The role of plant physiology and cultivar of chrysanthemum in the resistance against Western flower thrips

Western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), is an economically important pest insect in vegetable and ornamental cultivation worldwide. Little is known about host plant resistance in the vegetative and generative phases of chrysanthemum [Chrysanthemum x morifolium (Ramat.) Hemsl. (Asteraceae)] and the role of resistance factors such as flavonols, flower color, and flower shape. We screened a broad range of chrysanthemum cultivars across two seasons to quantify resistance against Western flower thrips. Resistance was based on silver damage on the leaves, and relative flavonol content was measured using a Dualex Scientific 4 hand-held sensor. There was significant variation in silver damage between cultivars, indicating different levels of resistance. There was no correlation between the relative flavonol content in middle leaves and plant silver damage. A clearer resistance level discrimination by flavonols in resistant and susceptible cultivars would be possible in the future by comparing multiple leaf positions during the ontogenetic phases of plant development. Moreover, the influence of flower color and shape on resistance to thrips was investigated by counting the adult thrips and larvae on flowers. The results showed significant differences in flower color preference by adults, but not by larvae. Flower shape influenced thrips larvae and females, but not males. The importance of the results for resistance determination is discussed.