Populations of the blackmargined pecan aphid,Monellia caryella(Fitch), were studied in the field, using sleeve cages to manipulate aphid populations and their natural enemies. Each cage enclosed 10 compound pecan leaves.M. caryellapopulations were observed to increase rapidly within closed cages from 1 aphid per leaf to 50 aphids per leaf at any time throughout the summer. Opening sleeve cages to allow natural enemies access to the increasing aphid populations always resulted in the decline of aphid numbers compared to their activity in adjacent closed cages. Release of 1 chrysopid or 1 coccinellid larvae per 10 leaves when aphid populations were increasing in closed cages always resulted in the prevention of an aphid outbreak. Laboratory feeding studies of selected chrysopid and coccinellid predators showed average feeding rates of between 25 and 60 aphids per day. Results indicate that natural enemies, particularly predators, play an important role in maintainingM. caryellapopulations at low levels in the field.