Monthly Mystery #13: Slant-lines and Lady’s Slippers

In May of 2009, I took this photo of a moth resting on a pink lady’s slipper in western Massachusetts and posted it on BugGuide.net:

DSC_6178

It was quickly identified as a “white slant-line” (Geometridae: Tetracis cachexiata)–a fitting common name–and I filed it away and didn’t think much of it.

The following May, this photo was posted of a similar scene in Pennsylvania, and I thought it was an interesting coincidence.

Then in May 2012, Paul Goldstein included a photo of the same thing in the slideshow for his keynote address at the 5th Nantucket Biodiversity Initiative Week.  It was tucked off to the side in a collage of photos and he didn’t say anything about it.  I asked him about it afterward, and he said he had thrown that in just to see if anyone would notice: this is a recurring phenomenon that he has seen repeatedly and for which he has no explanation. We both feel like there’s something interesting going on here and are determined to get to the bottom of it, but neither of us has come across a slant-line on a lady’s slipper since then.

According to David Wagner’s Caterpillars of Eastern North America, the caterpillar of this species is a stick-mimicking inchworm that feeds on a wide variety of woody plants; there is no indication that lady’s slippers or any other orchids would be food plants.

You might suppose that the moth is visiting the flower for nectar, but pink lady’s slippers have no nectar.  They smell sweet, tricking insects into thinking there is nectar inside, and only a strong insect like a bumble bee is able to push its way in.  Upon discovering there is nothing worthwhile inside, the bee exits, getting covered with pollen in the process.  (I got that information from this page, which I note does not list the white slant-line or any other moth as an associate of lady’s slippers.  There is a nice series of photos showing a bee emerging from a lady’s slipper here.)

Well then, you might ask, isn’t the moth just being fooled by the scent like a bumble bee? Maybe, but why does it rest on the flower for a long time, not fluttering and trying to find a way in?  And why just white slant-lines?  A Google search for “lady’s slipper moth” turned up, in addition to my own post on BugGuide, this one from Berlin, MA; this one from Pennsylvania; this one from South Carolina (in this case a pair of photos, with the photographer noting, “I’d really like an identification of this moth. In addition, I’d like to know why it is positioned on the lip cleft where pollinators normally enter.  These two moth images were taken on two different flowers early in the morning the same day about 1/2 mile apart.”); and the second photo on this page, from Tennessee.  There is also this note from Illinois, and I had previously found this shot with a yellow lady’s slipper (Cypripedium calceolus).  The only image that came up showing a moth other than a white slant-line was this one of a luna moth on a showy lady’s slipper (C. reginae).  I suspect that one was posed, since luna moths are nocturnal and aren’t typically found resting on prominent objects in the daytime.  They also don’t feed as adults–they don’t even have mouths–so I don’t see any reason one would be visiting a flower of any kind.

Any information or observations that might help illuminate this mystery would be most welcome!

[This paragraph added after discussions in the comments below:] Google searches for “Tetracis cachexiata flower” and “white slant line moth” turn up, in addition to several photos of this moth on lady’s slippers, a few photos each of wild blue phlox (Phlox divaricata) and the introduced European mustard known as dame’s rocket (Hesperis matronalis).  I also see one example of a white slant-line visiting mountain laurel (Kalmia latifolia) and one on some kind of honeysuckle (Lonicera).   A search for “white slant line moth on flower” adds white trillium (Trillium grandiflorum) and bearded German iris (Iris germanica) to this list.  So this moth does visit other, totally unrelated flowers, but the lady’s slipper association seems to be the most frequently observed one, and it still appears that no other kind of moth is attracted to lady’s slippers.

DSC_6209

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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30 Responses to Monthly Mystery #13: Slant-lines and Lady’s Slippers

  1. I’ve nothing to add that would help, only the note that I photographed the same thing in Shutesbury a few years back.

  2. Hi Charley! I’m afraid I have no insights to add to this, but when I saw this post in my email, I was immediately struck by the photo, because I’ve seen that moth on pink lady slipper before. In fact, the one you’ve linked to from Berlin, MA is my photo! The wonders of the internet. Fascinating to
    learn that others have found the same association.

    And now I just noticed that if you google images of white slant line moth, most of the photos showing it in association with a flower, show either pink lady slipper or a purple/pink phlox like flower. Interesting that there is not a lot of variation in flower species, and that the lady slipper figures prominently.

    • Hi Janet! I’m glad you saw my post. Yes, I had noticed one shot of the phlox-like flower before, and when I googled “Tetracis cachexiata flower” just now I saw two different ones. On closer inspection, the flowers in both photos have four petals rather than five, so I suspect they are the introduced mustard Hesperis matronalis (dame’s rocket). Very curious!

  3. Katie (Nature ID) says:

    I’m commenting, because I’m hoping you’ll get an answer. Very curious.

  4. Sam Droege says:

    Hey Charley

    Below are two shots of a Conchylodes moth with a Crane Fly Orchid pollinia planted on its eye. I think the relationship is known and the Crane Fly Orchid, of course, has a very different floral type. However, it may be the case that your Geometrid is also and orchid pollinator of other species and it generically attracted to the scent of orchids?

    http://www.flickr.com/photos/54563451@N08/9363497571/in/photolist-fgqnBi-fg3wtz-cXr9jU-csdipj
    http://www.flickr.com/photos/usgsbiml/9359234935/in/photolist-fgqnBi-fg3wtz-cXr9jU-csdipj/

    sam

    • Hi Sam– That would make sense, but I’ve seen no evidence that this moth visits any other orchids. It has been found on a few other flowers though (see above discussion with Janet Pesaturo); maybe they are somehow chemically similar, despite being otherwise completely unrelated?

      • Charley, in addition to Dame’s rocket and wild blue phlox, I see a pic of it on mtn laurel and another on a tubular white flower that I cannot identify. Try googling images of the common name, and maybe you get a wider search.

  5. Jim's Blog says:

    Hey Charlie! You are really on top of this. Thank you so much for extending the mystery of this phenomenon. My images (Cyp. acaule in South Carolina) represent the only time I’ve seen this moth. Although Cyp. acaule is not known to emit a fragrance, there is something going on here. Please keep me in the loop if you come across any more information.

    Jim Fowler, Greenville, SC

  6. Pingback: A short story of Cypripedium acaule (Pink Lady’s-slipper orchid) in an upstate South Carolina Heritage Preserve @ Jim's Blog

  7. Hi, Charley,
    The Tennessee photo is mine (Baskins Creek Trail, Great Smoky Mountain National Park) May 1, 2011. Thanks for the note. I’ll keep an eye out for other sightings. A search of “white slant line moth on flower” turned up images of the moth on Large White Trillium (Trillium grandiflorum) and German bearded iris. In most instances the images seem to catch the moth just sitting on the flower petals, though in my Pink Lady’s Slipper photo and one other I found, the moth appears to have its face in the fold of the slipper. By the way, I have your Tracks and Sign book with Noah and have spent many delightful hours perusing it. Thanks for such a wonderful book and your continued investigation into this fascinating world.

  8. Jim's Blog says:

    Hey Charlie,
    Sorry to split my comments, but I took a series of images of a Bombus species pollinating a Cyp. acaule in 2012, and I though you and your readers might be interested in seeing the collage. It fits your description of the pollination process quite well. Here is the link:

    http://www.flickr.com/photos/22032600@N04/7057144949/

    Cheers,

    Jim Fowler, Greenville, SC

  9. Alonso Abugattas says:

    I have heard of another lep, the European Skipperr, which tries to nectar at lasdyslipers and then get trapped and die, sometimes in large numbers. I will have to keep my eyes open to see what other things try and nectar at these flowers in the future…

  10. Robley Hood says:

    What a terrific post! In April 2012, I found the same kind of moth resting on a lady’s slipper. A photo of it can be found here: http://rmhood.smugmug.com/Other/365/21377925_629Kgb#!i=1801770877&k=QB442Tb I am fascinated and can’t wait to learn more!

  11. willardw@comcast.net says:

    I always say,” in through the mouth and out through the ears! “

  12. David McIntyre says:

    I have long suspected that this orchid produces a pheromone-mimic to attract pollinators, but I can’t find any confirmation online. As you point out, the orchid offers no nectar reward. Surely it takes more than just a random nice smell to trick a smart insect like a bumblebee (they’re all about learning how to get nectar from different flowers) into entering a flower not once, but twice, and in a habitat they don’t normally visit, to boot (*I’m* not accustomed to seeing bumblebees in the woods where my local lady’s slipper orchids grow, anyway). Granted, they don’t seem to be pollinated successfully very often, but still.

    If the orchid does produce a pseudo-pheromone, perhaps the chemical also, by chance, approximates the pheromone that white slant-lines use to find each other. And perhaps the showy lady’s slipper’s pseudo-pheromone (if it also has one) is similar to that of skippers. If so, this could be a case of amensalism, a relationship that harms one species (the moth, which wastes time waiting for a mate at the flower, or the skipper, which perishes), but has no effect on the other species (the orchid, which neither benefits nor suffers from the presence of the moth or skipper). It would be interesting to know whether the moths at the flowers are males or females. Unfortunately, I can’t find anything online to suggest that they’re at all sexually dimorphic.

    That said, I can’t help but point out that the moths *may* get photographed on lady’s slippers more often than other plants only because people pull out their cameras for lady’s slippers more often than for, say, Dame’s rocket. That no other moths seem to be associated with lady’s slippers suggests that the observation is not just a result of sampling error, but it’s worth bearing in mind that the relationship may not be as strong as it appears based on the photos posted on the web.

    • Thanks for your comments. I don’t think it’s particularly unusual to see bumblebees in the woods; I often see them visiting Solomon’s seal and other understory flowers. It definitely would be worth investigating whether both male and female slant-lines are visiting the lady’s slippers; I think the males would have somewhat broader, more feathery antennae, but these often aren’t visible in photos. I agree that sampling error needs to be considered here; when I first wrote this post I had hardly been able to find any photos of slant-lines on other flowers, but as the tacked-on final paragraph indicates, they have been observed on a number of other, unrelated flowers. It is still strange, though, that no one seems to have seen any other moths on lady’s slippers.

  13. Scott Gilmore says:

    Dr Rod Peakall (http://biology.anu.edu.au/rod_peakall/) has done a lot of work on sexually deceptive orchids in Australia. It is a fascinating subject and I wonder if that could be involved here.

  14. McMackin, Rebecca (BBP) says:

    Could the moths be gathering the scent from the lady slipper and using it for mating?

    Best,

    Rebecca McMackin
    Director of Horticulture
    Brooklyn Bridge Park
    718.724.6439

    • The thought has crossed my mind. Determining whether only one or both sexes are visiting lady’s slippers will be an important first step in testing some of these hypotheses. I believe I read once that the bees that pollinate Brazil nut flowers depend on a certain species of orchid for this very reason.

  15. Eric LoPresti says:

    I sense an experiment involving lots of tanglefoot on lady’s slippers next year.

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