Black swallowtail caterpillars on dill and fennel

Black swallowtail (Papilio polyxenes).


A black swallowtail caterpillar was discovered on a dill plant on September 13, as seen above.


A black swallowtail caterpillar was discovered on a fennel plant on September 14, as seen above. The dill and fennel plants were about 10 feet apart. Same caterpillar on different days. Or 2 caterpillars. I don’t know.


Larval food plants for black swallowtails include dill, fennel, parsley, and Queen Anne’s lace (Daucus carota). The caterpillars above were quite noticeable, and I immediately started worrying that a bird would lunch on them. Reading online about their defenses assuaged my concerns somewhat. 1st, the caterpillars have a special gland called osmeterium that emits a foul odor. 2nd, the caterpillars ingest oils from the larval food plants whose foul chemical taste repels predators, such as birds. Well, that’s comforting to know. But, Jeffrey Glassberg, in Butterflies of North America, tells us that only 1 or 2 butterflies out of 100 will survive the complete life cycle to become adult, flying butterflies. They are mostly done in by parasites, diseases, and environmental factors such as drought or too much rain, but predators like birds and crab spiders also take their toll.

If I want my garden to be butterfly friendly, and adult, flying butterflies live only 2-4 weeks on average, I guess I’d better pay attention to the other life cycle stages: eggs, caterpillars, pupas, and finally, adult, flying butterflies. In addition to life cycle stages, there’s also the wrinkle in butterfly gardening that different butterflies complete their life cycles at different paces. Some butterflies take a year to complete 1 cycle. Some butterflies complete 2 or 3 cycles in 1 year. Each cycle is considered a brood. Important to learn the lingo of butterfly lovers. Of course, much depends on location. I’m located in central Jersey, so I’m focused on here. Black swallowtails may have as many as 3 broods per year here. By paying attention to fly dates, which are listed in Glassberg’s Butterflies through Binoculars: the East, last published in 1999, it’s possible to get some idea of what particular butterflies are accomplishing in my garden. Of course, many environmental and climatic factors have changed in the ensuing 17 years, but it’s nice to have a starting point and some expert explanations about butterflies.

So, Butterflies through Binoculars: the East shows flight periods for black swallowtails in this area as late April through mid-June for the first brood, with a peak period in late May; late June through mid-August for the second brood, with a major flight period in mid-July; and  most of September for the third brood, with fewer flying butterflies being observed during September. NABA North Jersey Butterfly Club website gives extreme flight dates for black swallowtails in north Jersey of 4/7–10/19.

So my caterpillar(s), observed on September 13 and 14, which are in their final stage, or instar, of caterpillar-hood, could have pupated, which takes 1 or 2 weeks, and emerged as adult, flying butterflies at the very end of the September flight period for the 3rd brood of black swallowtails. Or they might have pupated and entered a resting state, or diapause, to overwinter as pupas.


In this year’s vegetable garden, I’ve made a point of planting parsley, dill, and fennel. I don’t have Queen Anne’s lace, yet, but winter carrots are up and growing. Above is a magnificent dill plant drying up in the garden. How many eggs, caterpillars, or pupas are, or have been, on it. I don’t know. I haven’t found any.


I have dill to spare, but fennel is a more painful situation for me. I have come to enjoy bulbed fennel, which is what is growing in the vegetable garden, as seen above–which looks somewhat like dill, doesn’t it. I have a recipe using diced and sautéed fennel bulbs mixed with olive tapenade and creme fraiche topped with sliced tomatoes and parmesan cheese that is so delicious. I want to find more spectacular fennel recipes, but the caterpillars are getting in my way. Or are they pupas now. Or have they metamorphosed into adult, flying butterflies. Or were they lunch for a bird. I don’t know.

I think I saw 1 black swallowtail in the garden this summer. I didn’t get a photo. It was black and flying fast, which the NABA Club website says that they do. This one was traveling for sure. No time to waste. I neglected to record the date of this sighting. Drat.

In the fall of 2015–September 18, to be exact–I photographed a black swallowtail in a dried-up bit of dill, which is shown above. I can’t help but think this female black swallowtail may have been the progenitor of black swallowtails in my garden this summer. If this female was laying eggs, eggs taking 3-5 days to hatch, the caterpillars would have gone through their various stages, or instars, of molting and growing, for about 3 weeks, and finally entered their pupa stage to overwinter. That would be roughly the end of October, which seems possible since our frost date is usually the end of October.

There were fewer butterflies of any kind this summer in the garden, excepting cabbage whites, of course. This year was very dry with moderate to severe drought conditions here. The pear tree did not produce fruit this summer. Butterflies, bees, and all other insects, squirrels and other critters, including Daisy, my yellow lab, all adore rotting pears lying on the ground. I think the drought and perhaps the lack of rotten fruit may have contributed to the dearth of butterflies and bees in the garden.  I had hoped for an increasing number of all kinds of butterflies, but not so.

So, black swallowtails overwinter as pupas in this area. I take that on faith since I haven’t found any pupas, but I plan to leave all the dill, fennel, and parsley–even the winter carrots–intact in the vegetable garden, and hope that I am rewarded with more black swallowtails next summer.

Hope springs eternal.

Black swallowtail on September 14, 2015.




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