Armyworms hatch out from eggs laid by Armyworm moths. The worms like to eat most of the plants in your garden but they don't like evergreen (conifer) trees. The only way they can get around is by creeping. They don't like walking over wood ash or diatom dust, or you could try making collars of tin cans or squares of tarpaper with a cross cut in the centre to protect, for example, prize tomato plants. There may not be so many next year, but it helps to clean up and recycle all the dead plants from your garden in the fall.