yellow aphids on milkweed
Aphids often infest milkweeds. This is of little consequence unless the plants are grown for ornamental or some other purpose. For instance, aphid infested plants are not suitable food for monarch caterpillars because the aphids attract ants and other insects that may kill the caterpillars or interfere with their development.
Spraying the plants forcefully with water may remove most of the aphids. These pests can also be controlled with insecticides but doing so may also kill other insects that feed on the plants or aphids.