Monarch season may be over, but aphid season is not. Aphids are one of the most problematic plant issues for milkweed. While these bugs may not harm your caterpillars, the little yellow (black, grey, green etc.) vampires will suck the juice out of your milkweed. It may not kill the plant but in substantial numbers they can certainly weaken it. Besides that, the monarch don’t want to lay eggs on a plant covered with aphids and I’m sure that you don’t want to try to find the eggs in that mess. I know I don’t. You go out to your garden one day and notice a few hundred aphids, but they are spread around and you have lots of plants, so no big deal. You go out a few days later and there are thousands! How does this happen so quickly? Here’s how.
Wingless female aphids (called stem mothers) are aparthenogenic. That means that they can reproduce without a male, so asexual reproduction. Rather than laying eggs like many other insects, the aphid nymphs are live born. They go through 4 instars, something like the monarch caterpillars. Instead of taking weeks to a month to go through all the phases, they are fully grown and reproducing in a week. When the plant becomes overcrowded, the aphids will start producing winged females which fly off to a different plant. The cycle continues and within a very short time, you’ve gone from a few aphids to thousands of aphids. According to entomologist Stephen A. Marshall in his book, “Insects: Their Natural History And Diversity,” in a perfect aphid world, lacking predators, disease or parasites, a single female aphid could produce 600 billion (yes, you read that right) descendants in one season. Try not to let that give you nightmares. However, it’s not a perfect aphid world and they have plenty of problems of their own.
What do we do? It’s not hopeless, but it can be tedious. There are several natural ways to get rid of aphids, depending on how much time and patience you have.
First, you can try companion planting. Heavy planting of marigolds, onions, chives, mint, rue, basil, etc. may help keep them from your garden. Sunflowers act as trap plants. The aphids love the bright yellow flowers and may choose to hang out there rather than on your milkweed. It you don’t have room for all those other plants, it’s going to be down and dirty with the aphids. I squish them and rinse the plants. If I do it early and consistently enough, it works fairly well. It’s not my favorite thing. I end up with my fingers being covered with yellow stains and it’s just gross. That being said, you eventually decide that the pop of a dying aphid is a satisfying sound. Think of it as tiny bubble wrap.
Theoretically, if you just blast them off the plant with a hose, their tiny legs are to weak to climb back up the plant. I have my doubts, but you can try it early on and see. You can use insecticidal soap, spray alcohol on them or other organic controls, but you have to be sure to rinse the plants thoroughly. Whatever physical method you use, be sure there are no eggs or tiny larvae on the plants.