These butterflies look beautiful, taste horrible

As summer draws to a close, we’re seeing lots of orange and black butterflies. Some are Monarchs (Danaus plexippus) migrating south, but many others are slightly smaller butterflies called Gulf Fritillaries (Agraulis vanillae).

Gulf Fritillaries have black streaks and spots on the upper surfaces of their wings, while the undersides are brownish with large silvery patches. On the forewings are three distinctive, black-rimmed, white spots.

These butterflies are familiar sights in the Lowcountry as they glide over fields and gardens, sipping nectar through straw-like mouthparts that coil up neatly when not in use. Lantana, verbena, and pentas are among their many food sources.

The spiny orange caterpillars, though, feed only on passionflower vines and can strip a plant of leaves in record time.

There are some 500 species of passionflower, including maypop (Passiflora incarnata), common in the southeastern U.S. The plants contain bitter-tasting alkaloids that Gulf Fritillary caterpillars not only tolerate but even store in their bodies through adulthood, making them unpalatable to many predators.

Biologists regard the bright orange and black pattern of the Gulf Fritillary as “warning coloration” –advertisement to predators of its toxicity. After sampling a colorful but nasty-tasting larva or adult, a bird presumably will avoid making the same mistake again.

I’ve seen some Gulf Fritillary butterflies with their wings badly tattered and worn, perhaps from first-time predator attacks. Amazingly, though, they could still fly and feed.

Warning coloration is attributed to other orange and black butterflies whose larval food plants contain toxins. These include Monarchs, whose caterpillars eat milkweed, and Viceroys (Limenitis archippus), whose larvae feed on willows. Since all of these butterflies are roughly similar in appearance, a bird that tastes an individual of one toxic species may avoid preying on other species that look much the same.

Gulf Fritillaries are common in the Lowcountry into early fall, but they can’t survive freezes. As temperatures start cooling down, large numbers of adults migrate down the coast to overwinter in southern Florida.

Once spring arrives, descendants of the original migrants straggle northward as the weather warms, gradually re-populating breeding grounds over the following summer.

This story was originally published September 13, 2016 at 9:37 AM with the headline "These butterflies look beautiful, taste horrible."