Secret to killing milkweed aphids, not monarch butterfly eggs

Los Angeles reader Millicent Stoller wrote to our SoCal Garden Clinic saying she was trying to plant for monarch butterflies. Stoller said she was growing two milkweeds, but both had become inundated with aphids.

“When I try to eliminate the bugs with a water spray or a soap-water spray, I end up destroying butterfly eggs,” she wrote. “Is there another way?”

For an answer, we turned to Barbara Eisenstein, research associate at Rancho Santa Ana Botanic Garden in Claremont, founder and manager of a nature park stewardship program in South Pasadena and horticulture chairwoman for the San Gabriel Mountains Chapter of the California Native Plant Society.

Eisenstein’s response:

The bright yellow aphids found on milkweeds are destructive, non-native pests. It is important to remove and dispose of them at first appearance or they will quickly infest the plant, making it difficult for monarchs to use the plant.

Monarch eggs are more tightly attached to leaves than aphids, so with just the right amount of pressure you may be able to wash off aphids without destroying the eggs.

Sprayed water may only dislodge the pests, which can climb back up on their own or be returned by aphid-harvesting ants. Soapy water may dislodge and kill more aphids, but it also is more damaging to the monarchs and can build up on the plant.

Though tedious, dabbing aphids with cotton swabs dipped in isopropyl (rubbing) alcohol is most effective. That kills them outright. Alcohol, however, also is lethal to monarch eggs and larvae, so care must be taken when dabbing.

Natural controls for the pests, sometimes called oleander aphids, include the ladybug, especially in the larval stage, as well as the lacewing, syrphid fly larvae and the tiny wasp Lysiphlebus testaceipes. Some of these are available in stores and online.

The Garden Clinic welcome questions and comments at home@latimes.com. Include “Garden Clinic” in the subject line. Because of the volume of mail we receive, we can respond only to select questions.