You probably know (if you're North American at least) that monarch butterflies lay their eggs on milkweed plants, the idea being that when the caterpillars hatch, they feed on the milkweed, which is poisonous for most animals. Monarchs, though, are resistent to the milkweed's toxins, and so can safely ingest it, and in the process become poisonous and bad-tasting themselves. Birds and other predators know this, so the story goes, and so avoid attacking and eating them.
This is well known, as is the ancillary story that viceroy butterflies developed the evolutionary trick of mimicking the monarch, even though their caterpillars do not feed on milkweed (although it turns out, this story is actually not quite as simple as most people think).
But, from my own anecdotal observations, it seems that a whole host of other insects also feed on milkweed - beetles, ants, bees, wasps, you name it. The most common one I see is the milkweed beetle, which is also red to warn off predators. (In fact, I rarely actually see monarchs on them!) But many other insects, moths and birds like hummingbirds do eat milkweed, although perhaps not as exclusively as monarchs.
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Red milkweed beetle |
So, how poisonous can milkweed really be? Have all these other insect also developed a resistance to milkweed's toxins? Have we been mislead all these years?
Well, yes, milkweed is indeed poisonous. It contains a toxic substance called cardenolide which, if ingested in large enough quantities - and there's the key - can cause cardiac arrest in humans, livestock and other animals. But is also tastes really bad, so instances of cattle poisoning, etc, are very very rare. Livestock will not eat milkweed unless they are very desperate. Similarly, pets will find your garden milkweed very disagreeable to eat, and so will typically stop before any harn can occur.
Several different species of insects are commonly found on milkweed plants, including: large and small milkweed bugs; swamp, red and blue milkweed beetles; milkweed aphids; and the showy milkweed tussick moth caterpillar. Like the monarch, these insects have developed a resistance to milkweed toxins, and some can even sequester the toxins in certain special glands.
So, poisonous? Yes, to an extent. Exclusive to monarch butterflies? Nope.