The West Virginia White (Pieris virginiensis) is highly localized in Ontario. This one is nectaring on a non-native mustard. The presence of non-native mustards is partly responsible for the decline of the this woodland butterfly across much of its range.
Photo: Randy L. Emmitt (Wikimedia Commons)
Eastern Onatrio's Frontenac Arch is home to a wide variety of species at risk, including an unassuming woodland butterfly, the West Virginia White (Pieris virginiensis). It's a butterfly that's notable for a number of reasons: it's truly sylvatic, whereas almost all other Ontario butterflies are found in open habitats; it's restricted to only a handful locations in the province, and it has apparently disappeared from many of those sites in recent years; and perhaps most importantly, it has a taste for mustards (family Brassicaceae). It's a combination of all three of these things that has earned the West Virginia White the designation of a species of special concern in Ontario.
I have fond memories of living among West Virginia Whites, which I could find flying in May only a short bike ride away from my cabin on Opinicon Lake. My favourite West Virginia White colony, was found under a forest canopy composed of some thirty species of trees, where salamanders seemed to reside under every fallen log, and spring wildflowers bloomed in ridiculous profusion. Among the abundant wildflowers were two toothworts: Cut-leaved (Cardamine concatenata) and Two-leaved (Cardamine diphylla), and it was upon these forest floor mustards that my West Virginia Whites fed. It's been several years since I last had the opportunity to visit my old colony, but I wonder with genuine concern how those rare butterflies are fairing.
In the past, the most pressing threat to West Virginia White populations throughout Ontario would have been loss or fragmentation of the mature forest habitat they and the toothworts require. Happily, my colony was safe from any kind of forestry, tucked away as it was on a large swath of protected land. The there was something else to worry about back then, in those heady days with my butterfly colony, and it's a worry that persists today: the insidious Garlic Mustard (Alliaria petiolata).
Garlic Mustard was intentionally introduced to North American from Europe as an edible herb during the mid 1800's. It has since expanded across the Great Lakes region and throughout much of the northeastern United States. Once established, it spreads like wildfire, choking out native vegetation and turning large areas of forest understory into Garlic Mustard monocultures. It's a terribly invasive species, more deserving of the appellation "weed" than almost any other woodland plant.
Just like the toothworts, Garlic Mustard is a member of the family Brassicaceae, the mustards. The mustard family includes such human favorites such as Broccoli, Cabbage and Cauliflower (Brassica oleracea), Turnip (Brassica rapa), Canola (Brassica napus), Common Radish (Raphanus sativus) and Horseradish (Armoracia rusticana). Many of these garden species are popular foodplants for introduced Cabbage White (Pieris rapae) caterpillars, but for West Virginia Whites the options are fewer because not many mustards grow in its dark forest habitat. The toothworts do and, unfortunately, so does Garlic Mustard. Indeed, I've spent more time than I care to recall pulling up Garlic Mustard in my woodlot. It's an annual chore, but it works to keep the forest floor more or less free of the nefarious weed. I fear however, that no one has been pulling Garlic Mustard at the old colony I used to visit years ago, and that may be cause for concern.
Garlic Mustard can out-compete the native toothworts for sunlight and soil nutrients, effectively eliminating them from the forest floor, leaving West Virginia Whites without their native foodplant. Interestingly, the butterflies will switch to using Garlic Mustard as a foodplant in the absence of toothworts, and may even prefer Garlic Mustard over toothworts. At first glance it seems like a fair trade off: replace one mustard with another and carry on. I mean, wouldn't stand to reason that the spread of Garlic Mustard would lead to a flourishing of West Virginia Whites, just as the planting of Crown Vetch (Securigera varia) along roadsides throughout southern Ontario has led to a drastic range expansion of Wild Indigo Duskywings (Erynnis baptisiae). In the case of the duskywing, which was once restricted to a small portion of southern Ontario, it has recently taken up the non-native vetch as a new larval foodplant and expanding its range as a consequence of this new widely available resource. In western North America, the naturalization of non-native Fennel (Foeniculum vulgare) has allowed Anise Swallowtails (Papilio zelicaon) to produce produce two flights every year, when previously it produced only one. This shift from unvoltinism to multivoltinism in the swallowtail was assisted by the tremendous abundance of Fennel which augmented the supply of natural carrot family (Apiaceae) foodplants. In strong contrast, however, the abundance of Garlic Mustard has actually had a negative effect on West Virginia Whites.
Lab experiments and field observations in the American portion of the West Virginia White's range have shown that caterpillars which feed on Garlic Mustard never reach maturity, usually dying after several days. Garlic Mustard, though attractive as an ovipositing site to adult females, is actually a death sentence to any caterpillar unlucky enough to hatch there. The Garlic Mustard's chemical defenses and a suite of growth inhibitors in the plants kill all caterpillars by the time they reach their second instar. Or at least that's the case in most populations. In at least one Massachusetts population, however, caterpillars are able to survive until the pupal stage by feeding on Garlic Mustard alone. Though the caterpillars took longer to pupate and were a third smaller than caterpillars that grew up on toothworts, they did survive. Perhaps other populations of West Virginia Whites may also be able to adapt to use Garlic Mustard.
There may be some hope for West Virginia Whites if other population can shift to using Garlic Mustard successfully. But that's not likely to happen soon enough to stop the decline of this species all across its range. Evolution is a long and often risky road, after all. It seems that combating Garlic Mustard where it has become established and keeping it from spreading into pristine woodlands in the first place is the best defense we can give West Virginia Whites. So I keep pulling and spraying Garlic Mustard in my woods and I keep hoping that the old colony of mine is still flourishing under those big trees half a province away, and not just in my memories.
