Ok, I’m going to do some very crude, novice data analysis, but bear with me here.
Here’s the number of Pipevine Swallowtails (Battus philenor) observed in the US:
More than 26,000 observations.
Now, here’s the number of Aristolochia spp. plants observed in the US, its host plant: https://www.inaturalist.org/observations?place_id=1&subview=map&taxon_id=54464
…around 4,500 observations.
That’s around 6 times the swallowtails per host plant (over the entire course of iNat history)
That doesn’t seem to match up. I’ve never heard of Pipevine swallowtails feeding on anything but Aristolochia species. Yet their host plants are just so… elusive.
Compare that to the milkweed butterflies (Danaus spp.), who similarly feed on a limited range of plants:
Danaus spp.: https://www.inaturalist.org/observations?place_id=1&subview=map&taxon_id=48663
Asclepias spp: https://www.inaturalist.org/observations?place_id=1&subview=map&taxon_id=47906
159k and 171k. That’s relatively even.
The disparity between the number of Pipevine Swallowtails vs their host plants both amazes and confounds me. Where I live, around central Texas, pipevine swallowtails are quite common. If there are so many Pipevine Swallowtails, where are all the pipevine plants supporting them? Do the butterflies have more host plants. Are they that hard to find?
I mean you have to take into account that not everybody uses iNaturalist, nor do they post their plants. I am one of those who only posts the insects in my garden and have not posted any of the plants in my garden.
I’m in several gardening groups and many people plant the host plant in their gardens to attract the butterfly and a lot of those people don’t even use inat.
Well, I’m definitely aware that not everyone is a iNat plant nerd like me .
Usually with iNaturalist we would be talking about wild plants, so captive/cultivated garden plants wouldn’t be included these numbers.
But even so… the disparity is large. Take the monarchs and milkweeds example into account - those are relatively even …although that’s quite biased because most people know what monarchs are and milkweeds are often put in the spotlight. As I said, crude data analysis, not research paper
I believe anything in the Aristolochiaceae is a suitable host for the butterfly. So the common Asarum canadense in my neck of the woods works; and indeed there’s many more observations of that the butterfly in the state (NJ).
Hmmm… I was considering that. Any supporting sources? I’m checking right now.
https://plants.usda.gov/home/plantProfile?symbol=ASCA Check out the factsheet for the species here. It states that it is indeed a host plant for pipevine swallowtail.
Ah, I see it! So it can feed on other plants in the Aristolochiaceae family. It also looks like Virginia Snakeroot isn’t in Genus Aristolochia as well… although that appears to be due to a change in taxonomy
It seems like most websites don’t seem to mention that other plants besides the normal pipevines can be used. A lot say that they can only feed on Aristolochia
On further note, it does look like you are right: there are fewer pipevine swallowtail than their host plants in New Jersey.
But… checking the Aristolochiaceae Family in Texas… it appears there is quite some geographic variation with the distributions of these butterflies/plants, considering the size of the US:
So… I don’t know. Maybe there are just less plant observers, and those that are already have trouble finding them.
Here in Wisconsin, Pipevines are not native and exist in a wild state only as the rare garden escapee. Pipevine Swallowtails, however, are found in many areas in the southern part of the state where pipevine is a common garden plant. So in these cases, when the host plant is observed, it gets marked as captive, whereas the wild butterflies don’t.
Seems that you are right there:
Interesting to see how a species that I take for granted down here in Texas can be uncommon of even elusive in other states… America is a large country.
Edit: After checking through multiple states, the numbers definitely seem less crazy. In some places the Aristolochiaceae family plants outnumbers the swallowtails. Maybe I should change the question to specify location a bit.
I still wonder, though, how it could be that I’ve seen so many pipevine swallowtails around my area, yet have still never found a pipevine.
I just uploaded an observation from my hike today showing a Pipevine Swallowtail feeding on some blue dicks (Dipterostemon capitatum).
Some further things to consider:
- Juvenile populations may be concentrated while adults diffuse - and more specifically human observers may not cross paths with the host plant as often as they are likely to encounter the adult butterfly
- Observation bias: most users are likelier to observe conspicuous organisms like birds or butterflies, or plants with conspicuous and colorful blooms. If they encounter a host plant when it’s not blooming or fruiting, and especially if it’s been partially eaten and isn’t in top condition, they may look right past it.
- Identification bias: plants are harder to ID than butterflies, full stop. Plant ID skills can be very localized- if an observer doesn’t know enough to place an observation beyond “Plant” and there’s nobody locally or category-level who can narrow it down, it may never hit a more specific ID.
Aristolochia tomentosa and macrophylla, the most common large species of native pipevine in much of the eastern US, are not particularly showy and tend to climb high up out of notice.
I agree with you on everything but the last point. Who can say what’s most difficult to identify? This is too subjective to make a definitive statement on. I personally find species like Carex easier to identify than common birds.
Totally agree with these points, especially the second one. Pipevine swallowtails are one of the most well known and conspicuous butterflies that even many amateurs know about them. The same cannot be said about the plant. In fact, I doubt many would have even heard of the plant if it weren’t for the fame of the butterfly. I see plenty of insect enthusiasts (including myself) go out of their way to search for the caterpillars and post dozens of observations of them without once posting an observation of the plant that they were found on.
I have kind of been trying (not actively pursuing, but learned about them and am keeping an eye open) here in Southern PA, and I’ve yet to see wild Pipevine or a Pipevine swallowtail… so frustrating if they are that common!
But having said that, I think dumplingbug is right… I bet there are many hanging out at some higher level ID that no one knows about. I could not ID it without flowers myself…
Pipevine Swallowtails are poisonous/distasteful. Showily advertising that fact is evolutionarily advantageous and makes them more likely to be observed.
Interesting. In New Mexico, there are only 4 observations on iNat of Aristolochiaceae (Birthwort family) for the whole state but there are 51 observations of Pipevine Swallowtail that are fairly widely distributed across the state. I don’t recall ever seeing the butterfly in New Mexico (I’ve only seen in Arizona) and am unfamiliar with the host plants.
In Indiana, my experience was that Aristolochia serpentaria is ubiquitous but inconspicuous. The leaves don’t grab your attention and you rarely see flowers. So it’s probably dramatically under-represented in iNaturalist.
The situation in New Mexico, where I live now, is more confusing. Aristolochia watsonii and Aristolochia wrightii are both known in only a handful of sites in the southwestern corner of the state. Aristolochia is not commonly cultivated, either–I’ve never seen it in cultivation or for sale in NM. The few caterpillar observations are all in the southwest corner of the state at sites where it makes sense for Aristolochia to be found. Adults have been found as far north as Santa Fe. I guess there are four possibilities: these are misidentifications; the caterpillars will eat various other plants that have not been documented; the adults are flying distances of up to 300 miles; a few people in the state are cultivating Aristolochia and in most of the state the swallowtails are relying on those plants.
I think Virginia Snakeroot plays an even more major role in their range. According to the NC Vascular Plants site:
As strange as it may seem, this is the ONLY native hostplant species for the Pipevine Swallowtail in the Piedmont and Coastal Plain; watching a female flying back and forth through a powerline clearing looking for a plant you simply cannot find yourself, and then stopping to oviposit on a small plant, it is amazing that the plant stopped at is always Endodeca!