We had some questions about the bright yellow aphids show up on milkweeds each year.
Where did the aphids come from?
Oleander aphids, Aphis nerii, are thought to have been introduced throughout the world from the Mediterranean region, where oleander is native (the species name nereii comes from Nerium, the genus name for oleander). The aphids are now found over much of North America, where they also use milkweeds and a few related plants as hosts.
How do the aphids reach my plants?
Each spring, when the temperatures and other weather conditions are just right, winged females are blown long distances on the wind. When the air becomes calm, the aphids can make directed flights to host plants. Experts have failed to find male oleander aphids in North America, so they believe the winged females produce young without mating (called parthenogenesis). The first offspring are female aphids without wings. The aphids continue to give live birth to more aphids until conditions cause winged forms to be produced. The winged forms move on again.
Why are they yellow?
Like the monarch and related butterflies, these aphids pick up the toxic cardiac glycosides from the milkweed in their bodies. Their bright yellow-orange color serves as a warning to anything that tries to eat them. The chemicals also are secreted from their cornicles (the tiny black tubes on their rear ends).
Does that mean nothing will eat them?
Unlike many other introduced insects, oleander aphids have a number of parasites and predators. You may have seen the results of tiny wasps (for example, Lysiphlebus testaceipes). The female wasp lays its egg by inserting its ovipositor within the young aphids. The wasp's larva then eats the aphid's insides. When the parasite goes through metamorphosis into a pupa inside the body of the aphid, it causes the body of the aphid to turn brown, tan or black, and stiffen. This immobile aphid is referred to as a “mummy.” When the wasp emerges from the pupa, it cuts a hole in the back of the aphid’s abdomen and flies away, leaving the aphid’s empty body.
Flower fly larvae and small lady beetles will also eat oleander aphids.
Flower fly adults feed on flowers and are pollinators (see below).
What should I do about oleander aphids on milkweed?
Let's face it, aphids have a negative reputation. Virtually every gardening book you pick up will list aphids under the "pest" section. These labels are made by humans, however, not the plants or insects. The first thing to realize is that there are a lot of different kinds of aphids, and each species has its own special way of doing things. Often the aphids that are pests are introduced species, with no or few natural enemies, living on introduced plants growing under less than optimal conditions. Other species of aphids are much more benign. If you are willing spend some time observing the insects you find on milkweed, I think you may be surprised. Believe it or not, you might even find that you like them better once you get to know them.
I have had oleander aphids on my milkweed plants (Asclepias subulata) for 20 years. Every year I do absolutely nothing to them. Every year the wasps appear, the mummies build up and the aphids disappear. The main plant is 20 years old, huge and healthy. The other milkweed plants are also growing well. I don't see any evidence that the aphids are doing harm. Why not leave yours alone for a week or two and see what happens?
What about monarch and queen caterpillars? Will aphids hurt them?
Frankly, you are much more likely to do harm by knocking off eggs and small larvae while trying to control the aphids.
You also might consider that although we humans think that monarch caterpillars eat only milkweed, no one told the monarchs that. Several species of caterpillar are carnivorous (eat insects), including a species that feeds on sundew plants and eats the insects the sundew traps. I would not be at all surprised to find out that monarchs eat an aphid or two from time to time.
You can read all sorts of things on the Internet these days. My advice is to be interested and patiently observe what is going on in your own landscape. Do the aphids go away? Do the plants look okay? Do you have a lot of monarch larvae survive? Each situation is unique, and with time and willingness to investigate with an open mind, nature can answer your questions.
P.S. Of course I am interested in ants and aphids. I have never seen ants tending oleander aphids here. Have you ever seen ants tending this species?