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 resi stance to insects. The existence or development of aggressive biotypes can lead to a rapid break-down of host plant resistance. Therefore th e study of biotypic variation should be included in breeding programs for resistance to insects. In the present study we measured the reprod uction of randomly collected females of ten different populations of t he insect herbivore Frankliniella occidentalis (Pergande) (Thysanopter a: Thripidae) on one susceptible and two resistant cucumber (Cucumis s ativus L.) accessions. Significant differences between thrips populati ons were observed on all three cucumber accessions. None of the popula tions had a significantly higher reproduction than the Dutch reference population NL1. For three populations, the Dutch population NL1, a po pulation from New Zealand (NZ), and an Italian population (IT), partia l life history parameters, such as reproduction rate, developmental ti me and survival were determined and the relative rate of increase r(r) was calculated. On all three cucumber accessions the r(r)-value of po pulation NZ was lower than of populations NL1 and IT. It is concluded that there is biotypic variation in F. occidentalis with regard to per formance on cucumber plants with different levels of resistance. Repro duction is a good criterion for differentiating biotypes of F. occiden talis on cucumber.