During yesterday's OSU Master Gardener Volunteer Diagnostic Workshop in Licking County, OH, one of the participants asked about the hordes of yellow aphids sucking juices from common milkweed (Asclepias syriaca) reserved for monarch butterfly caterpillars. These are oleander aphids (Aphis nerii) and their appearance reminds us that nature makes no food reservations.
Although the non-native oleander aphid is considered a "cosmopolitan feeder" because it may be found on a wide number of plant host species, the aphid still confines its feeding to members of the dogbane family, Apocynaceae. Oleander belongs to the dogbane family along with other hosts for this aphid such as members of the genera Vinca and Asclepias. Favored milkweed hosts include native species such as common milkweed (A. syriaca), butterfly weed (A. tuberosa), and non-native species such as tropical milkweed (A. curassavica).
Oleander aphids are parthenogenetic meaning that there are no males; all the aphids of this species are females. This partially explains why this aphid can rapidly develop high population densities. They also enjoy some chemical protection against the depredations of predators. The sap of oleander and milkweeds contains cardenolide glycosides which are very serious toxins. As with a number of other insects that feed on plants in the dogbane family, the aphid incorporates the glycosides into their flesh as protection against predators. It is speculated that the bright coloration of the aphid warns predators against taking a taste.
On the other hand, there are some beneficial insects that are not thwarted by the aphid's chemical shield such as Syrphid fly larvae, lacewings, and even some lady beetle larvae and adults. A common nemesis of oleander aphids as well as a number of other aphids is the parasitoid wasp, Lysiphlebus testaceipes. The wasp lays eggs in the immature aphids; one egg per aphid. Parasitized aphids are called "aphid mummies" for their swollen, dark brown bodies enveloped in a dry, parchment-like exoskeleton. The combined plundering by beneficial insects is known to provide significant control of oleander aphids; however, their impact requires patience. Exceedingly high oleander aphid infestations can be reduced with an insecticidal soap application; this approach will help to preserve the beneficial insects and will not harm pollinators visiting the flowers. However, gardeners should not allow the insecticidal soap sprays to contact monarch caterpillars.