Three species of caterpillars commonly attack cabbage at Ottawa, namely, the imported cabbageworm, Pieris rapae (L.), the larva of the diamondback moth, Plutella maculipennis (Curt.) and the cabbage looper, Trichoplusia ni (Hbn.). All three species are readily controlled with DDT sprays or dusts but the timing of the applications is imporrant. Growers sometimes begin dusting or spraying too early, wasting one or more applications, or they delay the first application until mid-August, when severe damage has already occurred. This paper is a report on four seasons' experiments at Ottawa, 1947 to 1950, to determine the number and timing of applications of DDT required for control of caterpillars on late cabbage.