Awesome photos. Although the 'season' in Iowa stretches a fair amount longer in either direction, I imagine you'd be hard pressed to go a day without seeing a crop duster popping up on the horizon from mid July till mid August. I've travelled there for summer work for the last 10 years or so and typically base out of a muni airport straight north of you a little (Rockwell city).
Most of the western states you list will be very similar in season for typical field crop, but several will have other forms of crop dusting (timber, rice, produce, etc....) that will have seasons offset by as much as six months. For example, while my home area (SW AZ) has at least some spraying to be done virtually year round, the produce (read; salad veggies) is really a fall through spring deal, with Oct-Dec, and then again Feb-Mar being the busiest months. With produce, there are logistics that make the work hours just as backwards as the season. Something most visitors find kind of interesting.
Between migrant workers, and snowbirds the population in our area more than doubles in the winter months, and we even have snowbirders from IA as neighbors
Halfast recently stopped by to visit and hung around until I had to go to work. He shot these with his phone, and I thought they were pretty cool.


You didn't mention what part of CA, but that is one of the most diverse states spraying wise, with the SE portion of the state being very similar to our area.
Take care, Rob