SELECTION FOR NONDIAPAUSE IN AMBLYSEIUS-CUCUMERIS AND AMBLYSEIUS-BARKERI AND EXPLORATION OF THE EFFECTIVENESS OF SELECTED STRAINS FOR THRIPS CONTROL

In Europe and North America the western flower thrips, Frankliniella o ccidentalis, is an important pest in various greenhouse crops, such as sweet pepper and cucumber. Two species of predatory mite are commerci ally applied for biological control of this pest: Amblyseius cucumeris and A. barkeri. Thrips control is generally successful from March onw ards. During winter, however, thrips control by these predatory mites is less effective. An important reason for this is that the commercial ly applied strains of both mite species enter reproductive diapause un der short-day photoperiods, whereas the western flower thrips does not enter diapause. In this paper we report on selection experiments for non-diapause in strains of both mite species, aimed at obtaining preda tors that do not enter diapause under light- and temperature condition s prevailing in winter. Additional experiments were done to estimate t he potential of the selected lines as control agents of F. occidentali s. Selection for non-diapause proved highly successful in both predato ry mite species. In a New Zealand strain of A, cucumeris diapause inci dence decreased from 41% to 0% in about ten generations; in a Dutch st rain of A. barkeri diapause incidence decreased from 67% to 0% in abou t six generations. Furthermore, selection for non-diapause had no infl uence on predator performance, measured as predation rate and oviposit ion rate on a diet of first instar thirps larvae. Rates of predation a nd oviposition were the same for selected and unselected lines in both species; rates of predation and oviposition were higher for A. cucume ris than for A. barkeri. After 18 months under non-diapause conditions , no less than 92% of a sample of the selected non-diapause line of A. cucumeris did not enter diapause when tested under diapause-inducing conditions. This indicates that 'non-diapause' is a stable trait in th ese predatory mites. Finally, a small-scale greenhouse experiment in a sweet pepper crop showed that the selected non-diapause line of A. cu cumeris established successfully under diapause-inducing short-day con ditions.