigua). It, too, is flowering now, quietly offering up its inconspicuous, yellow catkins. I may get irritable with the wind, but the willows sway with it. (They whack me in the face as they do.) For insects, the leaves seem to be offering a bit of shelter from the gale. Even on this gusty afternoon, I spot a small, black bee on the flowers. This is a species I didn’t see on the milkvetch. The flowers are also sheltering several tiny flies and click beetles, feeding on pollen as they sway back and forth. Sandbar willows are a moisture-loving species, often growing right at the edge of streams and ponds. But irrigated lawns are a lot like floodplains: a small patch of sandbar willows could do well in a residential habitat.

In a bare patch, I find a Rocky Mountain beeplant (Cleome serrulata). This native annual will bloom from now until frost. In the afternoon’s wind, all I see on it are a few ants. But on other days, I’ve found bumblebees, small wasps, Becker’s white butterflies (Pontia beckerii), and many other insects on these flowers. Beeplant likes disturbed soil and is easy to grow from seed.

Globemallow, goldenaster, and thistle

Along a dry roadbed of dirt, away from the stream, I see a patch of scarlet globemallow (Sphaeralcea coccinea). This low-growing perennial is inconspicuous except when it flowers. Then, its broad red corollas catch our attention, as well as that of bees. Several bees specialize on globemallow flowers, including species of Perdita and Calliopsis.

Out in the arid grassland, a sea of yellow is blooming. It’s hairy goldenaster (Heterotheca villosa), another tough, low-growing perennial. These plants can keep blooming from now to the fall. And last September, I found a very special bee fly visiting these flowers. A grasshopper predator, Anastoechus barbatus lays its eggs on the soil. There, its larvae crawl along, searching for grasshopper eggs, which they destroy. I may think grasshoppers are cool – but anyone who’s ever had a garden devoured by them would probably beg to differ. This bee fly is a welcome addition to any garden – and so are the flowers that support it.

What’s that brush of light purple on the hillside? It’s our common, native wavy-leaved thistle (Cirsium undulatum). Sometimes people mistake it for one of our weedy, non-native thistles and kill it. That’s unfortunate, since these flowers host bumblebees and a range of other pollinators. Even in today’s wind, this single flowerhead holds 60 tiny gray beetles (probably soft-winged flower beetles, family Melyridae) and two large orange blister beetles.

And… more native flowers!

I’m having to search far and wide to find these flowers, but the diversity is a good sign for pollinators. In moister areas along the stream, I find yarrow (Achillea millefolium). These flowers are shallow, easily accessible to insects.

Blanketflower (Gaillardia aristata) is just starting to bloom, its striking heads like miniature sunflowers.

Prairie flax (Linum lewisii) has a few open flowers and many slender, nodding buds. Earlier this summer, I noticed several small and medium-sized bees visiting these blooms.

Here’s a patch of wild licorice (Glycyrrhiza lepidota). This native relative of cultivated licorice (Glycyrrhiza glabra) has bur-like brown seedpods. I’ll admit that some people don’t like this plant because of its burs. But bumblebees love the white flowers, which will open up in a few more weeks. And last summer, I found a patch where over 50 ladybugs were feasting on aphids. Do you have aphids in your garden? Maybe a patch of wild licorice would attract some ladybugs.

Attract more bees

Today I’m picking and choosing, finding the native flowers scattered here and there. As I wrote last week, a sea of non-native smooth brome (Bromus inermis) dominates the habitat along this stream. But what if our streamsides held an abundance of all these flowers?

There would be patches of two-groove milkvetch for leafcutter bees, aphids, and ladybugs. We could tolerate some leafy spurge for ichneumonid wasps and biocontrol beetles. Yarrow, scarlet globemallow, and sandbar willows would feed other pollinator species. We could plant hairy goldenaster and wild licorice for mid-summer flowers. Rabbitbrush and goldenrod would feed insects in the fall. With all of these flowering plants (and a few dozen others we could think of), how many species of pollinators could we support? Hundreds?

Eventually, this is what I hope my yard will look like. And when it does, I have a feeling I won’t be worrying about aphids getting out of hand. (I might go crazy trying to understand all of that insect diversity, though!)

Learning from the milkvetch

Two-groove milkvetch is an interesting plant. Popular with a variety of bees, strikingly beautiful in bloom, and capable of accumulating selenium to levels that are toxic to many organisms, it’s something of an enigma. But although questions remain, a close look at the milkvetch reveals some general patterns that we can take to heart here in the midst of Pollinator Week. Here are some takeaways:

• There’s a lot more going on in our plant communities than first meets the eye.
• For pollinator diversity, floral diversity is a good thing. Milkvetch will attract some species, leafy spurge others, and globemallow still others.

It might not surprise you to read that I’m pretty excited about our local plants and the pollinators they support. And as I imagine habitats brimming with native flowers and filled with bees, I’d love to hear from you! Have you tried to add some native plants to your yard or neighborhood? Are there native plants that seem especially important for pollinator diversity? Have you observed anything related to two-groove milkvetch and selenium? Let me know!

Until next time, let’s take a moment to thank our local plants for supporting all of our pollinators. Maybe we can make space to add a few more to our yards or our neighborhoods.

Further reading

Wilson, Joseph S. and Carril, Olivia M. (2016). The bees in your backyard. Princeton, NJ: Princeton University Press.

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