Meeting the Whites

I was over in the park last week, hiking through the forest, the first time since last fall. I had gone specifically with the intent of looking for wildflowers, but of course anything can turn up while you’re out. I was paused photographing some Spring Beauties, kneeling in the dry leaf litter, when a pair of butterflies fluttered over and flopped onto a leaf a couple feet away from my knee. They were joined together in coitus and having trouble flying. I took the opportunity to snap a few photos of them.

They were whites, which was fairly obvious, but I wasn’t certain which species. Cabbage Whites have dark dots on the uppersides of their wings, which these didn’t have, so they were something I wasn’t familiar with. When I came home, I pulled out my Kaufman guide to butterflies and had a flip-through. After studying the plates and doing some poking about online, I think these are Mustard White, Pieris oleracea. However, there’s a very similar species, the West Virginia White, Pieris virginiensis, that also occurs in our area. The difference is subtle, and seems to be in the hint of yellow to the underwings, and the distinctness of the shading along the veins (guides talk about the shading being dark green in Mustard White, and gray to brown in the West Virginia White, but I can’t really see any colour one way or another). I held out hope initially that these were West Virginias, but I’m thinking now that the tinge of yellow probably makes them Mustard.

This is mostly disappointing only because it would be cool to have a rare butterfly flutter over and flop at your knee. While Mustard Whites are fairly common in the province, West Virginias are much rarer, found in only a few dozen scattered localities, with their population concentrated in just three main areas: Manitoulin Island, the Halton/Hamilton region along the Niagara Escarpment (incidentally, where I grew up), and the Frontenac Axis. They prefer moist, mature deciduous forests, and possibly part of the reason for these three areas being their strongholds is that they happen also to be the main areas where deciduous forest is still extensive in southern Ontario.

Interestingly, they were virtually unknown from the province until the mid-1970s when somebody found a population in the Halton Forests near Milton, Ontario. The forest habitat there was threatened with destruction by the expansion of the gravel quarry that still exists in the escarpment there. In 1977 the Toronto Entomologists’ Association (despite the city name, it is active over much of the province, as reflected in its website’s name, Ontario Insects) undertook a detailed study, commissioned by the provincial government, on the local population. They studied population demography, movements, size, parasites, and other ecological information about the butterfly. Their findings were used to support the case for listing the species as Endangered in Ontario.

A few years later, during the 1980s, additional populations of the butterfly started popping up – one in the Frontenac region, another on Manitoulin, and then smaller ones in other varied locations around the province. Eventually, in 1990, the government downlisted the butterfly from Endangered to Vulnerable, and the quarry expansion was allowed to go ahead (I can’t really begrudge the quarry this; we as a society demand a lot of gravel, and it has to come from somewhere).

Unfortunately, that’s not the only threat to the species. Both Mustard and West Virginia Whites lay their eggs on members of the mustard family, Brassicaceae. In the case of the West Virginias, in Ontario their laval foodplant is known to be almost exclusively Cutleaf Toothwort, Cardamine concatenata and other toothworts. Mustard Whites use toothworts and some other species such as rockcress (Arabis sp.).

The invasive Garlic Mustard, Alliaria petiolata, also belongs to the Brassicaceae. In some regions, Garlic Mustard has become widespread, taking over the forest floor and pushing out native mustards including the host plants of the two whites. Even in areas where native mustards remain, often the butterflies will lay their eggs on Garlic Mustard rather than toothwort or rockcress (in the case of the West Virginia, in some areas it appears they are choosing Garlic Mustard preferentially over natives, for unknown reasons). It turns out, however, that Garlic Mustard contains some sort of compound that proves toxic to the larvae of these butterflies, and caterpillars feeding on the plant die before their second instar. Obviously this spells a quick end to those populations. There are many Garlic Mustard control programs in the province, particularly in the West Virginia’s population strongholds, that should hopefully help the butterflies. Fortunately, I don’t recall having seen any Garlic Mustard around here last summer, so hopefully it hasn’t moved in yet.

I’m off for the weekend, heading back to Halton for a couple of days. The weather is supposed to be wonderfully gorgeous, sunny and warm, and I’m hoping to see lots to post about!