Q.When To Add Insecticides
What’s the best thing to do for my apple trees this time of year January in California

Certified GKH Gardening Expert
Generally speaking, this will depend on the type of insect and the pesticide that you choose in the end! Other factors that will determine when the best time to add the pesticides will be the weather, and maturity of your trees. This article will get you started:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/when-to-apply-pesticides.htm
This collection of articles will offer much more information on apple care: