Sunday, August 16, 2015

Milkweed Bugs

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
"The milkweed bug undergoes incomplete metamorphosis. The nymphs look like adults but do not have full wings and their color pattern is different. They have five instars before they reach adulthood. Black wing pads appear early in their development. Eggs are a light lemon yellow changing to a reddish color. Incubation period is about four to five days. Each molt lasts five to six days. An adult will live for about one month. The insect overwinters as an adult."

Milkweed bugs suck the liquid from the plants using their long proboscis which extends down along the abdomen.  The toxic chemical they ingest from milkweed is transmitted into the eggs. They are extremely productive, laying 30 eggs a day and up to 2,000 in their lifetime.  This trait makes them ideal subjects for insect laboratory research because they also have a short life cycle and are easy to manipulate.


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. 

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