Saturday, June 28, 2008

The Milkweed Insect Tribe, Part I

Because both milkweeds and the insects that inhabit them interest me so much, I tend to take a fair number of photos of all of them. I've shared some of my milkweed plant photos. Now I thought I'd share a few of my milkweed insect photos.

In this first section, I thought I would share photos of some of the insects that depend on milkweeds for all or part of their life cycle.

To the left, this handsome red beetle with white rings on its antennae is the milkweed longhorn beetle, Tetraopes femoratus.

To the right, this look-alike is the red milkweed beetle, Tetraopes tetrophthalmus. Note that it doesn't have white rings on its antennae, and that the spots on its elytra (wing covers) are elongated rather than round. This pair of species takes careful observation to tell apart.

The genus name, Tetraopes, means "4 eyes" and refers to the fact that all beetles in this genus have their compound eyes divided into an upper and a lower eye by the base of their antennae. Effectively, then, they have four eyes. There are 13 Tetraopes species in the United States, and 24 overall in North America. Many of the species in this genus are host specific, meaning they utilize only one species (of milkweed, in this case) in their life cycle.

These two milkweed beetles lay their eggs on milkweed stems near the ground or just below the soil surface. The larvae bore into the stems to eat, and overwinter in the milkweed roots. In the spring the milkweed beetles pupate, emerging as adults in late spring to summer. Adults eat the leaves and flowers of milkweeds.

Another interesting milkweed inhabitant is this fuzzy orange caterpillar. (I hadn't realized there were other caterpillars besides monarchs that ate milkweeds.) It turns out that this guy is the larval form of a white moth with the rather impressive name of orange-margined dogbane moth, Cycnia tenera. These moths eat both milkweed and dogbane plants; interestingly, the caterpillar can be either white or orange, depending on the species it is eating. I've seen them primarily on green antelopehorn, Asclepias viridis. All of the caterpillars I've seen have been orange.

Of course the classic milkweed insect is the monarch butterfly, Danaus plexippus. I've seen quite a few adults feeding at milkweed flowers this year, like this one on common milkweed, Asclepias syriaca, but I've only seen one caterpillar so far. It was on green antelopehorn. Unfortunately, I didn't have my camera with me, so I didn't get a photo. It was large and healthy, though, and probably just about ready to pupate.

The last of the milkweed-obligates that I've been able to photograph so far this year are the large milkweed bugs, Oncopeltus fasciatus. Lately I've seen some of the bright red young, called nymphs, that have recently hatched. They tend to feed in groups, at least at this stage, like the ones I photographed feeding on this green antelopehorn leaf and seed pod.

I've been seeing the adults, like the one to the right on a green antelopehorn bloom, for quite some time now. They seemed to appear shortly after the green antelopehorn started blooming. Again, like almost all of the milkweed specific insects, they are marked with warning colors: black and (in this case) orange.

In doing a little research, I learned that some of the northern dwelling large milkweed bugs actually migrate south far enough to overwinter as adults. Any adults or nymphs left far enough north to freeze during the winter will die. In the spring, the remaining adults start moving north again. Thus, the farther north you live, the later in the summer it will be before you see this species. I don't know whether the first adults I see here each spring spent the winter locally or moved up from more southern areas.

It takes the large milkweed bug nymphs about 40 days (and 5 molts) to reach adulthood. Only adults have wings, therefore only adults can fly. Both the nymphs and adults have the defensive habit of dropping to the ground if they are disturbed, which can make it hard to look at them closely. They'll remain still on the ground for several minutes, appearing dead, which is another defensive behavior, since many predators prefer to eat living animals.

In my next milkweed insect post, I'll show some of the other insects I've observed feeding at milkweeds. It's an interesting potpourri.


Pam said...

Thanks for posting these pictures! I am just starting with my garden and it took me a while to figure out what the orange eggs were on the underside of my baby red milkweed plants, which I started from seed this year. I loved seeing all the different kinds of Milkweed!

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Gaia gardener said...

All I can say is thanks. I appreciate you guys stopping by (and taking the time to let me know that at least some of my posts are being useful to someone).

Gaia Gardener: said...

Five years after posting this, I've gone back to and it now appears that the orange caterpillar here is probably the caterpillar for a different Cycnia moth, Cycnia inopinatus, also known as the Unexpected Cycnia.