Like many insects that forage on milkweed, Oleander Aphids have adapted the ability to tolerate the toxins in milkweed plants.
The good news is that aphids are not a direct threat to monarch eggs or larvae. Aphids will feed on the milkweed plant only; they won't spread to your other plants. They only tend to be problematic is the plant is very small or weak. In these cases, they may weaken the plant even further and greatly decrease the nutritional value for your caterpillars. We have had larger caterpillars eat the plant aphids and all!
It is nearly impossible to get rid of aphids. Lady bugs are nice because they eat the aphid but not the larvae. Of course catching enough lady bugs to solve the problem is more than time consuming- and it can be pricey to purchase them from a biological control supplier. Also, the lady bug larvae do eat the monarch eggs.
The easiest way to control aphids is to use the hose to blast them off every couple of days. You won't completely get rid of them, but it helps. You can spray them with hot, soapy water. This should kill them on contact.
Try this "Contact Only" mix (This was shared with us by Vic Jost @ Jost Greenhouses through Elliott Duemler at Taylor Creek Nursery).
• 1 part (e.g. 1 oz) Blue Dawn
• 1 part Isopropyl Alcohol
• 1 part white vinegar
• 128 parts (e.g. 1 gal) water
"Contact only" means that the insects have to have the mixture applied to their body for it to work.
Once you see that the mixture is working, as an extra precaution you can then rinse the plants so that they are safe for monarch larvae.
As a last resort, you can wear rubber gloves to smash the aphids. The gloves keep your fingers from getting stained orange.
Don't smash the aphid mummies! They contain beneficial wasps!