Do the trees recover later in the year? If they do, spraying would just damage the beneficial insects that also

If one tree seems heavily infested, see this page for a cheap remedy. It will work if the caterpillars - as they usually do - migrate up and down the tree.

Here is a link that might be useful: Cheap vcaterpillar control

I have had the same problem with these worms on all of my Palo Verde's as well. I used malathion, which initially killed them, but in a week the entire trees were covered in worms again. Were you able to get rid of them, and what did you use?

Try BT. Green Light is one brand. Check your big box stores and nurseries.It leaves most benneficial insects alone. Here's some info from the U of Colorado:

Here is a link that might be useful: BT