There is a good crop of caterpillars in this patch of sticky, stinky shrubby desert stuff* at the arboretum. I think they might be some kind of earworm, but I'm not sure.
Maybe these are what the Ammophilia wasps are after. I have taken a couple home to try to raise them and see what they turn into.
*Update: the plant has been identified as Hemizonia (tarweed). Thank you, Chris.
Also, you can see the lovely moth it turned into here.