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.
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.
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.