Orchards host living communites of insects. Some are beneficial, other detrimental and many others may or may not have have any effect. April and May are key times when you can affect insect populations by applying insecticices. My strategy is based on sustainability in its most practical sense (money). Using insecticides on a systematic way requires equipments and costly products that may not pay, so I only apply insecticides when and how it is clearly needed.
In mature orchards, I have seen how untreated trees overcome aphid attacks, depite their unpleasant look in May. Ladybirds and other parasitoids do that for free. Almond trees in my region are fully affected by lacebugs in late summer, which is aesthetically ugly and causes (harmless) itches to the operators. In these cases I do not treat trees just because I do not want to bear unnecesary costs. What is more, perhaps I am saving beneficial insects.
In April, 23, 2016, I saw 10% of trees affected by aphids in a young orchard. As you can see in the picture, the leaves and young shots shrink, which affects the tree development. Treating just the affected trees is a worth doing exception, because a potential outbreak is prevented with a very low cost using a manual sprayer. I have used insecticide with soap (soap has insecticide effect and makes the treatment more effective. I have used a conventional soap today, but I usually prepare 2% potasium soap). With just 2 litres of this solution I was able to check and treat arount 15 young trees in an orchard of 150. I left untreated the trees with no symthoms. The total cost of this was less than 1 € per hectare.
I usually see other insects in April, like those black or brown wasps in the final picture. I have seen that trees where they live in great numbers are healthy. I am not an enthomologist and can not say if they are parasithoids of dangerous insects. What I can tell you as a practical farmer is that there is no need to kill any insect in the orchard unless you have a clear reason for doing that. One of my favourite philosophers said: “the wise man sets a good regime and leaves the rest to nature“.