Onions, Allium cepa L., are one of the most important vegetables crops in New York state, and the most important insect pest of onions is onion thrips, Thrips tabaci Lindeman, which feeds directly on leaves causing blotches, premature senescence of leaves and distorted and undersized bulbs. Onion thrips (OT) feeding can cause substantial yield losses, even greater than 50%. Additionally, a virus disease, Iris yellow spot virus (IYSV) transmitted by OT, was confirmed in the summer of 2006 in New York state. This virus poses a serious threat to onion production in New York, as previous reports have indicated it can cause yield losses up to 60%. OT are found mainly in the narrow spaces between the leaves, which makes it more difficult for control with conventional insecticides. For these reasons and the importance of onion production in New York, studies on onion host plant resistance to OT and to IYSV were performed. Several onion varieties were screened for resistance to OT and IYSV. Visual damage ratings for thrips feeding and for IYSV symptoms presence were used. The number of thrips larvae was counted weekly in each variety. In another experiment, yield losses caused by OT and/or IYSV interaction were studied; visual ratings were obtained as well as counting thrips larvae. There were two treatments, protected with insecticide and the other not protected. When IYSV was observed in the field, plant samples were taken to the lab to confirm the presence of the virus.