Papilio cresphontes lays an egg in Oxford Mills, ON

Female Giant Swallowtail (Papilio cresphontes) laying an egg on Prickly Ash in Oxford Mills, ON

I have been out in the field a fair amount this summer collecting data for the final chapter of my PhD. When I am in the field I always keep a watchful eye for Papilio butterflies - partly because I find them beautiful and interesting, but also because it validates the ecological relevance of my fieldwork with artificial caterpillars (see example here). This was the second day in a row (June 3 2013 was the first) where I had observed a Papilio cresphontes butterfly. This species is very new to the part of Ontario where I live, so I wanted to log my sightings on eButterfly, a website where you can report butterfly sightings to researchers interested in phenology and range shifts. Photos are particularly important for reporting key sightings because the scientists can use it to make sure your ID is correct. Check out my earlier posts about Giant Swallowtail butterflies here and here.

When taking the photo at the top I was patiently waiting for the butterfly to land. As you can see, snapping a photo while a butterfly is flapping its wings makes for a blurry photo even in full sunlight. I took this photo when the butterfly landed, even though it was still beating its wings, just to make sure I got something. After she flew out of range I looked back at the photo and realized what she was up to - she was laying an egg! When I went back to the branch I was excited to find the egg that she had just laid:

Papilio cresphontes egg laid June 4 2013 on Prickly Ash in Oxford Mills, ON.

The previous day I had been contacted by Ross Layberry (Anne Hanes Natural History Award winner and co-author of the book The Butterflies of Canada) about a Papilio cresphontes sighting I had posted - he wanted to validate the location of the sighting. I sent these photos to him and asked if he knew the tree species. He ID'd the tree as a species called - Prickly Ash (Xanthoxylum americanum - also spelled Zanthoxylum americanum). Interestingly, this species has been labelled as a weed by the Ontario Ministry of Agriculture and Food. Prickly Ash is a member of the Rutaceae family which contains many of the host species used by Papilio butterflies. This seems to be the only tree species used for egg laying by Papilio cresphontes in this area. Here is another photo of the leaves:

Prickly Ash (Zanthoxylum americanum)
Branch with the egg marks so that I can find it later

The tree branch has been marked and I hope to see a fully grown caterpillar this summer. Note that she (the adult female) chose new growth leaves which is often the case when females are selecting egg laying sites. Giant Swallowtail caterpillars look like bird droppings, but unlike Papilio canadensis and many other Papilio caterpillars it stays this way throughout its caterpillar life stage. Here is a photo of what they look like:

Giant Swallowtail (Papilio cresphontes) caterpillar resting on a lemon leaf (Photo: TokyoJunkie via

Keep an eye out for these guys in your area, the butterflies and caterpillars. If you see one be sure to log your sighting at

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