Pillbugs (Armadillidium vulgare), sometimes called sow bugs or roly-polies, aren't really insects. Instead, they are officially classified as terrestrial crustaceans related to shrimp, lobsters and crabs. Because they are not technically insects, most traditional and organic pesticides won't kill them. Fortunately, these pests do little damage, and you can usually control them by following the proper cultural practices.
These oval, blue-gray to black pests breathe with gill-like structures and require high moisture levels to survive. The approximately 1/2-inch-long, wingless bodies bear a series of hardened, overlapping segments that look similar to the exterior armor of armadillos. This segmentation allows them to roll into tight balls whenever disturbed. These slow-moving creatures are usually nocturnal, typically hiding from the heat of the day to prevent water loss.
Pillbugs overwinter as adults and emerge to mate in the spring. Adult females lay between 25 and 200 eggs, which they carry beneath their bodies within brood pouches. The eggs hatch in about eight weeks, but the young bugs remain in the pouch for another six or seven weeks before leaving their mothers. Pillbugs can live for up to three years.
Pillbugs have weak jaws and prefer feeding on decaying or dead plant material. You'll often spot these scavengers hiding in mulched flowerbeds and gardens as well as within piles of grass clippings, compost and leaf litter. Pillbugs don't bite, and are completely harmless to people and other animals. They are typically just minor nuisance pests in gardens and yards, but populations occasionally become large enough during excessively wet weather that the insects start feeding on young, tender garden and landscape plants.
Pillbugs primarily damage plants by chewing jagged holes between the major leaf veins. They sometimes injure young seedlings by girdling the stems or cutting the roots, but the damage isn't usually serious enough to warrant the use of insecticides.
Pillbug feeding activity helps decompose organic matter in gardens, so many homeowners leave the insects alone. However, if you wish to reduce pillbug populations, simply make your landscape inhospitable by reducing humidity levels and removing shady hiding spaces.
Watering plants with a garden hose rather than a sprinkler helps reduce air moisture levels, as does watering early enough in the day to allow the soil to dry by nighttime. Instead of using a thick layer of heavy mulch, place a 2- to 3-inch layer of light mulching material in your garden and flower beds. The University of California Integrated Pest Management Program suggests using black plastic mulch because it makes the area too hot for pillbug habitation. Eliminating unnecessary piles of bedding mulch, grass clippings and fallen leaves also reduces the number of hiding places.
It's difficult to control pillbugs with insecticides because of their crustacean bodies, nocturnal feeding habits and the fact they hide in secluded areas. The University of California IPM Info website recommends using the dust form of a carbaryl-based insecticide to treat severe pillbug infestations.
Following the directions on the product's label, lightly scatter the pesticide until a thin, even layer of dust covers the infested plant material. Repeat applications every seven days until you obtain control. Water heavily mulched areas immediately after treatment to allow the chemicals to penetrate down to the pillbugs' hiding places. Don't allow children or pets to enter the treated area until the dust has settled.