Friend Terry planted milkweed (Asclepias syriaca) last year. Doing great. Until she noticed the stems packed with a bright yellow bug with black markings and appendages that looked like an aphid. Yup. Oleander aphid (Aphis neril ), aka milkweed aphid, also feeds on wax plant (Hoya carnosa). Adults are all female, reproducing asexually; males do not occur in the wild. Winged adult females (alata) are yellow and black with dark wing veins, wingless forms (apterae) are yellow with black cornicles, antennae, legs and cauda (tip of abdomen). Nymphs look similar to apterae, except smaller. Invasive. Photos at: https://commons.wikimedia.org/wiki/File:Milkweed_-_or_oleander_-_aphid,_Aphis_nerii.jpg http://entnemdept.ufl.edu/creatures/orn/shrubs/oleander_aphid.htm

Feed exclusively on plant sap. Pierce, suck, and penetrate stems, leaves and sometimes flower buds. May congregate on 2 stems, leaving rest of the plant untouched. Favor tender shoots in upper portions of the plant. A colony can get very big, very fast and plant may suffer die-off. If they grow wings, the colony is getting too large and they are preparing to move on--not much left of the plant to sustain them. Bright yellow indicates high toxicity, so spiders and birds tend to stay away.

The real issue is that milkweed serves as host plant for Monarch butterfly larvae. Terryfound Monarch eggs but wasn't sure if she could save them with such a large aphid colony. Monarch eggs are more tightly attached to leaves (usually undersides) than aphids. Her best choice was to move the eggs to an unaffected area of this or another plant. It's unclear if she caught it in time to save the eggs.

Just the right amount of water pressure on a localized area may wash off aphids without destroying Monarch eggs. But you have to scrutinize the leaves and know what the larvae look like. And "dislodged" aphids may climb back up or be returned by aphid-harvesting ants. Soapy water or horticulture soaps on localized area might kill more aphids but is damaging to the monarchs. Cultural controls include reduced levels of watering, pruning and fertilization to reduce tender shoot production. Beneficial insects for control include ladybug (especially in larval stage), lacewing, syrphid fly larvae, and the tiny wasp Lysiphlebus testaceipes. A bright, all-yellow bowl with water and touch of vegetable oil draws insects and is helpful in a small or confined area. In agriculture fields, they use a waterpan stand: https://ucanr.edu/repositoryfiles/ca2310p18-65528.pdf.

Barbara Eisenstein, horticulture chairwoman San Gabriel Mountains Chapter of California Native Plant Society, suggests a tedious but effective technique: dabbing aphids with cotton swabs dipped in isopropyl (rubbing) alcohol kills them outright. Alcohol, however, is also lethal to monarch eggs and larvae, so care must be taken when dabbing. http://articles.latimes.com/2012/nov/09/news/la-lh-milkweed-aphids-monarch-butterfly-eggs-20121104

156-page agricultural manual on milkweed, aphids and monarchs (pg 44+) and pathogens (pg 63+).

http://www.xerces.org/wp-content/uploads/2014/06/Milkweeds_XerSoc_june2014.pdf.

Monarch info and links: http://sfbaywildlife.info/species/monarch_butterfly.htm

FYI, and don't use where Monarchs are present: some advocate garlic as a natural pesticide for bugs, diseases and fungi. However, it will kill beneficial insects and bacteria. Test on small area first. Recipe:

http://green-patches.blogspot.com/2013/06/aphid-control-using-garlic-pesticide.html