Our City Arborist (tree specialist) has been inspecting these trees and working with both the County of Los Angeles and the University of California -- Davis experts about white fly control.
Here's what they say:
Message from Public Works Department:
“We are still struggling with the overwhelming abundance of white flies. Weather conducive to early and rapid replication created a population of white flies beyond our preparatory efforts for biological control.
"We confer with the University of California Agriculture and Natural Resources and follow their Statewide Integrated Pest Management Program. The recommended process to control white flies is through the placement of natural enemies such as lacewing larvae.
“External application of pesticides are not overly effective and the most effective pesticide is Malathion, which we don't want to use. A more recent alternative that has come to our attention is Trunk Injection. This process injects an insecticide into the phloem system of the tree. When the white fly feeds on the tree's sap, they become infected and die.
"This process should not be used on trees that will produce food within the year following the process.
"For Council District 5 in particular, we are focused on Ash trees, so this caveat is not of concern. We have conducted a pilot on a couple of Ash trees on Ladoga and Deborah Streets. The insecticide takes 7-10 days to work through the tree to the sap and we will be checking for efficacy next week. If this process works better than a biological approach, we may go citywide on our 1,500 Ash trees.”
---Michael Conway, Director, Public Works Department
I am including a slide presentation (click here) (done in Florida -- which has the same problems) that you might find helpful, as well as links to articles and websites. (Click here to get article on home garden pest management)