Milkweed is well known as the host plant for the monarch butterfly. The monarchs sequester toxic cardenolides from the milkweed plant for defense against birds and other predators. Milkweed is host to numerous other insects. Some of these insects also sequester cardenolides and have warning coloration.
One of the smallest insects on milkweed is the aphid, Aphis nerii. Its orange and black colors make it easy to see and send a message to predators: “I am toxic.” In the photo below, the cornicles (the black tubes sticking up out of the aphid abdomen) are clearly visible. Golden droplets are visible on the cornicle of the large aphid in the center and a smaller aphid in the upper right. In many aphids, the cornicle secretions contain alarm pheromones, signals from one aphid to another that a disturbance is nearby. In this case, picking the milkweed leaf and orienting it upside down in the sunlight may have triggered the release.
Aphis nerii are tended by ants and secrete honeydew from the anus of the digestive system. Interestingly the honeydew contains toxic cardenolides which may limit the amount of feeding and tending by ants. Milkweed with high aphid densities often collects honeydew droplets that mildew on the leaf.
Milkweed Aphid
Living With Insects Blog 
Great post! I learnt a lot more than I previous knew about aphids.
Do you know if Aphis nerii is global in its distribution? I found aphids that appear identical to yours in Bangalore, India: http://thedailycritter.posterous.com/arthropod-husbandry
Oh, and notably, there was not an ant to be seen in the vicinity, when I found the aphids. 🙂
Nice blog, btw. I got interested in insects and spiders from a photography angle, and am now trying to learn as much as I can about their fascinating lives. It’s nice to read about them from a “harder” entomological perspective that your blog projects. 🙂
Pavan, at least in the U.S., this aphid A. nerii is introduced from Europe, so I would bet it might be all over- and also feeds on tropical Oleander. Best, Kenny
Thanks Kenny. You’ve rumbled it: the plant the aphids were on was definitely tropical Oleander!
This definitely makes great sense to anyone
What an all around amazingly written blog..
Thats some informational article!!
This may be your best piece on the net.
I have loads of ants all over the leaves and stems on my common milkweed plants but no aphids.( I see the aphids on my tropical milkweed however but no ants. Do I have to worry that the ants will ingest the Monarch butterfly eggs and larvae?
To my knowledge, ants don’t attack monarch larvae.
Eggs may be protected by toxins as well.
Monarch numbers in Indiana are low this year.
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