Rates of visible caterpillar parasitism
This project has accumulated a lot of observations of visibly parasitized caterpillars. Some caterpillar species have hundreds of qualifying observations.
So now that we have this mass of observations, what can we do with them? One simple thing is to get a feel of rates of parasitism among different caterpillar species. Just compare the number of parasitized caterpillars in this project to the total number of caterpillars, species by species.
Here's that math for most-reported caterpillars in the project:
Carolina Sphinx (Manduca sexta)
386 parasitized of 6017 total = 6.4% of caterpillars visibly parasitized
Cecropia Moth (Hyalophora cecropia)
115 of 1690 = 6.8%
Regal Moth (Citheronia regalis)
124 of 767 = 16.2% (yikes!)
Saddleback Caterpillar Moth (Acharia stimulea)
70 of 1952 = 3.6%
Spiny Oak-slug Moth (Euclea delphinii)
51 of 661 = 7.7%
Of course, there are some potential issues with this methodology. Below is a quick list of some conceivable criticisms. Are any of these likely to change the rates significantly? Are there others that I'm overlooking?
- For my counts of parasitized and total reported caterpillars, I am looking only at observations where the intended subject was the caterpillar. So I could be overlooking observations of parasitized caterpillars that were submitted as observations of the parasite. Also, for simplicity, I am counting each observation as a single caterpillar, regardless of how many caterpillars are photographed.
- Are parasitized caterpillars significantly under- or over-reported on iNat? Maybe parasitized caterpillars are more likely to be photographed, either due to impaired concealment, or because they might arouse more interest in the observer? Or is the opposite true; parasitized individuals might become less active, and therefore less noticeable?
- One source suggested that a particular tachinid fly parasitizes only final-instar caterpillars. So photos of early instars don't necessarily represent individuals that will never be parasitized; they haven't yet reached the stage where they could be parasitized. If this is true then the percentages above are underestimates. (And in general, this is probably true even if parasitism isn't restricted to the final instar.) But maybe the percentages above are still meaningful in a relative sense, and you could still accurately make a statement like, "E. delphinii caterpillars are twice as likely to be parasitized as A. stimulea caterpillars."
And of course I am explicitly not addressing parasitism that happens in other life stages, like while pupating.
A big thanks to @claire146963 and many others for annotating the life stages in these thousands of observations.
Doug
Comments
Thank you for this thought-provoking data analysis. I appreciate the questions you pose. Another factor that influences the number of parasitized caterpillars observed is chosen host plant. How common is the host plant and how easy is it to photograph? Our current most observed parasitized caterpillar Manduca sexta frequently feeds on tomato plants—a favorite crop of the home gardener and well within camera range. Yet some other caterpillars are found feeding on tree leaves like oak and maple. Unless caterpillars are visible on lower branches their presence might not even be noted.
I am sorry for not being more encouraging, but there are MANY pitfalls with what you want to do. Still, if you want to do it, go ahead, and I suppose that it would have some value nevertheless... But really, any person who has reared caterpillars more seriously (either to get the adult Lepidoptera or, as in my case, to rear the parasitoids) would object to drawing any valid conclusion on data from pictures taken and uploaded into iNaturalist (or any other platform).
Among the many problems, people just observing/photographing but not collecting the caterpillars are bound to miss LOTS of information about those species. As someone already told you, a "healthy-looking" caterpillar might already be parasitized when you photograph it. But in your account it would score as "non-parasitized" simply because when you photograph it there was no any signal of the parasitoid(s) yet. Similarly, a person photographing but not actively searching for caterpillars may only encounter the most visible specimens -in spite many other caterpillars (of the same or different species) being around you but not being noticed by the photographer.
Searching and collecting caterpillars takes some practice and training, a casual stroller taking pictures will miss 90% or more of the specimens. Then, by not taking the caterpillar and properly rearing it, the actual status of it (being parasitized or not) cannot be properly assessed.
Then there is the problem of missidentifications -perhaps not as bad with caterpillars but indeed a problem with parasitoids... [One example I am very aware of: most of the "Cotesia congregata" records in iNaturalist are wrong, as most people provide that ID for cocoons on any sphingid-looking caterpillar. Still, there is some value in the data gathered, but more curation of such data must be done before results have a real value].
All of the above does not mean that you cannot pull the data and try to draw some conclusions, but the validity of that for scientific purposes is very low. You could, at best, find some general patterns, e.g., what are the most abundant caterpillars in a specific area/region... But, even there, that is totally inaccurate (because species represented by small, non-conspicuous larvae are rarely seen and photographed by casual observers!). Your parasitism percentages are also misleading. .. So, what could be useful? For once, the power of many people being out and observing nature, that is really valuable, and by sheer force of numbers (more observers) you are bound to find more data than what professional entomologists can do (a few dozens versus potentially many thousands that contribute to iNaturalist). So, there is indeed, valuable info here. For example, detailed distribution of species (new locality records) abound here. As a parasitoid "expert" I have also seen some new host/parasitoid associations, worth reporting in a scientific note.
But drawing general conclusions on the parasitism rate of species X... I am not sure that is worth it. Perhaps as some general, crude idea, but I am afraid that not much than that...
Of course, I may be wrong! And I do not want you to take my word as definitive. One thing you could definitely do is to check what percentage of parasitism has been recorded for some of those Lepidopteran in previous scientific studies (those done by properly collecting caterpillars and rearing them in the lab to study their natural enemies). And see if your numbers are similar. For example, check this paper on parasitisim of the Cecropia moth in North America (there are other two Lep species mentioned there): https://conbio.onlinelibrary.wiley.com/doi/full/10.1111/j.1523-1739.2000.99193.x?casa_token=BwYbVAK9NhoAAAAA%3AeLpLNIhyrCi4MCEkVwSYg6SH4TMsmTtByNfJXyJqmxWXcJwTlig91WFLurTXk1A7QNbu1tM1aNDS9A.
Also this one: https://academic.oup.com/ee/article/32/5/1019/338294?login=true
For Manduca sexta there is so much data already available! You can compare how the results reported in iNaturalist compare to those. By dates, localities, etc.
All in all, I very much respect and appreciate the value of the data gathered by Citizen Scientists. That is the main reason why I come back to iNaturalist as often as possible and try to help providing identifications (and explaining on what those IDs are based).
I am willing to help as much as possible. But I only wanted to be honest and say that data gathered under non-standard format, by different people with different skills and motivations, may be hard to translate into a scientific paper down the road...
Cheers, Jose
Thank you very much for your comments, @botanicaltreasures @josefernandez-triana, and for the references. I appreciate you taking the time to respond.
Regarding your suggestion to compare the parasitism rates with previously published results, I had already started looking into that, just to see if those numbers are anywhere close. A very old paper by Frank L. Marsh (1937) seems like a fair candidate to compare my homebrew iNat results against because: (1) he was studying a visually detectable parasite (Winthemia cecropia* oviposits externally on the host), and (2) the parasite oviposition takes place during the caterpillar stage. Marsh reported that "at least three percent" of the Cecropia moth population was parasitized by W. cecropia in the Chicago area. The iNat-derived number was 6.8%. For the reasons I mentioned in the original post, I would have expected the 6.8% number to be an underestimate, so I would not score this as good agreement.
While acknowledging that many methodological issues remain unaddressed, I thought I had mostly taken care of the "inconspicuous caterpillar species" problem by using parasitism rates instead of the number of caterpillars. It shouldn't matter that iNat participants would notice and submit 5% of one caterpillar species and 20% of another; the computed rate of visible parasitism for each species should remain unaffected, subject to the caveats listed in the original post.
*Marsh thought that he was working with W. datanae, but re-examination of his material by Sabrosky (1973) proved that Marsh was actually describing W. cecropia.
Hi Doug, All of your points are fair, so please do not be discouraged by my comments. Perhaps you can get the data to show some other interesting information, so go ahead! Cheers, Jose
I wonder whether overall there is a higher incidence parasitism amongst moth species because of a species population than amongst butterfly species given how many moths are crop pests: tomato hornworm (Sphingidae), cutworms (Noctuidae), diamondback moth (Plutellidae) etc. However, there is also the cabbage butterfly (Pieridae) and the long tailed skipper or bean leaf roller (Hesperiidae). Follow crop pests find parasitoids.
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