In southern CA, when is the best time to spray for insects on fruit trees?
Call your county agricultural agent for the best response.
Having lived in SoCal, we had to spray 4 times a year for white flies.
The neighborhood has them and our trees would get re-infected.
it really depends on where you live in Southern CA. I am in Castaic where it is freakishly blazing hot - still in the 90's and upper 80's and the "plus" if you could call it that, is an absence of many pests. I have never had to spray my citrus or figs - yet. Now that I said that, some odd bug will drop out of the sky on my trees! Here are some sites for you to get started. When I moved into this house almost 20 years ago, I had a cottonwood tree in the front yard. After a couple years it really started to look bad so I called in a local arborist for a consult. I had some sort of bug (beetle), which eats the core of the tree. I was advised to cut the tree down before high winds blew it over, possibly on my house or the neighbors. Not wanting to have to deal with that, I had it cut down and stump ground out and I treated the surrounding soil. Arborist was the least expensive portion of the project.
Contact your local Ag Extension for free advice, or consider calling in an arborist, they are not as expensive as one might think.