Q: Please tell me what this is! I don’t know if I should spray this or leave it.
A: They are milkweed aphids. I see them every year on my milkweed plants , looking like a bright yellow crust on the plant stems. Notice that they seem to sometimes jerk backwards in unison…as if they were all given a slight shock at the same time.
The easiest way to control them is with a water hose. A strong blast of water every few days will knock them off so that beneficial insects can eat them.