I have got rid of the little white bugs, aphids many times, always on crepe myrtles. I always used neem oil or just soapy water, both worked about just as good. probably stick with the soapy water in this heat. either way you have to spray both sides, underneath the leaves and on top. In the evening time after it's cooler, then spray off in the morning with a good stream of water. One application tyipicaly won't do much. you need to do it at least two or the times. In the evening and then spray off in the morming. And maybe repeat in a week or so.