Aphids have legs?

My common milkweed plants, Asclepias syriaca, have aphids. They usually do, but this year it seems worse than usual. Should I be worried?

Common milkweed with a sooty mold outbreak

According to a recent essay in the Purdue Landscape Report, it depends. Oleander aphids (Aphis nerii), which is what I think my plants have, actually benefit monarchs by impairing the plant's defenses; larva will grow bigger faster. Oleander aphids are a bright yellow color and the hind legs are completely black.

Oleander aphids

In the past, I think I have also seen Aphis asclepiadis, Dogwood-milkweed aphid. They are slightly duller yellow than Oleander aphids, and the hind legs are not completely black. They are often attended by ants, which is what I have observed.

Oleander aphids, up close and personal

The good news for us lazy gardeners is no treatment is necessary. In fact, using chemicals will harm the monarch caterpillars more than the aphids. However, you can try to reduce the population by spraying with a stream of water. Or, like me, let the natural predators like ladybugs and wasps have their way with the aphids.