You want to wait till that first frost kills the warbles.
That's an old wives tale but if it makes you feel better then more power to you. I have very seldom found a warble/wolves on a squirrel. I have found them much more often on rabbits.
Around here, just about any squirrel you kill in hot weather is apt to have at least one. I generally use the "first frost" rule of thumb myself. I know you can eat them regardless but those squirmy, squishy little bastards are just too disgusting to deal with...
