Oleander Aphids

Bright yellow pests on milkweed? It’s oleander aphids!


Many of us have planted region-specific milkweed plants to support Monarch butterfly populations. What we didn’t know, was that we would also be inviting a new pest into our gardens: oleander aphids. These pests can suck the life out your milkweed plants before the Monarchs ever have a chance.

Oleander aphids on milkweed (Kate Russell)

Oleander aphid description

Like other aphids, this species is small (1.5 to 2.6 mm), pear-shaped and soft-bodied. Oleander aphids, in particular, are bright yellow, with black legs, wings, and cornicles. Cornicles are tiny spikes on an aphid’s back that can excrete defensive fluid. [My dog did that once, after he got into some old pork bones. It wasn’t pretty.] Actually, these defensive fluids are cardiac glycosides that the aphids take from their host plants! Cardiac glycosides are known heart poisons. Luckily, these pests cluster on new stems and are easy to spot. And those defensive fluids won’t hurt you.


Aphid lifecycle

As much as I dislike aphids for their plant-damaging and disease-carrying capabilities, I have to give credit where it is due. These soft-bodied bugs really are amazing. Female aphids (and almost all of them are female) are viviparous and parthenogenetic. Wait! Come back! Let me explain. Viviparous means that offspring develop within the mother, the way we do. Parthenogenetic means fertilization by a male is not needed to produce offspring. Not like us. Most aphids do not have wings. But, when they become too crowded, or when a plant starts senescing (dying), some adult aphids emerge with wings. That would be something like all human beings being born 40% smaller, simply because we start running out of room and resources. Hmmm… But I digress. Let’s get back to aphids.]

Oleander aphids on milkweed (Kate Russell)

Oleander aphid management

If you’re not squeamish, you can squish the aphids between your fingers. Or, if you see a stray lady beetle wandering around elsewhere in the garden, gently scoop them up and show them where the feast can be found. If those are not options, you can use a strong spray of water from the hose to dislodge the interlopers, or use insecticidal soap. On plants with heavy stalks, you can interrupt support for aphids from ants by painting the stalk with a sticky barrier. This won’t get rid of the aphids, but it will make them more vulnerable to their natural enemies.

Unfortunately for the Monarchs, nearly all of these methods will harm their offspring along with the aphids. Lady beetles will eat Monarch eggs and larva, the insecticidal soap will kill them, as well. There is a parasitic wasp, Lysiphlebus testaceipes, that feeds on oleander aphids, as do syrphid flies, so avoid using broad spectrum pesticides. If you see dried up, brown husks of aphids, you will know that you have these helpers in your garden.

Oleander aphids on milkweed (Kate Russell)

Oleander aphid damage

Like other sap-sucking insects, oleander aphids pierce plant parts to tap into the phloem of the host plant. Think of it as diabetic mainlining. This nutrient rich food source blows through an aphid’s body, creating a sticky sweet residue called honeydew. Honeydew is a petri dish for bacterial and fungal growths, such as sooty mold. Also, the aphid tendency to feed in clusters stunts growth and deforms flowers and leaves, crippling milkweed and oleander plants. Vinca, periwinkle, and frangipani are also affected.