Plants grown to attract monarch butterflies may become infested with bright yellow oleander aphids. Colonies of these small sap-feeding insects live on stems, foliage, and seedpods of oleander, butterfly weed, and milkweed.
All of these aphids are females; however, they produce live young without mating, so numbers can build very rapidly. Sap removal by a heavy load of aphids can stunt growth and may be severe enough to kill some plants. Oleander aphids produce large amounts of liquid waste called honeydew. Ants and other insects visit infested plants to collect this nutritious fluid. Some of these insects may harm monarch eggs or caterpillars. Also, aphid feeding reduces the content of defensive chemicals in milkweed, which increases the effectiveness of a small wasp that attacks monarch caterpillars.
Keeping oleander aphid numbers as low as possible is helpful to monarchs. Depending on the situation, crushing aphids on plants, washing them off with strong sprays of water, or spraying them directly with insecticidal soap are control options that have minimal impact on the butterfly. Check plants weekly for resurgence of surviving aphids or arrival of winged forms to start new infestations.
By Lee Townsend, Extension Entomologist