The Urban Pantheist

Photos by urbpan. Location: common milkweed plant on the shore of Leverett Pond, Boston.

Urban species #310: Oleander aphid Aphis nerii

There are a few species of aphids which are generalists, feeding on a range of different plants. Most, however, drink the juices of a small subset of similar plants, such as the lettuce aphid (which feeds primarily on lettuce, but may also feed on artichoke and petunias) and the oleander aphid. The oleander aphid is thought to have originated in the Mediterranean, but has now spread worldwide, having been exported along with its host plant. Of course, oleander is a subtropical plant, so in order for the oleander aphid to spread to northern regions, it needed to find an alternate host. It has found that milkweed plants serve quite nicely. In places where oldeander will not grow, this insect may be called "milkweed aphid."

One thing that oleander and milkweeds have in common is that they are both quite toxic, bearing chemicals called cardenolides. Oleander aphids are able to take this poison from their host plant and store it. When a predator attacks, cardenolides are emitted from "cornicles," which are appendages on the top of the insect's abdomen. Usually this repels the predator, but if not, the poison does its dirty work. In an aphid predator such as a lacewing or ladybug, the poison can render the insect sterile, or interfere with wing development. The aphid presents a fair warning in its coloration: bright yellow-orange with contrasting black appendages. This caution sign combination works for all kinds of bees as well as the monarch butterfly (another milkweed feeder that uses cadenolides in defense).

Oleander aphids are thought to be all female, reproducing through parthenogenesis. Generation of wingless daughters and granddaughters are born and spread until they reach a point when the plant can support no more of them. Then a generation of winged aphids is born and they fly to new host plants. Like many other aphids, oleander aphids produce honeydew as excrement, but their honeydew contains the same poison they use to protect themselves. Some species of aphid are protected ("tended") by ants, who consume the honeydew. The giant milkweed we examined in Antigua had both aphids and ants; are there certain ants that can cope with the poison? Or since worker ants are both wingless and sterile, is the poison's effect moot? I hope this question is explored in the entomological literature, but I could not find it. It does seem that while the aphid causes little harm to the host plant, the uneaten honeydew accumulates and grows a fungus called "sooty mold," which is considered unsightly.

Oleander is a widely planted ornamental in cities throughout the tropical and subtropical world. Milkweeds such as butterflyweed are likewise planted, in temperate cities and suburbs. And common milkweed and giant milkweed survive in roadsides, parks, and vacant lots everywhere. Wherever these plants are, they are accompanied by their aphids.

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