The Giant Swallowtail Butterfly

Throughout the summer season, I have been fielding questions from customers who had planted Prickly Ash and Hop. They were all understandably excited since Puslinch county was reporting a strong presence of giant swallowtail butterfly. Mixed with their excitement was worry at the large number of caterpillars and their fate. I thought it would be easier to answer everyone’s questions through this article.

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Giant swallowtail butterflies (Papilio cresphontes) were first identified back in 1777. They are a widespread butterfly ranging from the southern states up to southwestern Ontario. In Canada, it is our largest butterfly. Since they are such a large caterpillar, experts say, that camouflage is not their main form of defense from predators. Most species of caterpillars blend in with their larval leaves or flowers. Instead, the giant swallowtail resembles bird droppings.

If one of the many predators, namely, birds, reptiles, amphibians, spiders and ants, finds the caterpillar, it shoots out this bright, orange osmeteria from behind its head and releases a foul smelling odor. The osmeteria looks like an orange, forked snake tongue.

I will cite the conventional data but I truly believe it is outdated and we are seeing an emergence of a northern giant swallowtail butterfly subspecies in Canada. In time, I suggest, 2 distinct subspecies will evolve – a northern and southern population. You make up your own mind as you read along.

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In the southern states, the giant swallowtail larval food is citrus. Also, these butterflies experience 3 flight seasons, that it, 3 full breeding/life cycles where the third generation goes into hibernation, as a chrysalis, awaiting the return of warmer weather. Here, the northern population, is different. The larval food for the caterpillars are Prickly Ash and Hop. The giant swallowtail butterflies experience only 2 flight seasons: late May into July, and late July to early September. In the summer, the chrysalis stage lasts 10 – 12 days. Chrysalis formed in winter will last till spring. The chrysalis overwinters in a winter diapause, a state of lowered metabolism where breathing is slowed and no feeding occurs.

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Temperature has a huge influence on chrysalis. Lack of September frosts correlate to increases of occurrences of giant swallowtails the next year. Apparently, caterpillars can survive multiple frosts, but low temperatures are stressful physiologically. Low temperatures decrease the activity of the caterpillars and the food quality of the host plants are diminished. This translates to caterpillars being slower to reach chrysalis stage and being more vulnerable to parasites and predators.

As we experience a more rapid onset of climate change, increasing September temperatures and lack of frosts will lead to more colonization by giant swallowtails in our area. It has been suggested that over the past century, giant swallowtails in the more north range have adapted to endure cooler temperatures. So I would suggest that conventional databases are not current since it seems the northern population are expanding further north. In 2012, they were reported as far north as Peel, Caledon, Erin and Orangeville area.

So as the giant swallowtail butterfly forges northward, what are the threats and what can we do? As their range expands, the breeding habitat decreases. The common Hop tree is a threatened species and the Prickly Ash is not common. Without larval plants no species of butterfly can sustain their population.

The nature of the breeding sites are important. These butterflies prefer open woodland and fence rows along with the associated fields. There are 3 times as many caterpillars on south facing vs. north facing fields. Sunnier field edges equate to more healthy, less stressed caterpillars. Placing host plants on south facing fields will benefit this butterfly.

Caterpillars will only travel 5 meters from their original host plants to pupate. Having brush piles and rock piles to safely pupate, or hibernate, are crucial.

I think we are witnessing a wonderful genetic response by this butterfly. The northern population is differentiating from its southern cousin by becoming more cold tolerant and adjusting its flight season to compensate for our climate. So enjoy these winged beauties in your garden this spring.