Late Summer Butterflies

Welcome back to Ask a Naturalist: your own personal “Google” for information on all things natural on PEI! I’ve been getting a lot of questions about all the Butterflies around right now, especially the white ones. So, let’s look at late summer Butterflies.

The Island has about 70 different species of Butterflies – mostly native – with varying approaches to getting through the cold northern winter. Those approaches influence when different species become most noticeable, but late summer is a time of peak activity for many.

A few of our species avoid winter altogether by heading south. While the Monarch is the most famous of our migratory Butterflies, Painted Ladies, American Ladies, and Red Admirals do the same. These insects are very active right now, with August and September being when adults are most often seen.

Some of our Butterflies overwinter as adults. The lovely-named Mourning Cloaks, Commas, and Question Marks are busy feeding and searching out cracks in rocks or wood in preparation for winter. Overwintering as adults allows them to get a head start in spring, and they are the first you’ll see flying once the days start to warm again. Spring and late summer are peak times for sighting these species, although they are not overly common on the Island.

Photo 1: A White Admiral, one of PEI's common Butterflies. This species prefers rotting fruit or animal scat to plant nectar.

Most of our Butterflies overwinter as larvae (caterpillars), and the adults are wrapping up the last few weeks of their lives. The White Admiral (Limenitis arthemis, Photo 1) laid its eggs during the summer, and they’ve been developing through several stages of caterpillars ever since. While the caterpillars munch on trees in the Birch and Willow families (and will use silk to bind leaves into an overwintering shelter), adult White Admirals prefer rotting fruit and animal scat to flower nectar. It’s common to see them in a sunny spot on the ground in August, as shown here.

Photo 2: Another of PEI's common Butterflies: a Viceroy. You can tell it from the look-alike and less common Monarch by the black bar across the middle of the hindwing.

The Viceroy (Limenitis archippus, Photo 2) is a common PEI Butterfly that closely resembles the much rarer Monarch (Danaus plexippus). An easy way to tell them apart is to look at the hindwing: Monarchs lack that horizontal line across the middle that you can see here. The original theory was that tasty Viceroys mimicked the foul-tasting Monarchs for protection from predators. Newer information shows that both species are distasteful to predators and so it’s hard to say who’s mimicking who. Like the White Admiral, Viceroy caterpillars overwinter rolled up in a leaf of their host plant, in this case Poplar or Willow. August is usually the best time to see Viceroy adults on PEI.

Photo 3: The Cabbage White is a common but non-native Butterfly on PEI. This is a female, with two spots on each forewing; males have only one.

The Cabbage White (Pieris rapae, Photo 3) is the one that most people are asking about. This insect was accidentally introduced to Quebec in the 1860s and is one of only two non-native species of Butterfly on PEI. (We do have a similar-looking native species – the Eastern Veined White (Pieris oleracea) – but it’s mostly Cabbage Whites I’m seeing now). It’s common throughout our region and its caterpillar is considered a pest of crops in the mustard family (including Broccoli, Brussels Sprouts, Cabbage, and Cauliflower). Unlike the White Admiral and Viceroy, Cabbage Whites overwinter as pupae (the final stage before adulthood) and so adults appear earlier in the spring – April for Cabbage Whites, compared to mid- to late June for White Admirals and Viceroys. While most of our native Butterflies will have one to three broods per year, Cabbage Whites commonly have five and sometimes even six.

Butterfly numbers fluctuate from year to year; 2023 was a great growing season for most plants and so our Butterflies fared well. A combination of this good season and late summer being a time of peak activity for many species means we are seeing lots of Butterflies right now. Cabbage Whites are always abundant, but their ability to produce multiple broods per season coupled with ideal growing conditions means they are super-abundant right now. How common they are next year will depend on the upcoming winter and the next growing season.

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