In 1989, the aphid Aphis helianthi severely damaged five of 10 scouted celery fields in west central Michigan. In 1990 and in 1991, A. helianthi again was a problem. reaching pestiferous levels in three of 10 and two of 10 scouted fields. respectively. This insect has not been reported previously as a pest of celery or any other commercial crop. Insecticide efficacy studies showed that some of the most commonly used insecticides are ineffective against A. helianthi. Resistance or tolerance to insecticides may explain its new status as a pest