Danaus Plexippus Caterpillar Gorging on Butterflyweed Leaves, an Excellent Food

Monarch butterfly caterpillar photographed at Raccoon Creek State Park, PA

It’s July 24th at Raccoon Creek State Park in southwestern Pennsylvania. Because it’s just minutes after 9 A.M. the sun’s rays are just beginning to warm up this stand of wildflowers.

Our Danaus plexippus caterpillar has gorged on Butterflyweed leaves (Asclepias tuberosa) and remains satisfied and sluggish. The following day it may have moved to Common Milkweed (Asclepias syriaca) and continue nourishing itself with such excellent Monarch caterpillar food.

These milkweed plants provide the glycosides that add a whole new dimension to this species. Those glycosides aren’t digested by the caterpillar. Instead, they remain unchanged and stable. The metamorphosis from caterpillar to adult ends with those same glycosides present in the adult.

That’s why I have never seen a Monarch caterpillar that bears predator damage. I have great difficulty remembering having ever seen a Monarch butterfly that had wings or other body parts that showed the ravages of a predator.

I have seen birds attack Monarch adults . . . and spit them out as quickly as they have mouthed them. The birds appear to be repulsed by something about the Monarchs.

What is that something? The glycosides. It must be a very, very unpleasant taste.

So our caterpillar rests in plain sight of any and all predators. Soon it will descend down the stem and remain out of sight until much later in the day. Some days later it will work to fasten itself to a plant stalk and then . . .

Have a look at our image of a Monarch Chrysalis.

In 2010 we saw reduced numbers of Monarchs. 2011 brought an increased number. What can we expect of 2012?