Environmental influences on feeding damage caused by western flower thrips (Thysanoptera: Thripidae) to chrysanthemum

The effects of test-location, daylength, thrips density, and choice or no-choice test on feeding damage by Frankliniella occidentalis (Pergande) were studied in chrysanthemum, Dendranthema grandiflora Tzvelev. Under all circumstances, tested cultivars differed significantly in the level of both silver and growth damage caused by thrips feeding on old and young chrysanthemum leaves, respectively. Thrips resistance to both types of feeding damage, measured under the different test circumstances was highly correlated. This means that thrips resistance in chrysanthemum measured in our standardized laboratory experiment gives a good prediction for chrysanthemum breeders who want to select cultivars for thrips resistance under commercial greenhouse conditions. Only test location had an effect on the level of growth damage. In the greenhouse, all chrysanthemum cultivars were more susceptible to growth damage than in the laboratory. The level of silver damage, however, remained constant. Silver damage was only influenced by daylength. All cultivars showed higher levels of silver damage under short daylength compared with long daylength. A density of 10 thrips per plant was found to be high because only 1.6 times more silver damage and 1.1 times more growth damage was found at a 4 times higher thrips density. At the highest density, many thrips died, which resulted in about the same thrips numbers for both treatments at the end of the experiment. Thrips tended to choose among cultivars. However, this did not result in a higher variance in resistance in the free-choice experiment compared with the no-choice experiment.