Prickly Ash

For several years now, we have been harvesting seeds and growing trees and shrubs that have been forgotten by the public and the nurseries. Such two plants are the Hop tree and the Prickly Ash. We purposely grow these since they are the caterpillar plants for the giant swallowtail butterfly. Since both these plants are scarce in the wild, the giant swallowtail has now become a member on the endangered list.

A fascinating bush that everybody has ignored and has faded away from the natural landscape. Here again, the common names are misleading for Prickly Ash is not a true ash but, rather, the most northern member of the citrus family. It is very fragrant when the leaves are crushed, like tangerines. The other common name is ‘toothache tree’. In colonial times the volatile, aromatic oil was used as a numbing emergency treatment for toothaches.

Like the Hop, it has been overlooked as a naturalization choice though it is an obvious choice. It is truly an adaptive plant where the nature and actual appearance of the Prickly Ash colony will conform to the planting location. If the colony is growing in moist, wooded conditions, the appearance of the colony is open with the Prickly Ash plants being well spaced at 10-13 feet apart. If the colony is growing in the wide open, dry soils, embankments or ditches, the plants form a thicket.

To avoid disappointment, plant both the Hop tree and Prickly Ash on the same site. My favourite arrangement is to plant a group of Hop trees surrounded by Prickly Ash. Though giant swallowtail butterflies can use both these plants as a larval/caterpillar source, they are odd as butterflies go in that they tend to reproduce on what they are accustomed to. For example, if a giant swallowtail lays her eggs on a Hop host, the caterpillars will be accustomed to Hop, not Prickly Ash, and tend to gravitate towards Hop when they become breeders. You maximize your chances of attracting giant swallowtail butterflies when you have both host plants.

So when you see your Hop trees and Prickly Ash having eaten leaves, REJOICE, don’t spray, for the giant swallowtail has made your place home. A truly satisfying experience.

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.

Adult Giant Swallowtail butterfly

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.

Giant Swallowtail caterpillar on Hop tree leaf

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.

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.