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.
Asclepias asperula, Antelope horn milkweed
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.
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| 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.
