Variation in Onion Thrips Number Per Plant Among Onion Plant Introduction Accessions

Onion thrips are a detrimental onion insect pest that reduces bulb size and yield and is difficult to control. They are attracted to onion leaves that are blue to bluish green in color and have a thick waxy coating. Some research suggests that onion thrips are less attracted to leaves that are light green in color and possess little or no wax on their surface. In the Western U.S., Iris yellow spot virus is a devastating foliar onion disease that is spread by onion thrips. Onion germplasm, that is less attractive to thrips feeding, would aid in the development of disease tolerant cultivars. Seventy-five onion plant introduction accessions from the U.S. germplasm collection were selected because they possessed a low amount of leaf bloom (degree of wax deposition) in a previous evaluation. Accessions were evaluated for the number of thrips per plant when thrips number was counted from ten plants per plot at four separate times throughout the growing season. No thrips control means were used during the growing season. In general, thrips number per plant increased from 12 to 14 weeks post transplanting while the number decreased afterwards up to 20 weeks. At this time, there were fewer thrips per plant than at 12 weeks post transplanting. At 14 weeks, PI 248753, PI 248754, PI 274780, and PI 288272 averaged less than three thrips per plant that less than most entries tested and less than the average number of thrips per plant for all entries, 21. These accessions produced dark green leaves that had a moderate amount of waxy coating. They may have potential use in developing onion cultivars that are less preferred by thrips and possibly more disease tolerant.