SEASONAL-VARIATION IN RESISTANCE OF CHRYSANTHEMUM CULTIVARS TO FRANKLINIELLA-OCCIDENTALIS (THYSANOPTERA, THRIPIDAE)

Seasonal variation in the level of host plant resistance can have impo rtant consequences for the repeatability of tests to measure host plan t resistance to insect pests. In the present study, the levels of resi stance to Frankliniella occidentalis of a susceptible and a partially resistant cultivar of chrysanthemum, Dendranthema grandiflora grandifl ora, were determined throughout the year. Thrips damage, reproduction and adult female survival were determined on excised leaves in Petri d ishes under uniform conditions. Strong seasonal fluctuations were obse rved in these three characteristics. On leaves from plants grown in wi nter, damage, reproduction, and survival were higher than on leaves fr om plants grown in summer. Clear differences in resistance were observ ed between the susceptible and the partially resistant cultivar on lea ves from plants grown in winter, while differences disappeared in summ er. Damage on both cultivars and survival on the susceptible cultivar were negatively correlated with mean daily solar radiation during plan t growth, suggesting that the level of resistance depends on light int ensity during plant growth. This was confirmed in an experiment carrie d out in summertime with shaded and unshaded plants. Leaves from contr ol plants, grown under high light intensity had a higher level of resi stance than leaves from shaded plants grown under reduced light intens ity. There were clear differences in resistance between the cultivars under shaded conditions (low light intensity), but not under unshaded control conditions (high light intensity).