Red Milkweed Beetles (from lower Michigan)

2014 August 16

When I was a kid living in the Lower Peninsula of Michigan, I used to find these colorful beetles on milkweed plants all the time. They were particularly amusing because if you pick one up, it will make a fairly loud “eeee-eeeee-eeeee” noise by rubbing parts of its exoskeleton together. However, they don’t appear to exist in the Upper Peninsula, so I took the opportunity to catch and photograph some while we were visiting Sandy’s parents near Manchester on August 1, 2013.

These are Red Milkweed Beetles, Tetraopes tetrophthalamus, a species of longhorn beetle that specializes in eating milkweed. And, judging from their orange color, making themselves inedible with milkweed toxins.

One of their interesting features is their eyes. For most beetles, their eyes have their field of view partially blocked by the bases of their antennae. Some species have gotten around this by developing elongated eyes that partially wrap around the antennae. But, these beetles have gone one step further – the antennae have actually divided the eyes in two, so that there is a “lower” set of eyes below and forward of the antennae:

and an “upper” set of eyes above and behind the antennae:

So, they have a full, unobstructed field of view.

Not that they actually make much use of this field of view for evading predators, though. They are ludicrously easy to catch. When you find them, you can just pick them off of the milkweed as easily as picking berries. Another indication that they just don’t care, because they are so toxic that nothing much is likely to eat them.

Historically, milkweed has been considered a noxious weed, and so these beetles are considered at least innocuous, and perhaps beneficial to the extent that they knock back the milkweed. However, lately people have been becoming concerned about monarch butterfly populations, and so have been intentionally planting milkweed to provide food for the butterflies. And so some people are actually starting to consider the milkweed beetles to be pests, because they are “killing the Monarch’s milkweed”.

For that matter, if the research during World War II had panned out, milkweed could have become a commercial source of latex for rubber. And in that case, milkweed would have become a cash crop, and both the milkweed beetles and the monarchs would today be considered pest species. Which just shows how the dividing line between beneficial insect and pest can be highly dependent on historical accidents.

One Response
  1. Carole permalink
    August 16, 2014

    Haven’t seen this beetle on my milkweed in Florida where milkweed plants are not plentiful.
    Fascinating eyes.

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