Posted: 07/08/2013 at 09:48
Andrew, it's always a good idea to start afresh with new mix in containers. Apart from avoiding any disease problems, a season of growing depletes the goodness in the mix. Every time you water, nutrients are leached out, including any you've added via fertilising. And caterpillar larvae can and will overwinter in the mix.
In short, your FIL is making life hard for himself!
It might or might not be Tomato Horn Worm. They're very distinctive:
Unless you want to pick them off by hand and send them to meet their maker, or if you have them in plague proportions, the best treatment for caterpillars is a product with Bacillus thuringiensis as its active agent. It's organic, a bacteria derived from soil and is harmless to everything but caterpillars. It's also systemic rather than a contact treatment meaning you don't have to hit the pest with the stuff. It remains active and effective. Until it rains. It's marketed under various trade names including DiPel.
Oh, and his mix is all right providing the manure isn't fresh.