Wild Senna and Sulphur caterpillars

Wild senna (Senna hebecarpa).

Something has been eating my wild senna (Senna hebecarpa) plant.

Above are photos of the same wild senna first taken on October 7, when its compound, even-pinnate leaves with 10-20 leaflets (this description from the NRCS Plant Fact Sheet) were on full display, and again on October 25, when nothing was left but the stems and dark brown seedpods. It’s amusing to me that before I became a butterfly gardener, I would have viewed this decimation in horror, but now I’m all excited about the possible signs of caterpillars in my garden.

Who was the culprit. Many sources list wild senna as a larval, or caterpillar, food plant for Sulphur butterflies. Butterflies of North America lists Little Yellow (Eurema lisa), Sleepy Orange (Eurema nicippe), Cloudless Sulphur (Phoebis sennae), and Orange-barred Sulphur (Phoebis philea) as possibilities.

Wild senna is a New Jersey native, but it seems to occur mostly in south Jersey. Although not listed as threatened or endangered in New Jersey, it is listed as such in Massachusetts, New Hampshire, and Vermont due to habitat loss. In Connecticut it is listed as of special concern.

NABA-North Jersey Butterfly Club website concurs that wild senna is a larval food plant for Cloudless Sulphur and Sleepy Orange, noting that a few wild sennas grow in New Jersey. Interestingly, this website lists legumes as larval food plants for Clouded Sulphur and Orange Sulphur. Wild senna is a member of the pea family (Fabaceae), which is a legume, so perhaps these butterflies could be included in my culprit caterpillar list.

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This photo of a Sulphur butterfly nectaring on a red zinnia in my garden was taken on September 27. It doesn’t seem big enough to be a Cloudless Sulphur, which is most often mentioned as using wild senna as larval plant food. The NABA Club website reports that Cloudless Sulphur butterflies are not known to overwinter in New Jersey and are more often seen as nonresidents in south Jersey. The same is true for the Sleepy Orange.

That leaves the Clouded Sulphur and the Orange Sulphur, which are resident in central Jersey, both of which are common and widespread. In fact, the 2 species are known to hybridize. The NABA Club website kindly counsels me not to be embarrassed to admit seeing a Sulphur, species unknown. So maybe I’m trying too hard here. Of course, there’s the problem that although these 2 species use legumes as larval host plants, no mention is made specifically of wild sennas. So everything is in limbo with regard to the culprits who ate my wild senna plant.

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Perhaps I should mention that wild senna foliage is a purgative or laxative. Livestock and wild animals such as deer leave it alone. We are not having a local deer problem at present anyway. Quail or bobwhites like to eat wild senna, but I don’t think there are any around here. The leaves didn’t fall due to frost damage since there were no fallen leaves present under the plant. Anyway, it looks like caterpillar damage to me.

You may be wondering what a wild senna plant was doing in my central Jersey garden if they aren’t known for growing here. In the fall of 2015, I determined to make my garden more butterfly friendly by planting more wildflowers native to this area, including more larval host plants of butterflies I saw in the garden in the summer of 2015. Wild senna was among the seed packets I ordered online. The NRCS Plant Fact Sheet recommends that wild senna seed be scarified–not frightened to death, but chipped to allow water absorption. I rubbed the seeds between 2 sandpaper blocks. The scarified seeds were started indoors. Only 1 seed germinated. This plant was placed under grow lights where it thrived. I called it the fancy plant because of its unusual leaves. In the spring, I planted it out with several other wildflowers I had started similarly. They were planted in a corner of the garden I call the wildflower corner–what else, which quickly became my favorite spot in the garden.

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Here is the fancy plant, uh, wild senna plant, in the wildflower corner on October 16. So it was decimated between October 16 and October 25. 9 days. Pretty fast work. To the right of the wild senna is a little holly tree and a New York ironweed plant. To the left are verbena, some asters, and some grasses.

Wild senna grows a deep tap root and will spread by rhizomes. It is drought tolerant. The dark brown seed pods split explosively and contain around 50 seeds. Wild senna can grow to 7 feet high. I’m fully aware that my lovely little wildflower corner may become a weed patch. That’s the way with wildflowers.

I hope to have more photos and more wildlife news about wild senna next summer. Somehow, I expect the plant to recover from its decimation, but maybe not. I’ll just have to plant more.

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A little more about Clouded Sulphur and Orange Sulphur butterflies. Larval food plants also include clovers and alfalfa, both of which are present in my garden. There’s lots of white clover and some red clover. There’s summer alfalfa, which is used as ground cover as well as green stuff for the compost pile. Oh dear, how many pupas have I ground to bits in the mulching mower.

Clouded Sulphur and Orange Sulphur have several overlapping broods and overwinter as pupa. Clouded Sulphur flight period is from April to October with extreme dates of 2/25-12/13 for north Jersey. Orange Sulphur from March to December with extreme dates of 3/17-12/31 for north Jersey. Orange Sulphur may also overwinter as larva, or caterpillars. All this according to the NABA Club website.

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Butterflies of New Jersey reports that Orange Sulphur, also known as Alfalfa Butterfly, was originally a southwestern U.S. species, but followed alfalfa cultivation east after 1870, invading the northeast U.S. since the 1930s to become common here. As mentioned above, Orange Sulphur have since hybridized with the Clouded Sulphur. But do their caterpillars eat wild senna leaves. That is the question.

A more vexing question for me is why I didn’t notice the decimation of the wild senna plant as it was happening. I would know much more if I had a photo of a caterpillar. Berating myself about my lack of observational skills calls to mind a passage in the Introduction to Walter Harding’s Henry David Thoreau: In the Woods and Fields of Concord, which illustrates Thoreau’s observational skills in an insightful and amusing way.

A Concord farmer named Murray was recorded as seeing Da-a-vid Henry, as he called Thoreau, standing by a mud pond as farmer Murray went out to his field in the morning. When Murray came home at noon, Thoreau was still standing by the mud pond. Returning to his field, Murray again observed Thoreau standing there, doin’ nothin’, as the farmer termed it, gazing down into the pond. When farmer Murray finally asked Thoreau, what air you a-doin’, Thoreau responded without turning his head. Mr. Murray, I’m a-studyin’–the habits–of the bullfrog. Farmer Murray then summed up Da-a-vid Henry as that darned fool for wasting his day a-studyin the habits of the bullfrog, and returned to his field.

Thoreau’s journals are punctuated by phrases like I sat awhile or I stopped to watch. When the reader thinks about it for a moment, Thoreau’s every encounter with nature and wildlife involves time spent in patient observation. And he didn’t have a camera, only paper for sketches or notes, and collected specimens that he often carried in his hat.

Patient observation must become a lesson I learn from Thoreau. It is not time wasted but an important aspect of gardening. And I have a camera.

Remember how space was supposed to be our final frontier. I think my little plot of suburban garden holds enough discoveries to keep me occupied for a long time if I take the time to look at them and figure them out. As does any square yard of natural habitat not cemented over and lost forever right under our unobservant noses.

By this time next year, I hope to have solved the mystery of who ate my wild senna plant. And if I do, I’ll write a post about it.

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Wildflower corner on October 16, 2016.

 

 

 

 

 

 

 

 

 

 

 

 

 

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