Yep, aphids. From here:
"Hop aphid infestations develop more rapidly during cool weather. Heavy populations of 8 to 10 aphids/leaf will weaken plants and reduce yields, and the application of an insecticide is recommended. Aphids should be controlled before or during the flowering stage to keep them from entering the young cones. Aphid predators, including lady beetles, green lacewings, and syrphid or hover fly larvae, can be used to help control aphids."