If you have milkweeds growing for Monarch caterpillars then sooner or later you’ll find yourself asking “what are all these yellow aphid things?”
They are the Milkweed Aphid, also known as the Oleander Aphid. Their scientific name is Aphis nerii Boyer de Fonscolombe.
Aphis neriiis an obligate, viviparous and parthenogenetic species which means that the adult aphids are all females and they produce live clones of the female nymph rather than eggs. The adult females may be winged or wingless. The optimal temperature for development of the aphid ranges from 28 to 30 °C, and no new nymphs are produced when the temperature is outside the range of 5 – 35 °C (Hwa and Yun, 1999).
The aphids form large clusters sucking the sap from their host plant. This can stunt the growth of the plant and also spread plant diseases.
As well as feeding on the sap from the milkweeds the aphids store the poisonous Cardenolides to provide protection from predators, in the same way that monarch caterpillars do.
The aphids secret a sweet substance known as honeydew, which makes them popular with ants who often guard and ‘farm’ the aphid colonies. However the ants may be after more than just honeydew. Different chemicals from the plant are also found in the honeydew depending on which bits the aphids are on, and ants prefer the aphid colonies from the flower tips 3-4 times more than colonies from the leaf terminals (Bristow, 1991).
Oleander aphid – Aphis nerii Boyer de Fonscolombe. H. J. McAuslane (2012), University of Florida. http://entnemdept.ufl.edu/creatures/orn/shrubs/oleander_aphid.htm
Are ant-aphid associations a tritrophic interaction? Oleander aphids and Argentine ants. C. M. Bristow. Oecologia, Volume 87, Number 4 / September, 1991 http://www.springerlink.com/content/v22m7345j61q4120/
Increased nitrogen availability influences predator–prey interactions by altering host-plant quality. John J. Couture, Jason S. Servi and Richard L. Lindroth. Chemoecology. Volume 20, Number 4, 277-284 http://www.springerlink.com/content/c5m7n250642l782p/
K.M.Hwa and C.T.Yun. 1999. Population Parameters of Aphis nerii and the Effect of Temperature on It’s Development. Chinese J. Entomol. 19:297-306. http://188.8.131.52/english/journal/19vol/no4/2.htm