Yellow aphids can appear in large colonies and are often a terrifying sight as they devour milkweed plants. They are a non-native insect and they can multiply very quickly. However, they are not a direct threat to monarch caterpillars because they feed on the milkweed plant only. They can indirectly affect caterpillar health by depleting nutrients in their only host plant.
What are oleander aphids?
The oleander aphid (Aphis nerii), sometimes called the milkweed aphid, is a common pest of milkweed plants. It is a non-native bug, most likely originating in the Mediterranean region where it's principal host plant, oleander, grows.
The Oleander aphid is a bright yellow insect with black legs, and stalks known as cornicles on the back of the abdomen.
Method One: Manual Removal
Although, time-consuming, the safest way to remove aphids is manually by squishing them between your fingers (use gloves to avoid staining your fingers) and then using a hose to dislodge them from the plant. Always check for monarch eggs and caterpillars before spraying because you could damage or dislodge them in the process as well.
It is best to catch the aphids before they become an aphid army, so even if there are just a few on the plant, remove immediately.
Method Two: "Contact Only"
This is a method recommended by Monarch Watch and was shared by Vic Jost @ Jost Greenhouses through Elliott Duemler at Taylor Creek Nursery. A mild solution of dish soap and water will also work.
• 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 directly to their body for it to work.
*Use caution with this method because it will also kill monarch larvae if they come in contact with the solution. Rinse the plant when finished so you do not injure monarch larvae and other beneficial insects.
Monarch Joint Venture
Natural History of Orange County and nearby places.