Tis the season...for yellow-orange aphids inundating our milkweeds. As many of you already know, they are Aphis nerii (oleander aphids), a cosmopolitan species.
I grow a lot of milkweeds specifically for monarchs and can tell you they will kill plants, make them unsuitable for larvae, and drastically affect flowering and seed production. I save A LOT of seeds each year to further milkweed growing efforts. If I let them progress, pollinators would be affected by lack of flowering and plants dying as would monarch larvae, and seed production.
We have lots of ladybird beetles, syrphid larvae, and lacewings but they can't keep up. Even the small Lysiphlebus testaceipes, a parasitoid wasp, can't keep up the their rapid generation rate. However, if you look closely at the aphid colonies, the parasitized ones are the ones that look slightly swollen, dull brown and smooth.
You can gently squish them. You can also lightly spray a fine mist of 70% alcohol. You can blast them off with the shower setting on a hose nozzle. If ants find living ones they will carry them back up. But I actually use all of the methods above at some point. Also, if you look at areas under these aphids, they excrete so much honeydew you get thick sticky leaf deposits that usually get colonized by a group of molds commonly referred to as black sooty mold. They are rendered unsuitable for larval feeding and are severely affected by a reduction in photosynthesis.
I don't use soap spray or alcohol on the flowers or plants when there are monarch caterpillars or eggs, and when they are in flower. This protects aphid predators, monarch larval leaf palatability, as well as keeping them accessibke and desirable by the pollinators. The milkweeds are very important food sources for a wide array of pollinators and are always being foraged. I also don't do it to protect the parasitoid wasp population.
If you do end up using alcohol or soap spray, do wash them off immediately after the damage is done so that the desirable critters can get back to doing what they do.
When I use the water blasting, I hold each leaf or inflorescence from behind and hit them with the water shower spray in the front. You need to sort of get used to how much pressure you can blast without negatively affecting plant tissue. You'll also have some fall and regroup on a lower leaf. They can be hit the following day. Ants may bring survivors up as well. The key is to treat it like a game of Whack-A-Mole. It gets easier.
