Sunday, September 16, 2012

Milkweed Bugs

If you take a close look at a milkweed plant today you will probably see crowds of yellow oleander aphids on the stems. Before long a monarch butterfly will come for a drink of flower nectar. Turn over a leaf and you'll see milkweed bugs.

Milkweed bugs, Oncopeltus fasciatus, suck juices from milkweed tissues and seeds; they have piercing, sucking mouthparts that are like sharp straws. These three are warming up on a metal fence railing in a milkweed patch. The little one is an immature stage called a nymph. The other two are adults. 

Milkweed bugs are bright red for a reason; it's a warning to anyone who might be thinking about eating one. Like monarch butterflies, milkweed bugs become poisonous from eating milkweed. A bird that eats a milkweed bug and gets sick will remember the bright colors and avoid them in the future. The strategy is called aposematic coloration.

Even among  bright red bugs this one stands out. It has just molted its skin; the remnants are still attached to its tail end. Newly emerged insects are lighter-colored than usual and somewhat soft.
Milkweed bugs have seven stages: the egg, five increasingly larger immature stages called nymphs, and the adult. They don't make cocoons. Each nymphal stage looks a little different, but none of them have wings and they can't reproduce. Like other insects the milkweed bugs have exoskeletons that make them rigid on the outside. After growing as large as the skin will allow, they shed it and come out bigger. At this time of year and you will probably easily find all the nymphal stages and lots of adults on any milkweed plant. Click here to go to a site that has a nice drawing of the stages. 

The newly emerged bug will soon darken, harden, lose that bit of old skin, and then get back to the business of eating milkweed. Mmmm... milkweed! Click on the photo to enlarge.

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