Aphids are quite unpleasant. They make trees look bad with sticky and bended leaves. I have patiently watched their development in the orchard. You can take your own conclusions out of the following two experiences.
In second and third leaf orchards, aphids have appeared in several confined outbreaks, affecting groups of 2 to 4 trees. That happened in April and May 2015. Leaves of young trees are so tender that aphid presence makes them bend to form a little ball. The plant lucks ugly and its development is seriously affected. I decided that treatment was necessary to heal the trees and prevent the expansion of the pest. In early May, I sprayed only the affected trees with a thiamethoxam insecticide at the recommended dose in a solution of potassium soap (1.5 %). The plague was easily controlled in this way. I did not treat the good lucking trees in order to save the populations of potentially beneficial insects, such as those abundant black and brown wasps that I had seen in early April.
I acquired a mature orchard in February 2015. This orchard had been neglected for years, so it has been a good laboratory to look into the natural tendencies and interactions of its living community. I found in early May that most trees were heavily affected by aphids. I did not panic and waited two weeks. I visited the orchard again and focused on the most affected trees. This time, I observed that almost every leaf had one ladybird. The plants looked better. The pest was self limited with no intervention. The trees that were heavily affected in spring have endured the summer and their fruit is healthy. I have noticed that the external hull has small dents, which have not affected the shell or the kernel size and quality.