Thursday, July 18, 2024

A viceroy butterfly alights in Groton on July 17. (Photo: Tom Murray)

Viceroy butterflies are mimics. They look similar to monarch butterflies but have a black bar across the wing and are slightly smaller. It was once thought that viceroy butterflies mimicked monarchs because monarchs store toxins from milkweed in their wings and are poisonous to bird predators –  so if viceroys looked like monarchs, birds would think viceroys were poisonous too, and leave them alone.

It turns out that viceroy caterpillars eat willow leaves, which contain salicylic acid – the ingredient aspirin is made of. In humans, aspirin can cause stomach upset or even ulcers. Similarly, salicylic acid stored in the wings of viceroy butterflies causes stomach upset in birds. So birds would avoid viceroy butterflies even if they did not look like monarch butterflies.

Viceroy butterflies have a black band near the edge of each hindwing … (Photo: Tom Murray)

It is now known that if a bird tastes either species of butterfly, it will avoid both. In other words, viceroys and monarchs are co-mimics. Mimicking each other is a shared protective device that helps each species survive: If both species are distasteful to birds, birds do not sample as many butterflies to determine if each species is edible.

In fact, viceroy butterflies are mimics in all stages of their life cycle. Viceroy eggs look like plant galls, a ball or lump on a leaf caused by insect feeding or egg-laying. Aphids, midges, wasps and mites can all create leaf galls. Viceroy caterpillars are mimics too: The caterpillars look like bird droppings.

… and monarch butterflies do not have that black band. (Photo: Jeanine Farley)

Viceroy butterflies do not migrate. They overwinter as caterpillars inside a rolled-up leaf. In late spring, these viceroy caterpillars reemerge, feeding on willow and aspen leaves. They develop into a chrysalis in about 10 days, emerging as adult butterflies in mid-June. A second brood of viceroy butterflies emerge in late July. Some of these butterflies lay eggs that develop into a third brood that emerges in September, but this third brood is small. Many of the second-brood caterpillars and all of the third-brood caterpillars suspend feeding as caterpillars and build a winter nesting site from a willow or aspen leaf.

To build its winter nesting site, a viceroy caterpillar first uses silk to attach a leaf securely to a branch, so the leaf will not fall off the tree in the autumn. Then it eats one end of the leaf and makes a tube called a hibernaculum out of the other half. To make the tube, it uses silk to roll the edges of the leaf together. It spends the fall, winter and spring inside this leaf tube, emerging in May to finish growing as a caterpillar. Then it develops into a chrysalis and finally a butterfly.

Viceroy caterpillars look much like bird droppings. (Photo: Tom Murray)

Like all butterflies, the emerging viceroy butterflies cannot eat. They use a long tube to drink from puddles, aphid honeydew, dung and carrion. Later-season viceroys get nectar from flowering plants such as mints, goldenrods, asters and milkweed.

Look for viceroys now. They reach peak numbers in mid-June and again in late July.

A pair of mating viceroy butterflies in Lancaster. (Photo: Robert Gessing)

In our region, viceroy butterflies look like monarchs. But if you were to travel to Florida, for instance, the viceroys would look different – monarch butterflies aren’t common in Florida, so mimicking the look of a monarch there would do them no good. In Florida, viceroys look like the Queen butterfly, a common species there. Queens are darker than monarchs. Consequently, viceroys in Florida mimic queen butterflies and are darker than viceroys in our region.


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Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.