How to Control Armyworms. Armyworms feed on food crops and garden plants at night. When the food supply is gone, they move en masse to a new site. Hence the name armyworm. Armyworms can destroy an entire plant in just one evening, and there may be as many as three generations in one year.
Know what you're looking for. Armyworms are 1 1/2 inch caterpillars that are pale green when first hatched but then change to olive green with a white stripe later in the season. They turn into moths that are gray-brown with a white dot on the wing.
Look for armyworms on the undersides of leaves and on tender new growth. You will notice holes in the leaves of new growth, most often in the spring or early summer months.
Rake up fallen leaf debris to eliminate daytime hiding places for armyworms.
Encourage hungry birds to visit your garden to help control armyworms by setting out feeders, birdbaths or nesting material in the area where the worms are feeding.
Attract predatory wasps that will control armyworms by planting dill, fennel, coreopsis and brightly colored flowers near the armyworm-prone plants.
Use horticultural oil in July to kill the eggs of second-generation armyworms.
Spray Bacillus thuringiensis, an organic control for caterpillars, in the late afternoon or early evening hours when you see the first signs of armyworm damage in your garden. Bacillus thuringiensis (or Bt) is actually a bacteria and is safe to use around children and pets. There are also numerous chemical sprays available to control armyworms.
Spray during the dormant season (winter) with a dormant-season oil spray to head off recurring infestations - armyworm eggs may overwinter in fallen debris. This is a good preventative measure.