EFFECT OF THRIPS-TABACI (THYSANOPTERA, THRIPIDAE) OIL YELLOW ONION YIELDS AND ECONOMIC THRESHOLDS FOR ITS MANAGEMENT

The proper management of onion thrips, Thrips tabaci Lindeman, on yell ow onions requires the determination of their effect on yields and the establishment of an economic threshold. In field plot experiments, we observed no difference in onion thrips population densities among 6 y ellow onion cultivars during 1988 and 1989. Onion thrips infestations resulted in onion yield losses of 43 and 34.5% with maximum seasonal a verages of 149.2 and 172.0 thrips per plant in untreated plots in 1988 and 1989, respectively. No effect on onion yields was observed in 199 0 when seasonal thrips densities did not exceed 35.2 thrips per plant. The effect of thrips on onion yields was best described by nonlinear models and by the use of the cumulative number of thrips-days per plan t as a measure of thrips pressure. Plants were relatively insensitive to thrips feeding early and late in phenological development. Importan t differences in the onion plant response to thrips damage were observ ed between 1988 and 1989. These differences were likely linked with th e duration and intensity of the water deficit recorded during both sea sons. Economic thresholds of 2.2 and 0.9 thrips per leaf were estimate d during the summers of 1988 and 1989, respectively. These thresholds can be used for onions sold at harvest or stored because costs associa ted with storage are compensated for by a higher return price. The use of the 0.9 thrips per leaf economic threshold in 1990 would have resu lted in unnecessary treatments, making this threshold suitable for sev ere drought situations only.