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Black Swallowtails in the Boscobel Herb Garden – July-August

File this under “Entomology for Gardeners”: On July 9th the team working in the Boscobel Herb Garden were surprised and delighted to see that some of the recently added plants were a big hit with the butterfly caterpillar population – 13 distinctive green and yellow- and black-striped caterpillars were feeding on only four of the fennel plants. Another half dozen of the insect visitors were found on the parsley and dill plants elsewhere. The ten PGC gardeners present, who easily command 250 years of cumulative gardening experience, agreed that they must be Monarch caterpillars, and showed the display to Ed Glisson, Boscobel’s Director of Visitor Engagement. By the time the following picture was posted on PGC’s website later that day, some doubt had been cast on the ID so the Horticulture team ran with the non-committal caption “Fennel Feeding Butterflies To Be”.

7-9, Fennel Feeding Butterflies To Be.JP
7-9 Caterpillars1.JPG
7-9 Caterpillars2.JPG

Two weeks later, Ed was showing a group from the Washington Garden Club (Washington, CT) around the garden and pointed out the still busily eating caterpillars.  Marguerite Purnell from the WGC immediately identified the caterpillars as those of the Black Swallowtail Butterfly and sent the following delightfully informative email, illustrated with her own close-up pics. The Horticulture Committee is sharing Marguerite’s enthusiastic email in the interests of furthering appreciation of the insect life our plant life relies on and co-exists with. 


From: Marguerite Purnell
Date: Thu, Jul 25, 2019 at 12:49 PM
Subject: Caterpillar ID
To: Ed Glisson

Good afternoon Mr. Glisson,

Thank you so very much for your time yesterday. Our group thoroughly enjoyed our time at Boscobel. What a beautiful place!

I do have some information for you regarding the caterpillars we saw in the Herb Garden yesterday. Indeed, they were a type of Swallowtail: Black Swallowtail (Papilio polyxenes) to be exact. The host plants for the caterpillars are carrot, celery, dill, fennel, parsley, Queen Anne's lace and other carrot family plants; this corresponds with what we saw the caterpillars munching on yesterday (versus a Monarch (Danaus plexippus) caterpillar that will only be found on Milkweed plants). 


From yesterday (found on the fennel in the Herb Garden):


The good news is that female Black Swallowtail butterflies only lay a few eggs at a time, so it's unlikely that your Herb Garden caterpillars will ever rise to pest status. So, enjoy these wonderful caterpillars, and know that they will grow to be gorgeous ornaments of the garden next year, if left to their own devices. Please note: the pupae overwinter, so if you clean up rigorously around the herb beds, the pupae will be lost and thus no butterflies will emerge because the pupae were destroyed. This has happened in many suburban areas where yard cleanup is quite enthusiastic these days, to the point that many insect species (moths, butterflies, fire flies etc.) that overwinter as pupae are becoming fewer and fewer.


If you're at all interested, here's a website that provides some great information (replete with photos) about the life history of the Black Swallowtail: 

It's interesting (though perhaps a little yucky to some) that the earlier instars (caterpillars moult as they grow, and each version that emerges is called an instar) look like bird droppings. It's a fascinating evolutionary adaptation that provides camouflage for the younger and still small (aka easily eaten by others) caterpillars. They look quite different from the full-grown caterpillar instars (shown above) that were foraging on the fennel in Boscobel's Herb Garden. The one on the right, you were absolutely correct, is preparing to pupate. If you check out the website above, it will provide some fascinating additional information about what will now transpire...though you'll be able to watch it yourself in the coming days (that is if you don't harvest all of the fennel).


OK, I've rambled on quite enough now. I hope the information is helpful (and fun). Congratulations to the Philipstown Garden Club for the wonderful work they have done tending Boscobel's Herb Garden! What a treat it was for us to see it. I hope they will learn to love some of the less cuddly denizens of the garden (especially those early instars) and continue to nurture them (as they do the herbs). Reducing herbicides and pesticides is a wonderful first step! So, congrats to them and to Boscobel again. Keep up the great work!

We look forward to returning to Boscobel...

With appreciation,

Marguerite Purnell
Washington Garden Club
Washington, CT


As of August 6, at least one Eastern Black Swallowtail was discovered pupating on the dill in the Boscobel Herb Garden, and we await the metamorphosis. Do visit the garden to enjoy this phenomenon of a species which evolved nearly 40 million years before we did and about which we are still learning.  And please share any photos with the Horticulture Team.

8-6 Black Swallowtail Pupating on Dill.J
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