Swollen milkweed pods line country roads in late summer and early fall. When fully mature, the pods split and the seeds disperse courtesy of the silky parachute attached to each one. But a close inspection of these bulging seed pods often reveals a striking black insect with bright reddish-orange markings.
I checked the milkweed by my garage this morning (Sept. 24) and every stem was covered with milkweed bugs. Many stems were a peculiar yellow-orange color. When I looked closer, I realized the stems were covered with tiny yellow-orange larval milkweed bugs. Each was about one millimeter long. Adults grow to be about a half-inch long. Larger larvae and adults were concentrated on the seed pods, waiting for the pods to mature and split.
Milkweed bugs are seed eaters. Most seed-eating insects are less conspicuous because they eat seeds that have fallen to the ground. Milkweed bugs, however, gather on seed pods and are difficult to miss. Sometimes a single pod is covered with dozens of these "true bugs" (members of the Order Hemiptera). And often many of the individuals are immatures.
Milkweed bugs exhibit incomplete or gradual metamorphosis. That means that rather than going through four distinct life stages - egg, larva, pupa, adult - immatures (nymphs) grow gradually to adult size and resemble adults except that they lack full wings, and their color pattern is different. Black wing pads appear early in development, and nymphs have bright red abdomens. Milkweed bugs molt five times before reaching adulthood.
Seed-eating bugs face a dilemma: they have piercing-sucking mouthparts. They cannot chew through a seed's tough outer shell like rodents and they cannot crack seeds like seed-eating birds. Instead, they pierce the seed coat with their sword-like beak and inject saliva into the seed. The saliva digests the seed's insides, and after a time, the bug sucks up the soupy contents through its tube-like beak.
Larger amounts of saliva dissolve seeds faster and more completely. That's why milkweed pods become communal dining sites. Many milkweed bugs attacking many seeds mean more food for all. This technique also permits other seed-eaters to attack larger seeds than an individual bug could handle.
Despite the large number of milkweed bugs that attack milkweed seeds pods, only the seeds on the periphery of the pod are most vulnerable. The seeds that develop toward the center of the pod enjoy protection from attack.
The bright orange markings on milkweed bugs warn predators away. Like monarch butterfly caterpillars, which eat milkweed leaves, milkweed bugs are foul-tasting, thanks to chemicals in the milkweed plants. These chemicals protect milkweeds from most, but not all, herbivores.
Large milkweed bugs (Oncopeltus fasciatus) share another fascinating behavior with monarch butterflies - they migrate. University of Delaware entomologist Doug Tallamy describes in his book, "Bringing Nature Home" (2007, Timber Press), that large milkweed bugs migrate from their wintering grounds along the Gulf Coast to as far north as Canada.
Milkweed pods don't mature until late summer in the northern half of the continent, so milkweed bugs winter in the Gulf Coast states where milkweed seeds are available during the colder months. When summer returns, large milkweed bugs follow milkweeds as they mature from south to north. That's why we only see this species in late summer as milkweed pods mature.
Another seed-eating bug is easily confused with milkweed bugs. Box elder bugs, though just a bit larger, are also dark with bright red markings. As their name suggests, however, box elder bugs are found only on box elder trees (a type of maple) and never on milkweeds. Box elder bugs consume box elder seeds in the same fashion that milkweed bugs eat milkweed seeds. By mimicking the warning coloration of milkweed bugs, box elder bugs no doubt deceive at least some hungry predators.
Some people are more familiar with box elder bugs as a nuisance. In the fall, after the first frost, they gather on south-facing walls of houses, sometimes by the thousands. And like the "multicolored Asian lady beetles," they sometimes enter homes where they intend to overwinter. Fortunately, box elder bugs (like the lady beetles) are harmless and easily controlled with a vacuum cleaner.
- Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via email my web site, http://scottshalaway.googlepages.com