Oleander aphids (Aphis nerii) are commonly viewed with disdain by devotees of monarchs (Danaus plexippus). This is the time of the season when we see hordes of the non-native yellow aphids on common milkweed (Asclepias syriaca) plants that are "reserved" for monarchs. Of course, it shows that Nature takes no restaurant reservations, even for royalty.
It’s understandable to think that anything that threatens milkweeds by extension threatens monarchs. I’m often asked for a control recommendation for the aphids that will not topple the monarchy.
However, a study published in 2014 casts serious doubt on the need to control the aphids. In fact, the study showed monarch caterpillars benefit from the sap-sucking done by oleander aphids.
The authors of the study reported that “Danaus plexippus larvae feeding on plants together with A. nerii weighed 37.7% more compared to larvae on control plants without aphids.” The authors further stated, “… the presence of aphids did not influence the amount of leaf damage caused by monarchs suggesting that monarch feeding behaviour was not impacted by the presence of aphids, and increased weight was not simply due to greater feeding.”
To understand this counter-intuitive result, it’s important to reexamine the common belief that milkweed toxins do not affect monarch caterpillars. The impact of the aphids demonstrates that the caterpillars are tolerant of milkweed toxins, but not completely immune to their effects.
Regarding control measures for the aphids, if the scientific data isn’t convincing, one option is to simply let Nature take its course. Even though oleander aphids are non-natives, they are still the target of a plethora of enemies. The “3-Ps” (predators, parasitoids, and pathogens) can dramatically reduce aphid populations.
If you would like to take a more direct approach, you can send the aphids on a one-way water ride of doom by hitting them with a strong stream of water. Typically, very few waterlogged aphids find their way back to the start of their ride.