Variation in performance of western flower thrips populations on susceptible and partially resistant cucumber

Biotypic variation is of major concern in breeding for host plant resistance to insects. The existence or development of aggressive biotypes can lead to a rapid break-down of host plant resistance. Therefore the study of biotypic variation should be included in breeding programs for resistance to insects. In the present study we measured the reproduction of randomly collected females of ten different populations of the insect herbivore Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) on one susceptible and two resistant cucumber (Cucumis sativus L.) accessions. Significant differences between thrips populations were observed on all three cucumber accessions. None of the populations had a significantly higher reproduction than the Dutch reference population NL1. For three populations, the Dutch population NL1, a population from New Zealand (NZ), and an Italian population (IT), partial life history parameters, such as reproduction rate, developmental time and survival were determined and the relative rate of increase r(r) was calculated. On all three cucumber accessions the r(r)-value of population NZ was lower than of populations NL1 and IT. It is concluded that there is biotypic variation in F. occidentalis with regard to performance on cucumber plants with different levels of resistance. Reproduction is a good criterion for differentiating biotypes of F. occidentalis on cucumber.