University of Maryland Extension

Aphids Got your Milkweeds

yellow aphids
(Oleander aphids)

aphids and monarch caterpillars on milkweed
(Aphids and Monarch caterpillar)

Milkweeds are under assault from an alien insect, the oleander aphid, Aphis nerii. These little orange insects suck the sap out of stems, leaves, can cause flowers and pods to abort, and can even kill plants. They concentrate milkweed toxins in their tissue more effectively than native milkweed aphids, and studies have shown that beneficial insects are less effective at controlling them. As a milkweed gardener, what are your options?

  • Wait for beneficial insects to arrive - sometimes this approach works and sometimes it is an abysmal failure.
  • Right plant right place - reduce stress on your milkweeds and they are less likely to be plagued by aphids. Swamp milkweeds require constant moisture, butterfly milkweeds require excellent drainage.
  • Don’t fertilize milkweeds - in general, aphids are attracted to plants with higher nitrogen content.

When patience and prevention have been exhausted, it’s time to get out the big guns. The following options will kill aphids, but will also take some beneficial insects and even monarch caterpillars out, so use the utmost caution:

  • Squish - place affected parts of the milkweed plant between thumb and forefinger and drag along the stem.
  • Squirt - use water from a hose or strong spray bottle to blast the aphids off the stem (can be combined with previous option).
  • Spray - spray aphid colonies with either horticultural soap or oil, both break down quickly but the soap is more likely to burn foliage. To prevent damage to insects elsewhere on the plant, consider cupping the part of the plant you are spraying in the palm of your hand.
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