We had a colony of Large Milkweed Bugs covering a small cluster of milkweed. They are not a serious threat to the plants in general but this plant was being saved as a source for seeds. I received permission from my editor to study them for several days before they were removed.
As you might guess by the name, Large Milkweed Bug, Oncopeltus fasciatus, there is also a Small Milkweed Bug which we observed in upstate New York recently. Both have the bright orange aposematic coloration warning that says "I am full of milkweed toxin- consume at your own risk."
|Small Milkweed Bug|
|Large Milkweed Bug|
|All five instars of nymphs clustered together|
There are several subtle (to me) anatomical differences between the male and female. The easiest way to tell is to compare a mating pair, because the female tends to be larger than the male. Since copulation lasts for up to 10 hours there are usually multiple pairs on a single plant.
The large milkweed bug, Oncopeltus fasciatus, is a good species to study for an understanding of hemiptera ("true bugs"). They are said to be easy to raise with a short life span, making them the perfect "lab rat" for an entomologist or a classroom full of 5th graders. They can be fed cracked sunflower or watermelon seeds.* That said, I followed the directions carefully and all of mine went on a hunger strike and died. Obviously I am not up to the 5th grade level.
* Instructions for raising milkweed bugs are available at this link.
A "must watch" video from Cornell discusses how Monarch caterpillars handle milkweed defenses.