That larva has nothing to do with Aphids since Aphids bear live young that look just like Aphids. Aphids on Milkweed can be controlled with a sharp spray of water that knocks them off the plant. Some suggest hauling the garden hose around to do that while I find a one quart spray bottle easier.
That larva does not look like the larva of a Monarch Butterfly, to me, and is not a Milkweed Bug but more than likely the people at your local office of the University of California Cooperative Extension Service could identify what you have.

Here is a link that might be useful: University of California CES

if thats a milkweed flower.. you have really cool milkweed ... are you sure on that ID???

ken

Ken, it's A. curassivica, the tropical milkweed that became popular as an annual oh...five or ten years ago.

hm, well I googled it and it might just be a fly. lame.
http://bugguide.net/node/view/335543/bgimage

There are definitely a lot of flies around my milkweed though. It's been bustling with business ever since the aphids went crazy on that thing.

Yes, a syrphid fly larva. It & its siblings decimate aphid populations.

Yep, that larva has EVERYTHING to do with your aphids. Everyone needs to be familiar with what the larvae and adults look like.

Good for you, menk, for doing your homework!

I wish I had those (or more of them). I'm having to smash the aphids by hand when I see them on mine. I got the same milkweed. Yeah your research and others nailed it. Hover fly larva.

This post was edited by fieldofflowers on Wed, Sep 3, 14 at 20:14

Thanks everyone! I came home today and found this (see pic). Looks to be a syrphid/hover fly (at least according to internet pics). Probably emerged quite recently, as it looks a bit emaciated. I'll let it out later so it can go find nectar.

fieldofflowers - I think your aphid population has to reach some critical mass or something in order to attract more predators. My aphids were a bit of a nuisance, but it wasn't until I saw huge patches of aphids in several parts of the plant that I also noticed a bustling of other insect activity around the milkweed. I had at least 2 ladybugs crawling on it, yielding 4-6 clusters of ladybug eggs the following few days, plus a bunch of flies circling the thing and crawling on it, plus these orange/black insects that kinda look like non-shiny long ladybugs. I'm assuming those are predators after the aphids. However, if you just want to kill aphids en mass, I recommend filling a tiny spray bottle with alcohol (cut with water if you want), and spray the aphids with that. It doesn't seem to damage the plant at all and doesn't seem to harm ladybugs or their eggs (I accidentally sprayed a cluster of eggs since they blended with the yellow aphids... ladybug larvae still emerged).