Milkweed, the sole host plant for the Monarch butterfly, often becomes infested with the bright yellow Oleander Aphid (Aphis nerii). These non-native pests reproduce rapidly, quickly covering stems and new growth. The milkweed ecosystem is uniquely sensitive because it supports the Monarch’s entire life cycle. Using conventional broad-spectrum pesticides, or even many organic sprays, can kill Monarch eggs, caterpillars, chrysalises, and adults. Therefore, removing aphids requires specific methods focused solely on eliminating the pest without harming the developing butterfly.
Immediate Physical Removal Methods
The safest way to immediately reduce an aphid population is through mechanical action. A strong, steady jet of water from a hose nozzle will dislodge large colonies of aphids. Use medium pressure, aiming carefully to knock the insects off without damaging delicate stems or dislodging Monarch eggs or larvae. Target new growth and the undersides of leaves, where aphids congregate.
Manual removal is also effective. You can use gloved hands to gently squish clustered aphids, or wipe them off with a damp cloth or paper towel. For stems completely covered in dense colonies, prune the infested section entirely. Dispose of these cuttings immediately away from the garden to prevent aphids from migrating back to the host plant.
Applying Targeted Topical Solutions
If physical removal fails, non-systemic topical treatments offer a targeted solution, but they require extreme caution. Products like insecticidal soap or horticultural oil work by coating the aphid and suffocating it on direct contact. Since these solutions also kill Monarch eggs and larvae, they can only be used after all Monarch life stages have been safely removed from the plant.
Before spraying, thoroughly inspect the plant, and carefully relocate any visible eggs or caterpillars to an uninfested milkweed plant. Apply the solution directly to the aphids, ensuring complete coverage, especially on new growth and stem tips. To prevent harm to returning Monarchs, the treated milkweed must be thoroughly rinsed with clean water one to two hours after application. This step washes away the residue, which could otherwise affect Monarchs that hatch or crawl back onto the plant.
Utilizing Natural Predators
For sustainable, long-term aphid management, encouraging or introducing natural predators is a beneficial strategy. This biological control method utilizes beneficial insects to regulate the aphid population naturally. Effective predators include the adult and larval stages of lady beetles (ladybugs) and lacewing larvae. These insects are voracious aphid consumers.
You can attract native populations of these insects by diversifying your garden with nearby flowering plants that offer nectar and pollen. While you can purchase and release lady beetles or lacewings, they may not stay if the aphid population is too small, and they can occasionally prey on Monarch eggs or very young larvae. Biological control is best viewed as a slower, preventative measure that helps stabilize the milkweed ecosystem rather than an instant fix for a massive infestation.