The Latest Dirt - Sept 2022

Milkweed Is Overrun with Aphids

By Emma Connery

Client’s question: I'm growing milkweed in hopes of creating a habitat for monarch butterflies. Every day every plant is infested with oleander aphids. I pick them off but I'm wondering if there's anything else I can do that would not require such careful attention. Thanks so much for your help.

Photo by Kathy Keatley Garvey. https://ucanr.edu/blogs/bugsqu ad/blogfiles/35382.jpg

CoCoMG Emma Connery’s reply: Thank you for contacting the UC Master Gardener Program’s Help Desk regarding aphids on your milkweed. Thank you for helping to save Monarchs by providing habitat. Yep, those aphids sure do love milkweed! I am sure you are finding picking off aphids pretty tedious. This is a bit of a dilemma—manage one insect on a plant while causing no harm to the other.

Photo by Kathy Keatley Garvey. https://ucanr.edu/blogs/bugsquad//blogfiles/60503_original.jpg Lady beetle larva eating oleander aphid.

I checked with some universities, because above all others I trust their research and knowledge base. I did an internet search using keywords ‘monarch caterpillars and aphids university’ and the consensus is that almost anything you do to manage the aphids can also damage the eggs and caterpillars you’re trying to provide for. The most benign approach is a spray of water but even that can dislodge eggs and caterpillars. There are a lot of natural enemies that love aphids. If there are aphids, some natural enemies will find them, so leave the aphids alone. You might get a kick out of this YouTube video that shows a number of different natural enemies attacking aphids.

Aphids suck plant juices, which may decrease the vigor of the plant, but they will do no direct harm to eggs and caterpillars. Following are resources from universities on aphids and monarchs.

Purdue University: Milkweed Aphids are out in force this year. Should you try to manage them?
https://www.purduelandscapereport.org/article/milkweed-aphids-are-out-in-force-this-year-should-you-try-to-manage-them/

Oregon State University: Monarchs vs. Aphids
https://bygl.osu.edu/node/833

University of Illinois Extension: Aphids on Milkweed
https://extension.illinois.edu/blogs/garden-scoop/2021-08-07-aphids-milkweed

I hope you find this information helpful—or at least comforting. If you have additional questions, contact us again. Best to you and your milkweeds.