The first time I saw Aphids on my milkweeds I was worried that they would kill all of my new plants & drive the monarch butterflies away. I tried washing them off the stems & leaves with the hose & when that didn't work I tried scraping them off each plant, one by one. Now, I know better... I leave them alone & enjoy watching them! They do feed on the newer leaves & stems of the plants but they attract ladybugs & other beneficial insects to the garden & the monarchs don't seem to mind. This is the 1st time I've seen them this year..