Tri-State Neighbor: South Dakota Farm Newspaper

A pervasive rule in biology is that if an animal is bright, showy and easy to catch, then it either tastes awful or is deathly poisonous.

With this in mind, a quick visit to a milkweed plant reveals a handful of conspicuous insects that shouldn't appear on anyone's dinner plate.

Milkweed is a very common perennial plant, which can be seen in cropland, roadsides and backyards across the United States. Giant pods are currently swelling and will soon give forth the wispy seeds that float about as a sign of the approaching winter.

The “milk” from these plants contains a class of caustic chemicals called cardenolides that prevent most insects (and other herbivores) from eating them. However, those species that have figured out how to detoxify milkweeds, find a very abundant food source and few competitors.

Another upside to feeding on milkweed is that the chemicals ingested often make the insects themselves toxic. Advertising their toxicity, milkweed insects are some of the more striking showmen found in nature.

The best known example of this is the monarch butterfly, which actively flits about before taking off for Mexico in the fall. These favorite of summer visitors are inextricably tied to milkweed plants as their larval host.

A quick look at a milkweed plant with obvious hunks taken from their leaves often reveals an exquisite caterpillar (growing to almost 2 inches in length). The alternating black, white and yellow stripes flaunt the cardenolides that lie within, and monarch caterpillars have only a few natural enemies.

Another striking resident of milkweeds is the red milkweed beetle. Black-speckled and bright red, with long black antennae, these three-quarter inch long beetles should catch your eye as they munch on the veins of milkweed.

The milkweed beetles have four compound eyes, and like bespectacled children, it is a trait they are no doubt chastised for by other bugs. Close relatives within the long-horned beetles have C-shaped eyes wrapped around their antennae, but in the red milkweed beetle, the eyes are entirely interrupted, and thus appear as four.

A third insect commonly found on milkweeds are Oleander aphids, which feed on plants in the dogbane family, of which milkweeds are a member. These are bright yellow aphids with black legs that can form abundant colonies that cover large portions of the plant.

The aphids harmlessly suck the juices of the plant and produce sugary honeydew that ants love. In fact, some ants actually protect these aphids from enemies in exchange for the sugary food the aphids produce.

These are just three species of an array of showy insects commonly found in association with milkweed, but represent the colorful diversity that results when insects adapt to living on toxic plants.

A weed is best defined as a plant out of place. While our first reaction is to want to kill “weeds” with broadscale enthusiasm, a closer look reveals that these plants support a beautiful and rich fauna of creatures that have a real value. Unfortunately, it is much simpler to kill something when we don't understand it.

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