Because of the reproductive cycle of the aphid, aka 200 eggs per female every 2 weeks, you will need to spray at least weekly to keep the voulme down. You should spray everything that grows not just 1 or 2 plants. Aphids are a contant problem from spring thaw to first freeze and beyond.. Might suggest starting with 1 tbsp of baby shampoo per gal of water ( a little milder than Murphy's) Apply weekly for about 6 weeks. This is to get the bulk of the problem. Then start with neem, This is a reproductive interruptor. Just don't stop the battle, They can fly 150ft on a windless day, a 5 mph breeze can bring in a bunch from next door. One of the best aphid recductions is very simple. Reduced applications of Nitrogen. Example: Per NMSU the ideal nitrogen for a tomato is 2.5, when you apply 15-30-15 this is 5x more nitrogen than the plant needs. Aphids love this.. You will have a big reduction in aphids just by fertilizing with a N = 3 or 4 If you switch from hi to low, it may take 6 to 8 weeks for the current high to leach out. Byron Despite all our accomplishments, humanity owes its existence to a six inch layer of topsoil and the fact that it rains.