Butterflies and Larval Food Plants

Image Gallery This cutie is the caterpillar of the spicebush swallowtail butterfly. The big eyes are fake; they probably evolved to make the caterpillar look like a snake to predators.

WARNING! Butterflies can be addictive!
So say many new books on butterfly identification, and the number of “butterfliers” to join the sport in the last decade seems to confirm this. A new style of field guide (see listings, Page 67) that allows identification on the fly, rather than requiring the butterflies be netted and killed, is partly responsible. Anyone who has used a bird guide will be comfortable with this method, and with using a pair of short-focusing binoculars that allows you to study a butterfly as close as your foot without scaring it away.

At the start, new butterfly gardeners concentrate on attracting adult butterflies by adding nectar plants to their landscapes. But curiosity and logic soon drive them to wonder about the other side of the life cycle, and that’s where the fun really begins.

Gardeners and other plant lovers often find the study of butterflies particularly rewarding because it offers fascinating glimpses into why plants are the way they are. Like most herbivores, butterflies have a preferred meal plan — in fact, many species are so finicky about their choice in food plants for their caterpillar stage that their survival rates rise and fall on the success of a single plant species. The coevolutionary dance between plant and insect is an amazing story of defensive strategies attempted, overcome, improved and overcome once again.

Plants “retaliate” for being chomped on by developing special poisons or distasteful chemicals. We humans have many reasons to be grateful for this process — the herbivore pressure on plants has led to wonderful adaptations, both medicinal and flavorful. Mints, oregano, spicy nasturtium leaves and pine resin are good examples. Most of the aromatic smells, unique tastes and medicinal properties we enjoy in plants came about as defenses. Butterflies are only the visible tip of the iceberg when it comes to the world of insect herbivores driving these botanical experiments — but they are so wonderfully visible!

Plants’ chemical-defense plans generally work, but eventually an insect comes along that finds a way to neutralize the poison, or better yet, put it to a good use. The relationship between monarch butterflies and milkweed is a celebrated example. The poisonous glycosides in milkweed repel most herbivores. But monarch caterpillars not only eat the leaves, they harmlessly store the glycosides on into adulthood. Birds learn quickly to avoid these brightly colored butterflies after the first vile taste.

Over time, as the plant ramps up the concentration of the compounds, the insect eating it adapts. But success comes at a cost — eventually, the butterfly becomes dependent on those compounds, which it needs to trigger both egg-laying behavior and larval feeding. Apollo butterfly larvae eat stonecrop and store for defense a compound from it called sarmentosin. Red admirals nibble safely on nettle leaves. Pearl crescents need asters. Fritillaries only eat violets (though you won’t catch them at it since the caterpillars retreat into the ground during daylight). Skippers cue in on grasses. Cabbage whites eat, well, cabbages. (Leave the Bt spray off one of your cabbage or broccoli plants to encourage those delightful butterflies.) And so it goes, for most species.