Did you ever see so many aphids attacking a single plant? What a mess! But don’t reach for the spray can. The damage is much less than you might think. In fact, there’s no real damage at all.
These are milkweed aphids on a red milkweed (Asclepias incarnata) plant in my garden. Milkweed aphids attack only milkweed–like all aphids, they’re very specific predators. Aphids that eat roses don’t also eat milkweeds, and vice versa. And like all insects that attack milkweeds, they’re adapted to be able to digest the deadly cardiac glycosides that milkweeds produce. That lovely orange color is warning insects that like to eat aphids–and there are legions of them–to stay away. Hence the very dense population of aphids. Nothing’s eating them.
I grow lots of milkweed plants of two different species, and every year a few of them fall victim to various milkweed insects–aphids or bugs or, if I’m very lucky, monarch caterpillars (but not this year as far as I know, although I saw one caterpillar on a groups of milkweeds I planted on another site in the area). This year, a few plants had milkweed bugs on a few seedpods, and this one plant had an extremely heavy infestation of aphids.
Guess what I’m going to do about those aphids? If you’ve been reading this blog for a while, you probably already know I’m not going to do anything. In this case, a little thought to the time of year and the life cycle of plants and insects tell me that the plant has already flowered and set seed and will be going dormant within a month. At that point, the aphids will die as well, because there will no longer be any food for them. And since the aphids cannot attack any other plants, what harm can they possibly do in my garden? So I will do nothing except admire the density the aphids achieve on this single plant. You can actually see the orange color from almost 20 feet away.