July 31, 2022

I’m standing along Helena, Montana’s Centennial Trail on this hot summer evening, contemplating a mystery. The mystery is a plant: Mentzelia decapetala, ten-petal blazingstar. Tightly closed, luminous white flowers rise above the toothy, sandpaper-textured leaves. Mentzelia plants, with their massive blooms and unique foliage, would be striking in any garden. But here they seem to be thriving without anyone’s care, a group of native plants in the middle of the city, growing along an abandoned railroad track. Like bold, living sculptures, they pose questions without offering obvious answers. I can’t help but wonder: what lies beneath their showy appearance? What’s the story of these plants’ lives?

When I do a quick Google Scholar search, I’m surprised to find that very little has been written about the ecology of Mentzelia decapetala. These plants are truly a mystery: I can think of so many questions. The leaves are so rough that they stick to my fingers and pant legs. Why such sticky foliage? The flowers are massive and showy, but right now they’re tightly closed. When do they open? Which insects pollinate them? Around Helena, these plants are closely associated with barren habitats: rockslides, sparsely vegetated slopes, abandoned railroad tracks. It seems incredible that they even survive – let alone thrive – in such harsh places.

Before I make any more field observations, I decide to do a more thorough search of the literature. And here and there, mostly in articles about related Mentzelia species, I find some tantalizing hints. These bits and pieces all seem to be pointing to what I had originally suspected: the story of this plant is a remarkable one. 

Mentzelia pumila, the death trap plant

Through my reading I learn that a close cousin, Mentzelia pumila, is also covered with clinging hairs. If we zoom in with a microscope, we can see that these hairs form a thorny forest of hooks and barbs, lying in wait to snag passing insects. It’s a death trap where these insects perish slowly, their wings or legs caught on tenacious hooks. According to scientists Thomas Eisner, Maria Eisner, and E. Richard Hoebeke, during their study the Mentzelia pumila plants “seemed invariably to have numbers of dead insects stuck to them.” It appears that these miniature, deadly forests of hairs can catch almost any type of insect that visits this plant. They did find one exception, though: the aphid Macrosiphum mentzeliae picks its way slowly through the hairs, avoiding the deadly barbs. Hidden among a dangerous, microscopic forest, this aphid manages to make a living feeding on Mentzelia pumila.

Why is Mentzelia pumila covered in these gnarly traps? It seems to be a defense against herbivores. But in their article, Eisner, Eisner, and Hoebeke suggest that this plant might also be passively hunting insects, benefiting from the additional nitrogen that their corpses might contribute to the soil nearby.

Fly killers, bee lovers

Could the clinging hairs of Mentzelia decapetala act similarly? Eventually, I unearth a hint that they may. It’s a brief mention in an 1879 journal, hiding under the antiquated name of Mentzelia ornata. (This name was historically used for M. decapetala.) Watching this American plant in a French garden, where it had been planted, an observer found flies visiting the foliage, apparently attracted to a fluid secreted by soft, glandular hairs among the stiff barbs. The flies would frequently find themselves caught by the barbs. They would either die there, trapped, or pull their heads off in their frantic efforts to escape. Flies, don’t land here! This plant is a death trap!

I also find an article about another related species, Mentzelia nuda. This one discusses pollination and suggests that, unlike the leaves, Mentzelia flowers can have a more positive relationship with insects. This study documents a variety of bees and several small flies visiting Mentzelia nuda blooms. Among these bees are two Mentzelia specialists, Perdita wootonae and Andrena mentzeliae. There are also various generalist bees, including the golden northern bumblebee (Bombus fervidus). Mentzelia nuda flowers produce nectar deep inside, among the dense stamens. And in this species, nectar production actually lasts for about 10 days after the petals fall, attracting ants to the developing fruits. The ants seem to defend the seeds against the beetles and moths that attack them, increasing seed production.

More from the literature: from flowering to dispersal

Mentzelia nuda flowers open in the late afternoon and close near sunset. Not so for our Mentzelia decapetala, according to the only study I can find that discusses pollination in this species. This study, from Nebraska, tells of flowers that open about an hour before sunset and close again near midnight.

In the Nebraska observations, honeybees and sphinx moths were the most common insects that visited Mentzelia decapetala flowers. The researchers also found bumblebees, grasshoppers, and a few other moths on the flowers.

This patchwork of information gives us a tantalizing sketch about the life of Mentzelia decapetala. It’s a plant of contradictions, finding a way to make a living in barren, dry habitats. A flower of the dusk and the night, it feeds bees and sphinx moths. The leaves, on the other hand, are a miniature forest of wicked, barbed spines: a graveyard for unwary insects. Another paper reports that the seeds are winged, scattered to new homes by the wind.

It’s a fascinating sketch of a plant, but it’s far from complete. And is this sketch even accurate for our local Mentzelia decapetala patches around Helena? It’s time to head out into the field and find out.

Into the field: afternoon at Devil’s Elbow

August 2, 2022

It’s a hot, windy mid-afternoon. I’m standing on a steep, shaley, southeast-facing slope near Devil’s Elbow Campground, baking in the summer heat. There are hundreds of Mentzelia decapetala plants here, growing up out of the hot shale. All of the flowers are tightly closed right now. So far, this matches with the Nebraska study: this plant definitely isn’t an afternoon bloomer.

I start searching the foliage, looking for insects. Will I find any of them trapped here, like the large numbers that have been found on Mentzelia pumila? At first glance, I’m not finding trapped insects. Instead, I find spots of a dark, viscous, sticky substance on the leaves. It looks like molasses. I taste a little bit. Surprisingly, it’s slightly sweet like molasses, too, though there’s a bitter aftertaste.

I’m amazed to find a Mentzelia plant that’s been browsed, the tips of its stems bitten off. This is a total surprise – what would eat these hairy, sandpapery plants? Could it be deer? As a gardener, I know that deer have an incredible ability to eat seemingly unpalatable vegetation. But I can’t imagine that even a deer would enjoy such an unpleasant-textured plant.

I find a stem with marks of an injury, and there’s this same blackish liquid congealed near the wound. This seems to confirm what I’ve already been starting to suspect: this black substance is Mentzelia sap.

Not far away, I find a few more stems that have been bitten off near their tops. Surprisingly, though, even though I’ve checked about a dozen plants, what I’m still not finding is any sign of trapped insects. I haven’t yet seen a single invertebrate stuck among the hairs, unlike what the literature records for this plant’s close relative, Mentzelia pumila.

Deadly hairs

And then, as soon as I think that, I find my first insect victim. It’s a tiny fly, about 2 millimeters long, black with red eyes. It’s slightly shriveled and very dead. Its legs are stuck among the forest of hairs, on a bract below a flower bud.

Now my search for tiny creatures becomes more productive. A medium-sized spider skitters from among the flower buds, untrapped and very much alive. Clearly some invertebrates are able to get around in spite of the hairs. And at least on this species of Mentzelia, it’s not just aphids.

Next I spot a small, soft-bodied fly. This one is another casualty, stuck to a drop of sap below a flower.

Unfazed weevils and a struggling moth

Now I notice a small, blackish weevil. Like the spider, it’s not trapped at all, crawling nimbly along the stems and across the flower buds. In my literature search, I found an article by Kathleen Keeler reporting the weevil Orthoris crotchi as a seed predator on Mentzelia nuda. Perhaps the weevil I’m seeing is Orthoris crotchi or something similar: a seed predator on Mentzelia decapetala.

As I part the leaves to check for insects, they catch on my fingers. It’s an interesting sensation for me, though clearly a deadly one for some insects. Others navigate the hairy forest unimpeded. I spot a second weevil on a flower bud, crawling quickly across.

Next I find a narrow-winged moth, perching on another flower bud. It’s still alive, but it struggles as I get photos: a leg is stuck among the barbed hairs. Why is it that the Mentzelia foliage is a death trap for some creatures and not for others?

I walk past several more plants whose tops have been browsed. I wonder if a mule deer in the neighborhood has a stomachache right now.

More creatures among the foliage

I keep checking more Mentzelia plants, especially focusing on the clusters of flowers and buds where I seem to be finding the most invertebrates. But the search has slowed down again. I’ve found a few more weevils, none of them stuck. Insect prey trapped here at this season seems sparse.

I spot a tiny spider, gray with black speckles on its abdomen, hiding among the flowers. It jumps onto my finger and then I release it back where I found it.

It’s still early in the bloom period here. Many plants only have green flower buds. Various others are within their flowering window, the bulky white petals wrapped tightly together in the afternoon heat. Only a few plants have immature fruits yet, their white petals fallen to the ground and the green capsules swelling with seeds.

Attracting ants?

I check the tops of these few developing fruit capsules. If this was Mentzelia nuda, I would expect (based on the literature) to find the tops of these fruits still producing nectar, attracting ants to protect them. The literature told me nothing about whether Mentzelia decapetala might attract ants in the same way, so it’s up to me to check.

So far, it seems that the answer is no. I’m not seeing any ants visiting these fruits, and the tops of the capsules appear dry rather than sticky. Unless I’m missing something, it seems that Mentzelia decapetala stops producing nectar when it stops flowering.

On the hairy margin of a developing Mentzelia fruit, I spot another medium-sized black fly. It’s upside down, shriveled, and dead. So far, flies seem to be the primary victims of this plant’s clinging hairs.

Then I spot another trapped moth, this one tan with black speckles, its wingspan as wide as my thumb. It’s stuck to the flower bracts. 

The sun will shine on this slope for hours longer today, but already the shale is blistering hot. It almost burns my bare leg as I kneel on it. 

Lessons in the heat of the day

I haven’t found a single open flower here this afternoon. The smooth, waxy white petals are clenched tightly, just like researchers reported in Nebraska.

This Mentzelia population is a mix of sizes and shapes. There are the tall, branching plants I’ve been checking for insects. Between them, low to the ground, there are smaller plants. So far, these ones are just rosettes of deeply toothed leaves. And scattered among these two forms, there are also the dead, bleached skeletons of plants from previous years.

This mixture of rosettes, flowering plants, and dead skeletons says “biennial” to me. And indeed, the Manual of Montana Vascular Plants reports this life cycle for our populations of Mentzelia decapetala. Biennial plants typically live for two years. In the first year, they start out as a low cluster of leaves. In the second year, they grow taller and flower – and then they die. But they live on through their seeds, which give rise to the next cohort of young plants.

It’s sweltering out here, and I’m ready for a break. But before I go, I find another flower with a small, midge-like fly trapped on it. I collect this one and bring it back so that I can take a closer look in the lab.

What have we learned out here? It seems that, at least at this season, trapped insects aren’t very common on these plants. Would this picture change later in the year? Or is Mentzelia decapetala just less of a fly-catcher than Mentzelia pumila? It’s impossible to tell without watching these plants over a longer time. But it’s clear that these hairs are an effective trap for some insects – especially soft-bodied moths and flies. 

From dawn to dusk: Mentzelia along the tracks

August 3, 2022

It’s almost an hour before sunrise, and I’m back among the Mentzelia decapetala plants along the Centennial Trail. At this hour, the flowers are all tightly closed. They glow softly in the blue-white glare of the streetlights.

Closed flowers in the afternoon, closed flowers before dawn. It matches with the Nebraska observations, the only published information I found on the flowering of Mentzelia decapetala. So far, it seems that Helena’s Mentzelia behave like Nebraska’s Mentzelia, opening shortly before sunset and closing sometime during the night.

Hairs under the microscope

Later in the day, I stop at Carroll College. I want to use a high-powered dissecting microscope to take a closer look at the Mentzelia leaf and flower that I collected the day before. Under the microscope, I can see that the plant’s protective hairs have tiny barbs along their entire length. The hairs seem to be longest and most prominent on the bracts that surround the flowers. This is also the region of the plant where I’ve found the most trapped moths and flies so far. Are the hairs a system of protection for the flowers, in particular? Each hair looks like a miniature, extremely narrow-crowned fir tree – or perhaps a weapon from a nightmare. As we’ve seen, for certain flies and moths, perhaps “nightmare weapon” is indeed the best description.

Strangely enough, the small fly that was trapped here yesterday has disappeared! Only a few leg fragments remain to tell me that I didn’t imagine the whole thing. Where has it gone? Frankly, I have no idea. My best guess is that another invertebrate – perhaps a spider – had been spending the day inside the Mentzelia flower. Stored in a plastic vial overnight, I imagine that this hypothetical predator emerged from the flower and ate the trapped fly.

Mentzelia in the evening

It’s 8:00 pm when I return to the Mentzelia patch along the Centennial Trail. The sun is sinking, but the evening is still hot: it’s 90°F right now, and the heat is made just slightly more bearable by a gentle westerly breeze. The Mentzelia flowers are beginning to open: striking, starlike white arrays with bouquets of glowing yellow stamens inside. I sniff one of them. It does have a fragrance, but the smell is delicate, just a little whiff of perfume.

Already, I’m noticing bees on these flowers. But unlike the Nebraska Mentzelia decapetala, which honeybees visited in the evening, the common visitors to Helena’s Mentzelia patch are all bumblebees.

A plethora of bumblebees

I’m carrying my insect net tonight. In short succession, I catch three bumblebees as they go from bloom to bloom. They seem to be focusing their efforts near the tips of the stamens, clambering about on them and apparently collecting pollen. 

I walk over to another large, bushy plant, growing right along the abandoned railroad spur line. This one is barely open yet, just the first few flowers beginning to unfurl. Here I spot another bumblebee. She hovers briefly near the closed flowers, just long enough to decide they aren’t worth bothering with right now, and then lands among the stamen bouquet of a barely-open bloom. 

I didn’t expect that I would be doing a repeat of last week’s bumblebee observations tonight! But I did bring a cooler of ice with me, so once again I’m netting every bee I can catch and cooling them down to identify them. 

Another bumblebee arrives at this Mentzelia plant and I net her as well. So far, every single bee I’ve seen tonight has a bold orange band across her abdomen. I’ll need to take a closer look and double-check, but it seems that these are all Hunt’s bumblebees (Bombus huntii) – the least picky flower visitors of the seven bumblebee species we found near Helena last week.

Life and death in the Mentzelia patch

It’s 8:20 now. I continue moving from plant to plant. Most of the flowers are still closed. And of the blooms that have opened, most have still only unfolded halfway. They’re white cups right now, not the white platters they’ll soon become. 

But they’re open enough for the bumblebees. I’ve caught seven individuals now, all with rusty-banded abdomens. And so far, bumblebees are the only flower visitors I’ve seen. I’ve noticed a few small, blackish weevils crawling on the outer surfaces of the flower buds – presumably the same weevil species I found during yesterday’s midafternoon visit to Devil’s Elbow. Now I spot a pair of these weevils mating on a Mentzelia leaf. Just like yesterday, they seem completely unfazed by the barbed hairs that cover the leaves.

Tonight I’m mostly focused on flower visitors, though I’m also trying to notice any insects trapped on the foliage. I’m not seeing many dead bugs: like the pattern I noticed at Devil’s Elbow, the trapped creatures seem few and far between. Nevertheless, I do notice a bee fly (family Bombyliidae) stuck to a flower bud, its wings extremely frayed. It’s been dead here for a while. 

It’s interesting to reflect on the dual nature of this plant. Its leaves can be deadly – but the weevils and the spiders seem to navigate them without trouble. Its flowers, on the other hand, seem relatively safe. I haven’t found a single trapped bumblebee anywhere. Unlike showy milkweed, whose flowers sometimes trap and kill honeybees, Mentzelia flowers seem to be trap-free for pollinators.

Approaching sunset

Like the plants at Devil’s Elbow, the Mentzelia here have just a few fruiting capsules developing so far. And again, the disks on top of these capsules are dry, without any sign that nectar is still available after the flowers have withered. There are no ants on these fruits – although I do notice a reddish ant trying to crawl into a tightly closed flower. The pattern I saw at Devil’s Elbow is getting reinforced: unlike Mentzelia nuda, it seems that Mentzelia decapetala does not attract ants to protect its maturing seeds.

It’s 8:30 now. The wind has died down and the sun is just a finger’s width above the western mountains. The evening light is becoming a rich golden. I walk over to a patch of plants on a rubble slope near the old railroad tracks. Here, the flowers are mostly open now. The bumblebees are staying very active. There are at least three just in this patch, each one diligently rummaging among the deep yellow anthers.

Bumblebee ID

Now I return to my cooler and check on the bumblebees. Am I right that they’re all Hunt’s bumblebees, or is there a surprise here? The bees I caught earliest are mostly chilled now, just twitching slightly. I run through the identification process with them. They all have moderately long cheeks, yellow hairs on the face, and bold yellow stripes across the thorax. I’ve done my due diligence now – and as I had suspected, these are Hunt’s bumblebees (Bombus huntii).

They all look similar, so I run through the identification process quickly, even managing to confirm the identity of those that have not yet cooled off fully. They’re all females, and all Hunt’s bumblebees. Most of them have very full pollen baskets, heaped high with a tawny yellow mixture of nectar and pollen. Now that I’ve confirmed their identities, I can let them all go. Freed from their vials, they shiver to raise their body temperature. Then one by one, they fly off into the fading light.

Hoverflies at sunset

Now it’s 8:40. The sun has just set behind the mountains. The Mentzelia flowers are fully open now. I notice a small hoverfly (family Syrphidae) perching on a Mentzelia stamen. It flies off as I approach. Several Hunt’s bumblebees are still active, their gentle buzzing barely audible against the background noise of traffic from Benton Avenue nearby.

I spot a larger hoverfly, also visiting the tip of a Mentzelia stamen. His abdomen is black with narrow, curved white markings. Like most hoverflies, this one is wary. He levitates above the flower before I can get a photo of him. He hovers a few feet away, controlling his position as carefully as a helicopter. I inch my insect net into position and swipe. I’m expecting he’ll dart away, evading the net – but surprisingly, I manage to catch him. I suspect he’s a white-bowed smoothwing (Scaeva affinis), a relatively large and distinctive hoverfly I’ve observed before around Helena. (And later, with a closer look, I’m able to confirm that I’m correct.) I only see this single individual, but it’s still a record of another species that is showing an interest in Mentzelia flowers. The white-bowed smoothwing is known to visit a variety of flowers; the larvae hunt aphids.

Mentzelia moths?

As the twilight fades, I’m starting to think about moths. So far, the Nebraska observations of Mentzelia decapetala seem to be a fairly good match for the patterns I’m seeing. There, the common flower visitors on these evening flowers were honeybees and sphinx moths. Here I’ve found Hunt’s bumblebees instead – a native species that, like honeybees, visits a wide range of flowers. Will I find sphinx moths, too?

By now, nearly all of the Mentzelia flowers have opened. I’m kneeling along the abandoned rail line, at the edge of an extensive patch of these white starbursts. Three Hunt’s bumblebees are still going from flower to flower here. It’s quite a way to spend an evening. I’m watching a Helena summer spectacle: one whose beauty I couldn’t really imagine until this week. Mentzelia decapetala pollination at sunset: it’s a striking contrast. I’m kneeling in a wasteland, a few feet from a car seat that someone has discarded, yet I’m in the midst of this patch of smooth white petals, exuberant yellow stamens, and sandpaper green leaves growing where no one cares about it. Still no moths, though.

Will any show up? I really have no idea. I alternate between kneeling at the edge of the patch and pacing among the plants. I’ll give the moths a bit more time.

By 9:10, the last of the bumblebees have finished their foraging for the evening. The blazingstar patch stands quiet, the luminous yellow and white flowers catching the fading light like beacons. Their scent seems stronger now, though it’s still just a delicate perfume.

I’m still waiting and wondering. Is this all for the night? Or is it just the beginning of the show?

White lines in the twilight

It’s 9:20 when I spot my first sphinx moth. It’s massive, larger than a hummingbird, far bigger than I had imagined. Its abdomen is as thick as my index finger, spotted with white. Its wings flash white and rosy as it moves soundlessly from flower to flower. I edge closer and sweep my net. Incredibly, I manage to catch it. The moth flutters in the net bag and I start walking towards my car to retrieve a butterfly observation cage. As I walk, I spot a second sphinx moth, similarly patterned, silently probing the glowing white flowers.

Watching the moth flutter in the butterfly cage I’ve retrieved, I immediately realize that its massive size isn’t the only striking thing about it. Its forewings are intricately patterned. There’s a broad, tan stripe running through the middle, intersected by narrower white veins. Occasionally I can catch a glimpse of the incredible, salmon-colored hind wing. It’s a white-lined sphinx (Hyles lineata). Looking it up, I read that the larvae of this widespread moth can feed on a variety of plants, including willowherb (Epilobium) and evening-primrose (Oenothera). And although it’s a common species, this is the first time I’ve seen one around Helena. No wonder. The adults are mostly active at twilight and after dark – times when I haven’t been out searching for them.

White flowers in the night

It’s 9:40 now. The last hint of the sun is a peachy glow to the northwest. I can see a number of tiny micro moths flying around the Mentzelia, but I can’t tell if they’re actually visiting the flowers. Another white-lined sphinx is flying around, quietly dipping its proboscis into the showy white blooms. In spite of its bulky body, it’s a fast, agile flier.

I’ve forgotten to bring a headlamp with me tonight. The flowers are still glowing, though, illuminated by the floodlights from the Batch athletic fields in the distance. But without a headlamp to help me, it’s getting difficult to see what’s visiting the flowers now. Are there other insects flitting from bloom to bloom in the night? Perhaps other moths, or nocturnal bees? It’s a question that will have to wait for another time.

But just as I turn to go, I spot another moth alighting among the stamens of a Mentzelia flower. This one is medium-sized: still substantial but maybe a quarter of the size of the white-lined sphinx. Before I can catch it for a closer look, it flutters off into the night.

There’s something I find captivating about this plant. For some insects, it’s a prickly killer, trapping small flies and moths with its miniature barbs. For others, like the weevils, it seems to be a home. And on these warm summer nights, it’s a spectacle that few people get to see: showy white flowers that open near sunset, attracting orange bumblebees and enormous sphinx moths. Among the discarded trash along this abandoned rail line, it’s nice to know that this kind of magic still exists.

Further reading

Brown, D.K. & Kaul, R.B. (1981). Floral structure and mechanism in Loasaceae. American Journal of Botany 68(3):361-372. Retrieved from http://www.jstor.com/stable/2442772

Eisner, T., Eisner, M., & Hoebeke, E.R. (1998). When defense backfires: detrimental effect of a plant’s protective trichomes on an insect beneficial to the plant. Proceedings of the National Academy of Sciences 95(8):4410-4414. Retrieved from https://www.pnas.org/doi/abs/10.1073/pnas.95.8.4410

Gray, A. (1879). The beheading of flies by Mentzelia ornata. Botanical Gazette 4(10):213-214. Retrieved from https://www.journals.uchicago.edu/doi/pdf/10.1086/325279

Keeler, K.H. (1981). Function of Mentzelia nuda (Loasaceae) postfloral nectaries in seed defense. American Journal of Botany 68(2):295-299. Retrieved from https://www.jstor.org/stable/2442861

Keeler, K.H. (1987). Survivorship and fecundity of the polycarpic perennial Mentzelia nuda (Loasaceae) in Nebraska Sandhills prairie. American Journal of Botany 74(6):785-791. Retrieved from https://www.jstor.org/stable/2443859

Lesica, P., Lavin, M. & Stickney, P.F. (2012). Manual of Montana vascular plants. Fort Worth, TX: BRIT Press. 

Schenk, J.J. (2013). Evolution of limited seed dispersal ability on gypsum islands. American Journal of Botany 100(9):1811-1822. Retrieved from https://bsapubs.onlinelibrary.wiley.com/doi/pdfdirect/10.3732/ajb.1300075

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