Nematodes for carrot fly

I see that, as well as the usual nematodes for slugs, Nemasys do one called 'grow your own' which covers a multitude of pests including carrot fly and caterpillars. Surely they will have nothing to feed on until the carrot fly comes along, lays it's eggs and they hatch ?  As for caterpillars, I presume you have to wait till they are crawling all over your prized greens then water the nemasys over them? Does anyone have any experience with this product?