By Irene Shonle, Horticulture Specialist, El Paso County Extension
Milkweeds are becoming a popular plant to add to the garden. The main drivers are that they are native and a host of the increasingly endangered monarch butterfly. However, they also attract a host of other pollinators (and even other milkweed specialists such as the milkweed beetle), and can add beautiful flowers and fragrance to the garden.
It is important to plant locally native milkweeds, such as Asclepias speciosa, A. tuberosa, and A. incarnata. There are also 16 other species of Asclepias in Colorado that are very cool, but harder to find, such as Asclepias asperula, A. cryptoceras, A. subverticillata, A. latifolia and more. If you can find seeds, some of these would be choice in the garden. I’m particularly fond of the antelope horn milkweed, A. Asperula.
Because milkweeds contain both a milky sap (giving them their name) and cardiac glycosides (or cardenolides), most insects that have not evolved to eat them are unable to process the toxins. Those that do, often sequester the toxins as a protective mechanism, and wear bright warning colors (aposematic coloration) to warn predators. This is why you often don’t see a lot of insect damage to milkweeds and if you do, it may be something you invited!
However, one insect that has recently come to the party that is an unwanted guest. These are the bright orange aphids with black legs, antennae and cornicles that you often see in clusters on the milkweeds. These are non-native aphids called Oleander aphids. They came in on introduced Oleanders, and the chemistry is similar enough in the plants to allow them to jump from one plant to another.
|Oleander aphid, (photo from U. of Wisconsin)|
We mostly see aphid populations controlled by birds, ladybugs, parasitic wasps, Syrphid fly larvae, and other predatory insects, but because the Oleander aphids also sequesters the cardenolides, they are often (but not always) avoided by these predators, unlike most other aphids. Oleander aphids are parthenogenic (meaning that there are only females, and they give birth without needing to mate), so populations can rise exponentially. Really extensive populations are hard on the milkweed because they suck out so much phloem, and coat the plants with honeydew. I have seen some Asclepias asperula fail to flower or set fruit, and sometime even just wither away due to the pressure of the aphids. Some of the more robust species, such as A. speciosa, probably can withstand higher aphid populations.
If you are seeing large populations and are concerned for your milkweed, don’t get out an insecticide, because that will also impact other insects. Instead, you can do a couple of things. Improve the health of the milkweed by giving it adequate water, but don’t fertilize it. You can also wash off the aphids with a stream of water (ensuring there are no desirable insects like monarch caterpillars), or even squish the masses. You could also use an insecticidal soap on the aphids, but be very careful to check for desirable insects. You’ll probably have to do this a few times during the growing season, but if you can get them early, you might have a chance at winning.