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:
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.
Added 5/31/2020: Apparently white slant-lines also have an association with mayapple flowers, which is equally puzzling because these flowers don’t produce nectar. See this post.