For the last forty years (on and off), I have searched for the larva of the Giant Swallowtail (Papilio cresphontes) which feed on Prickly-ash (Zanthoxylum americanum.) Prickly-ash occurs in Tenhave Woods, Oakland County and I have seen adult Giant Swallowtails in the woods for many years. I was helping identify and measure trees in the swamp forest when I found three larvae, the first that I have seen. They resemble bird droppings. A few days later, I found another group of Giant Swallowtail larvae at a highway rest area in Genesee County. This seems to happen to me often, that once I find an insect or plant for the first time I will start finding it at other places.

Giant Swallowtail Larva

Prickly-ash is not an ash but a member of the Rue family (Rutaceae) that includes the citrus. The odor of the flowers and fruit is similar to that of lime and the unripe fruit looks like a tiny lime. The plant is armed with prickles and a stand is painful to walk while doing fieldwork. The twigs and the main leaf stalk have prickles. Prickly-ash grows mainly in the Lower Peninsula but recently a few stations in the Upper Peninsula have been discovered.

Prickly-ash flowers and fruit

The adult Giant Swallowtails have an impressive six-inch (15 cm) wingspan. When it alights on a flower it must continue to flap its wings or the flower stem bends under the butterfly’s weight. This makes the butterfly easy to identify and difficult to photograph. The Giant Swallowtail ranges north to Saginaw. Go out and look for it in southern Michigan.

Copyright 2013 by Donald Drife

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