More about milkweed

In a previous post, I described two lovely species of milkweed (Asclepias) that I grow and explained the relationship between milkweed and monarch butterflies. But the ecology of milkweed is much more complicated than the simple equation of milkweed = monarchs. Lots of other critters are dependent on milkweed as well. There are milkweed bugs, milkweed beetles, and milkweed aphids, to name just a few, and all feed on milkweed and nothing but milkweed. All these critters can safely consume the cardiac glycosides in all parts of the milkweed plant. All have bright orange coloration so that their predators are warned away.

I’ve seen very few monarchs this season. Usually many arrive to lay eggs on the milkweed leave in July; this year I saw only one. Loss of wintering habitat in Mexico may be the problem. So may a dramatic increase in pesticide use.

As soon as i started growing milkweed, the milkweed bugs arrived. They perch on the seedpods as they form and destroy the developing seeds inside. Here’s one on a butterflyweed (A. syriaca) pod:

Milkweed bug feeding on seedpod of butterflyweed (A. syriaca).

As you see, there’s a low level of infestation this year. Some years all the pods on all the plants are completely covered with bugs. The seeds inside the pods never develop. But then guess what happens? The plants hold some pods in a kind of suspended state–the pods remain tiny. As soon as the bugs complete their life cycle and die off, the plants then grow and develop those seed pods. Pretty amazing, isn’t it? It happened just last year, when the infestation was particularly bad. I think my plants are enjoying a low point in the milkweed bug population cycle this year–generally a population will reach a high point, crash, and the gradually recover.

Milkweed pods, as you can see, have an unusual (and very graceful, I think) shape. When the seeds are ripe, the pod turns brown and splits open bottom to top, and the seeds gradually waft out on the breeze, each one propelled by a tiny parachute of milkweed down. Here’s one of the first red milkweed (A. tuberosa) pods to open this year:

Asclepias incarnata pod opening.

The seeds leave the pod one by one, slowly and gracefully drifting away. This undoubtedly increases the probability that they will land away from the parent plant. Plants have all kinds of strategies for spreading their seeds, but this is one of the loveliest I know.


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