The problem with aerial application is that it's a small industry, and there isn't a traditional route to the cockpit. The short answer when asking most applicators is that they'd like to see you come to them with ag experience, often experience in type, which often as not will be 502, 602, or 802 experience.
There are very few operators flying piston equipment any more; nearly everything is turbine, including many of the ag cat positions that remain. Getting insured in the equipment usually requires a minimum of a thousand hours of ag experience. Attendance at an ag school doesn't take that requirement away. Those who have less than a thousand hours of ag will have difficulty getting insured. It's more than hull insurance, too. One of the single biggest concerns is drift when doing ag; if one has no aerial application experience, one may stand a chance with an operator that does pesticide work, but will have a very difficult time getting hired to fly for an operation that does herbicide application, and most operators do both.
There's not a lot of turn-over in aerial application. Many of us that do it have done it for a long time. I know a number of operators that may hire once every ten or fifteen years, if that. It's not like making a career on the way to the airlines.
If it is something that you're targeting right out of flight school, then at that stage all of your experience and training should be geared toward tailwheel flying and low altitude experience. It's not usually something that you'll get into by simply attending an ag school. The days are mostly gone when one could come aboard mixing and loading and flagging (went away with GPS), and eventually work into a flying position in a small piston airplane like a Cub, or Pawnee, or Ag Truck.
A number of operators today do night work, and that's definitely not something that you're going to get into as a new pilot. Nor should you try. Night work is common in desert areas, as well as areas with highly susceptible bee and beneficial populations. It's also common with certain types of crops.
Some crops are easier to get into than others. Rice is straight forward but a lot of work, with a great deal of flying, but the seats that open up are almost non-existent; those who have them (and many are year-round) don't give them up. Corn and wheat work with pesticide are the most likely to open up to an inexperienced pilot, most of the time in the bottom-rung aircraft; those positions don't pay as well, and will often be the last seats filled and the first emptied during the season.
Many operators prefer to see a pilot with maintenance experience; holding a mechanic certificate with both airframe and powerplant certification is desirable. If you're going to get into ag, holding the A&P should be a goal, and something you should plan; most ag operators and pilots that I know are A&P at a minimum, many have their inspection authorization (IA) as well. That takes time and experience.
As far as a path to the cockpit; there's no traditional route or background, and there's not a wide career path of those who came before. It's a small industry.