No, you absolutely canNOT use malathion. Your insecticidal soap (or even homemade concoction) can also be very harmful to butterfly eggs and larvae.

Keep after the aphids with sprays of water and even your fingers and their population growth will eventually slow down. Those of us who grow some Asclepias for the Monarchs are resigned to the aphids.

What you saw with the Aphids is the next generations moving in after you knocked those you could see off the plant, they did not return. Note in the link where they talk about Aphids and Nitrogen.

Here is a link that might be useful: About Aphids

Thanks for the link and replies. I hope I did very little harm with the soap. Maybe not as the monarchs are still laying eggs. The aphids were so thick they looked like pollen. I have acquired a knack for wiping them of and then coming back with the water.
In the spring I had plenty of lady bugs that seemed to take care of the problem. Now in the fall I have only seen one. Maybe because of the drought. I haven't seen any honey bees either. Don't know what's happening there.

We have no idea where you are so can't comment on what environmental factors(weather, for example) might have an influence on your plants. This has been a dreadful year for people all over this country and others. If not excessive heat and drought, then torrents of rain and flooding. Sigh.

Here's the situation with ladybugs and the milkweed/oleander aphid....NOTHING likes to eat aphids that have been ingesting volumes of that bitter, toxic, sticky sap. Including ladybugs. They may try to tend to them in the beginning, but soon move on to far tastier aphids on other plants. Thus, this particular species of aphids go pretty much unchecked for much of the growing season.

I grow a few Asclepias for the Monarchs, too. I prefer the spray first and then squish technique, lol. I may have to try your method.

I'm about half way between Houston and Austin Texas. The drought here has been devastating. Our normal rainfall is 34"s. For the last year we have had less than 8"s. The pastures in the area are barren. There are a few weeds. Theses weeds provide the only natural food source available to the butterflies, That is other than flower beds around homes.
As far as getting them off most are pretty easy on the leaves just by pulling your hand/fingers over the leaves from bottom to top. The ones around the flower head are the ones I have a problem with. Tomorrow I'm going to try a paint brush. With persistence I seem to be making a little headway.
Thanks for the info. on the ladybugs. I never thought about their likes and dislikes before. I guess it makes sense.
On another note I have been walking my pastures finding other host plants for the Monarch. I have identified these as spider milkweed/Asclepias arenaria . They do not seem to have the aphid problem associated with Asclepias tuberosa. The plant and flower are also pretty nice. I will be collecting seed for propagation and trying some next year.