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.

The post How to attract more bees: plant milkvetch appeared first on Wild With Nature.

June 16, 2022

I think I may be the only person in Helena with a fondness for leafy spurge (Euphorbia esula). Yes, I’m talking about that leafy spurge, the noxious weed that everyone hates. I started to like leafy spurge last summer, when I noticed that the flowers seemed to be extremely popular with a diversity of colorful pollinators, especially wasps. The colonies I was watching were small ones along a stream. This was an area otherwise dominated by non-native grasses – grasses which offered essentially nothing for pollinators. Leafy spurge was one of the few nectar sources available – and it was extremely popular with the insects.

So is this noxious weed always horrible, as everyone seems to think? Or is there more nuance to this story? Today I’m going back for another look.

On this morning at the cusp of summer, the landscape is bursting with life. The hills are green from the recent rains. The meadowlarks are singing and the cottonwood leaves are out. Birds are everywhere.

The first spurge patch I visit is tiny, a few clumps flowering among a thick floodplain stand of smooth brome (Bromus inermis). Smooth brome is one of several abundant, non-native grasses that dominate much of the Helena Valley. Competitive, turf-forming, and prone to crowding out natives, it seems to harbor very little biodiversity. However, it makes a good pasture grass. Perhaps that’s why the State of Montana doesn’t consider smooth brome a noxious weed. Leafy spurge, on the other hand, is listed as “noxious.” According to the Montana Natural Heritage Program, noxious weeds are plants that “have a destructive impact on Montana’s landscape.” They highlight displaced native plants and lost wildlife habitat as particular concerns. Yet ironically, in my experience in the Helena Valley, smooth brome appears to pose a much greater threat to native plants and habitats than leafy spurge does.

Sleepy morning

Early on this sunny morning, these leafy spurge clumps are relatively quiet. But already, dozens of ants are busy on the stems. I spot a sleek, black-and-orange wasp flying from flower to flower. I swing my insect net and it tumbles in. 

Wasps are so diverse that to identify them to species usually means collecting them and studying them under a microscope. Judicious collecting has a negligible impact on insect populations (unlike habitat loss and insecticides), but I still try to minimize it. Today I will be collecting some of these insects, though. Over the winter, when I have more time, I’ll be trying to identify these creatures to species. Species identification is the key that unlocks any studies scientists have already done with these species. All winter long, I’ll be learning more about the summer landscape, trying to understand what all of these insects are doing in the ecosystem.

I transfer this wasp from my net to a killing jar, where it dies quickly from ethyl acetate fumes. Now I can look at it up close with a magnifying lens.

The wasp is surprisingly delicate in my hands. The black head and thorax contrast with the orange abdomen and legs. The slender, flexible antennae are made up of over 30 segments. These super-segmented antennae are found in only a few wasp families. This plus the pattern of wing venation tells me that this is an ichneumonid wasp (family Ichneumonidae). The ichneumonids are unbelievably diverse. There are well over 5000 species in North America, and many more that scientists have not yet described.  

What good are wasps?

“Wasps,” you may be thinking: “aren’t those the annoying creatures that hover around picnics and sting children?” Those wasps, the social vespids (such as yellowjackets and paper wasps) are actually just the tiny minority that give this whole, colorful group a bad name. The reality is that 99% of wasps go about their lives without any interest in stinging us. Many of these wasps still can sting – but you have to be trying really hard to get one to sting you. And because they don’t visit our picnics, you probably won’t notice them unless you’re looking for them. 

What good are wasps? To start with, they’re colorful, extremely diverse, and have fascinating life histories. They’re important pollinators (this is even true of the species that like to sting us, such as yellowjackets). And the vast majority are specialized predators or parasitoids. These wasps spend their lives hunting down specific insects. Depending on the species of wasp, they may attack cutworms, weevils, grasshoppers, aphids, other wasps, or even spiders – in short, basically any invertebrate imaginable. Many of these wasps help regulate potentially “pesty” herbivorous insects. All of them play a critical role in the complex food webs that surround us.

Wasps for farm and garden

What about the ichneumonids? These wasps are all parasitoids: females of each species seek a specific type of host insect and lay their eggs on it. Sooner or later, as the ichneumonid larvae develop, they kill their host. Because of this, ichneumonid diversity isn’t just of interest to nature-lovers and biologists. It’s also important to gardeners, ranchers, and homeowners – to anyone who has ever experienced an outbreak of some herbivorous insect.

It’s relatively easy to tell that a wasp is an ichneumonid: just look for the many-segmented antennae plus the pattern of wing venation. But from that point, identification is next to impossible. That’s why I’m very lucky to have some help from Brandon Claridge, an ichneumonid researcher who is working on his Ph.D. at Utah State University. Brandon’s research focuses primarily on certain groups within this massive family, but he has offered to take a look at any of the ichneumonids I can collect here. While identification will still be difficult, he stands a much better chance of making sense of these wasps than I do. And ichneumonids are very under-studied, so it’s quite possible that we may even find a species new to science here!

The next wasp I find on the spurge flowers is also an ichneumonid, its entire body brick-orange except for a patch of lemon-yellow under the abdomen. While I’m photographing it, I notice another, apparently identical wasp foraging on the spurge. It’s always reassuring to see that I haven’t collected the only representative of a species. Hopefully there are many more of them around.

More insects & weird flowers

Now a honeybee is visiting the bizarre yellow flowers of the spurge, buzzing steadily as it flies from one to the next. And although this patch is quieter than what I’ve seen in the past, already another ichneumonid species has shown up. This one is long and slender, another variation on the theme of orange and black. It has a strikingly yellow face.

I stop to watch one of the ants on a spurge flower, head buried in it, manipulating the stigmas. I see ants practically everywhere, so it’s easy to take them for granted. They’re surprisingly diverse and complex creatures, though. And I know practically nothing about them. Today I collect this ant from the spurge, hoping to learn more about this species in the lab this winter.

Now I take a closer look at the flower structures of the spurge, which are truly weird. A pair of greenish-yellow bracts makes a cup around a set of glistening, crescent-shaped glands, a platform from which a set of stamens and a single female flower emerges. Shooting off to the sides like miniature fireworks are two greenish cups, each housing another female flower and more crescent-shaped glands. There’s a reason these glands are glistening: they offer up sugar-rich nectar while the female flowers are open, attracting all of these wasps and ants.

Biocontrol beetles

Now I move to a slightly larger patch of spurge. Right away, I find a leafy spurge stem-boring beetle! After I get a few photos, it moves to the far side of the spurge leaf, hiding from my camera. Unlike the ichneumonid wasps, these beetles are fairly recognizable to species in the field (the fact that they’re perching on leafy spurge helps a lot). Intentionally brought here from places in Eurasia where leafy spurge is native (after careful study to make sure that they wouldn’t become invasive), these beetles (Oberea erythrocephala) are specialized herbivores that feed on spurge stems and roots. This is one of a number of biocontrol insects that have been released in Montana. The hope is that these insects will reduce the competitive edge of their host plants, allowing non-native plants like leafy spurge to “play better with their neighbors.”

A diversity of visitors

I manage to get photos of a small, docile red wasp visiting the outer, female spurge flowers. When I try to catch it, though, it eludes me. Sometimes I’m glad when that happens. I don’t like to collect, so sometimes I’m relieved when the insect gets away. This wasp has many-segmented antennae, too. I don’t manage to get a look at the wing veins before it flies off. In any case, though, this is another parasitoid, either an ichneumonid or one of their close cousins, the braconids.

The morning is warming up, but the wind is still gentle. These small patches of spurge are starting to get active. Ants are everywhere. A crane fly is nectaring on the flowers, awkward on its long legs. In a narrow patch of spurge along the stream, there are even several species of stoneflies feeding on flower nectar! I spot a small, black and yellow wasp (I suspect a predatory species in the family Crabronidae), but it flies off before I can catch it.

The diversity that is present here is astounding. One of the most common visitors is a black wasp (or is it a cleptoparasitic bee?) with a red abdomen. There are at least 20 of these visiting the spurge flowers.

More ichneumonids for farm and garden

There are so many ichneumonids! I find one that is jet-black with red legs.

Another is black with scattered patches of cream.

There’s a black ichneumonid with orange legs.

One has a strikingly patterned abdomen, orange at the base followed by black and white stripes.

I find an orange ichneumonid with dark-banded wings.

There’s one whose abdomen is the deep red of a Bing cherry.

This diversity is much more than just a pleasing kaleidoscope. Each species has its own life history, and each targets a specific type of insect as its host. If we could even begin to understand the complex ways that these wasps influence the local food web, it would be mind-blowing.

Flies and more

Although these parasitoid wasps are the most conspicuous and varied insects visiting leafy spurge today, they aren’t alone. I catch an elongate, hairy fly that I don’t recognize: another story to learn about later.

Another fly has a flattened body. Its thorax is covered with velvety golden hairs. I suspect this is a soldier fly (family Stratiomyidae). Many species in this family are flower visitors; the larvae are detritivores, breaking down decaying plants.

The diversity is overwhelming. I catch a large, extremely wary fly with a polished, metallic-blue abdomen: probably a blow fly (family Calliphoridae).

A few small, extremely active black wasps with iridescent blue wings are visiting the spurge. I catch one. This wasp isn’t an ichneumonid: the antennae are much less segmented (just 12-13 apparent segments). It’s a spider wasp (family Pompilidae). These fascinating predators hunt spiders, which the wasp larvae feed on.

At this point, I’m sitting in the shade of a chokecherry. It’s taken me several hours to photograph all of these insects and record their information. Now it’s mid-afternoon, and it’s becoming one of the first hot days of the year. The sun is shining. The breeze has become blustery, but it’s not strong enough to discourage pollinators from flying.

More pollinators on the landscape?

Done cataloging that batch, I head back towards the leafy spurge patches. But I have trouble getting there, because on the way I have to walk past the two-groove milkvetch (Astragalus bisulcatus). The huge, bushy purple clumps of this native legume are in full flower. They’re busy with activity, as well: honeybees, two species of bumblebees, and a variety of other hairy bees are going from bloom to bloom. Today I’m trying to focus on the leafy spurge, but it would be very easy to get sidetracked watching insects on the milkvetch…

The insect community on these purple flowers is strikingly different from the group visiting the spurge just a few feet away. Instead of slender wasps, the prominent species here are fuzzy bees. And the striking difference in these communities suggests an intriguing idea: might patches of non-native flowers actually increase the overall numbers of pollinators present in an ecosystem?

This question hinges on whether plants like spurge are creating additional pollinator habitat, or whether they are just “stealing” pollinators that are already present from adjacent, native plants. In 2003, Vincent Tepedino and several other researchers took a look at this question in a park in Utah. Focusing on bees, they found that native plants on their study site attracted different, more specialized pollinators than did non-native plants. Based on this work, they suggested that, at least under these conditions, the presence of non-native species may boost a landscape’s carrying capacity for bees.

Bees here, wasps there

Today in this habitat, as in the Utah study, it appears that the non-native leafy spurge and the native two-groove milkvetch are supporting vastly different pollinator communities. It seems to be providing habitat for a variety of species that otherwise wouldn’t be here. So far I’ve seen only a very occasional bee on the spurge, while the milkvetch is hosting many bees. Meanwhile, spurge nectar is proving very attractive for ichneumonids and other wasps. I’m not seeing these species at all on the two-groove milkvetch.

Some more wasps and flies are visiting the leafy spurge now. I catch two tiny, iridescent cuckoo wasps (family Chrysididae). Like so many insects, these beautiful wasps have bizarre life histories. The females of most species are parasitoids on particular bees and wasps. They’ll sneak into the nest of a host and lay their eggs. If they are successful, the cuckoo wasp larvae will develop by feeding on the young of the host. Gruesome!

Then there’s a bee fly – a member of another group of parasitoids (family Bombyliidae). This one is small and fuzzy, with a dark stripe along the front of the wing.

Leafy spurge and insects

Where would all of these insects go if the leafy spurge were gone? Presumably most if not all of these pollinators are native species. (I’ll find out for sure when I identify them this winter.) If so, they were able to survive on this landscape before leafy spurge showed up.

But it’s different now than it was then. The area along this stream isn’t rich in native plants. It’s mostly covered with smooth brome and other competitive, non-native grasses. When people try to manage weeds, they usually ignore these grasses (as well as any nearby native plants). The grasses are as competitive as spurge, though, and much more abundant. And in the midst of this sea of grass, these small colonies of leafy spurge are providing habitat for a complicated, diverse, striking community of insects.

Beyond kneejerk reactions

Let me be clear: leafy spurge can be a serious threat to native plant communities in some areas. It occupies millions of acres across the west. I’m not trying to deny that leafy spurge can pose threats to native plants – and there are undoubtedly times when thoughtful management to reduce spurge populations will be appropriate. But far too often, invasive plant management is nothing more than a poorly informed, kneejerk reaction. When we label leafy spurge as “bad” – without even asking what it’s doing here – and then spare no expense to destroy it, I would argue that we, not it, are the invasive problem.

And in the case of today, we just have a few small patches of leafy spurge in heavily grass-invaded habitat, with biocontrol insects already present. Here, I believe that the only responsible management option is to leave it alone. Let’s enjoy this leafy spurge, with its weird flowers and all of the wasps that are benefiting from it. This “horrible” weed is helping our pollinators. And instead of trying to eradicate a few little patches of spurge, let’s dig up a patch of smooth brome. There, next to the spurge, let’s plant some milkvetch and other natives, and watch the pollinators hum.

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