The Beautiful but Poisonous Milkweed

Its flowers smell like perfume, its colorful blossoms and abundant nectar draw a variety of insects to it.

But only a very few species can tolerate eating the milkweed, because a cut leaf or stem produces a sticky, milky, alkaline sap that is full of toxic chemicals. Monarch butterflies and their larvae seem to tolerate these nasty substances (called cardiac glycosides), and actually sequester them in their body, so that if a naïve predator like a young blue jay tries to eat them, the bird’s digestive system immediately rejects the caterpillar and the bird spits it out. This might not save that particular caterpillar or butterfly, but the bird won’t try eating another one of that kind and remembers “orange and black – don’t eat that”.

This week the first caterpillars showed up, one I had expected, and one I had never seen before.

This Monarch caterpillar is probably just a few days old. In a week or two, it will look more like this one below.



And then I walked over to another clump of milkweed across the yard and found this clump of larvae that had eaten the milkweed leaves literally to the "bone". They had almost completely defoliated the plant.

A little closer view of these half-inch long fuzzy larvae. Turns out these are the Tufted Milkweed Moth (Euchaetes egle). The female lays her monster clutch of eggs on plants without Monarch larvae, which is a good thing since this enormous group are voracious eaters. I captured this bunch and put them in a rearing cage with a couple of 2-foot long milkweed stems, and they managed to demolish all of those leaves in less than two days.

The milkweeds are forming small pods of seeds now, and soon another group will arrive to monopolize these plants – the milkweed bugs.



Milkweed bugs are seed predators and, like the Monarchs, they arrive by migration from points south, in time to harvest the seed crop. It’s always amazing to me that these insects can hone in on scattered patches of milkweed as they move through an urban area like this one. The picture above was taken last August when the pods had matured. Adults and nymphs feed communally on the seeds; they stick their mouthparts through the pod wall, secrete salivary enzymes to digest the seeds, and then slurp back the digested nutrients. We've only seen milkweed bugs on plants in Minnesota the last few years; perhaps its the change in climate that makes it possible for them to come this far north.