Kids call them roly-polies or pill bugs -- those little gray bugs that curl into balls at the least sign of danger. Surprisingly, these timid creatures can be a powerful natural defense against crop-eating stink bugs. Lester Ehler, a UC Davis entomology professor, recently found that the lumbering pill bug, previously thought to feed only on decaying plants, was devouring more than 90 percent of the stink-bug eggs in an on-farm study. The shield-shaped brown or green stink bugs, named for the odor they emit when threatened, feed on a variety of crops, including tomatoes, peppers, beans and pears. In tomatoes, stink-bug damage appears as yellow spots that turn white and corky as the fruit ripens. In California, growers typically use harsh chemical pesticides to control stink bugs that are a problem in Central Valley crops. "Pill bugs are slow and can't chase down prey, so they need a stationary source of food," says Ehler, an authority on biological control of insect pests. He discovered that pill bugs spend the day on the ground, but at night climb up into plants to feed on stink-bug eggs. On a farm along Interstate 5 near Sacramento, Ehler found that stink bugs spend the winter in leaf litter, under tree bark and in streamside blackberry brambles. In spring they come out to feed on wild radish and mustard plants bordering the highway, then move into the tomato fields in early summer. To control stink bugs without pesticides, Ehler suggests farmers and highway officials replace wild radish and mustard plants with native grasses, in addition to encouraging pill bugs. Home gardeners can support pill-bug populations by providing them with plenty of compost, supplemented with sliced potatoes, carrots and even pelletized rabbit food, he suggests.