Aphids love the milkweed that I plant. Since I grow lots of milkweed, I get lots of aphids. Up until approximately June 22nd, I have had a biblical plague of aphids on my milkweed plants. A few days later 90% of them were dead or had disappeared. What happened?
I asked that question of Dr. Lincoln Brower, a monarch expert and he suggested that it looked like an insecticide had killed them. Since I don’t use insecticide, I wondered if the mosquito spraying that they do in my city might have killed them.
According to one Wall Street Journal article, spraying for mosquitoes will kill bees and butterflies, so why not aphids.
According to an article in Mother Earth News, “Do not use pyrethrum in situations where lady beetles, honeybees and other beneficials are active. Used carelessly, pyrethrum can wipe out these and other beneficial insects. ”
According to St. Louis County mosquito control, they use a product called Aqua-Reslin. They also indicated that they only spray after their traps have certain types of mosquitoes. West-Nile virus is something they don’t want in our neighborhood. You can call 314-615-4284 in St. Louis County to see what communities are being sprayed that night. What I have learned is that they don’t spray all the communities listed on each night. For example, this last Thursday they sprayed in Kirkwood, Des Peres and Ladue. The other cities were not sprayed.
http://www.stlouisco.com/HealthandWellness/MosquitoControl/MosquitoControl
Their email address is Mosquito.DPH@stlouisco.com
Ed Spevak who is the Curator of Invertebrates at the St. Louis Zoo has also written a good article on mosquitoes.
While my initial thoughts were that the mosquito spraying was killing the aphids, if you look at the picture below you will see the same type aphid death, but I know for sure that there has been no spraying in our neighborhood, because it is the July 4th holiday.
Leaf #1 was June 30th. The same leaf is shown as #2 and the picture was taken on June 5th. What is killing these aphids?
One option from the University of California indicated that some wasps will lay eggs on aphids.
They also say, “Aphids are very susceptible to fungal diseases when it is humid. These pathogens can kill entire colonies of aphids when conditions are right. Look for dead aphids that have turned reddish or brown;” That is certainly St. Louis in the summer and the dead aphids I see are this color. Possibly our hot humid conditions are killing off the aphids.
What I will probably try to do in the future is to water my milkweed leaves more often to try and encourage this killer fungus and reduce my aphid population without endangering other insects.
Butterfly Gardening 
I appreciate your,concern about mosquito sparying. I share your anx about it.
I tend to think the humidity is a contributor to their demise.
They very soft bodied and are easily injured. A favorite way I use to use to decrease them was with a spray of water or “flushing “a plant which either floats them away or makes fungus easier to kill them off but now I just wait for Nature to rebalance. I think it was a rebalancing act that affected them, best guess anyway!
Aphids cannot exist in large numbers for long. They reach a population peak and are almost always completely wiped out. That’s their cycle. If you mass milkweed, you’ll mass the aphids, and something will have to come along and notice that high of a concentration. That something will eat them or lay eggs in them or otherwise make use of the banquet until it is exhausted. Ladybugs, wasps, and lacewings are obvious. Hoverflies are another possibility since they would already be around your surrounding nectar sources. Your milkweed aphid trap can keep all of these predators well fed assuming they can survive the spraying.
If you mass host plants for monarchs and black swallowtails anywhere in your garden, you’re pretty much ensuring the health of the rest of your plants. The milkweed traps the pests. The carrots release all the chemicals to draw in the predators. The predators eat the pests, and patrol the rest of the garden while they’re there. It’s clever and easy.