Selection for non-diapause 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 occidentalis, is an important pest in various greenhouse crops, such as sweet pepper and cucumber. Two species of predatory mite are commercially applied for biological control of this pest: Amblyseius cucumeris and A. barkeri. Thrips control is generally successful from March onwards. During winter, however, thrips control by these predatory mites is less effective. An important reason for this is that the commercially applied strains of both mite species enter reproductive diapause under 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 predators that do not enter diapause under light- and temperature conditions prevailing in winter. Additional experiments were done to estimate the potential of the selected lines as control agents of F. occidentalis. Selection for non-diapause proved highly successful in both predatory mite species. In a New Zealand strain of A, cucumeris diapause incidence decreased from 41% to 0% in about ten generations; in a Dutch strain of A. barkeri diapause incidence decreased from 67% to 0% in about six generations. Furthermore, selection for non-diapause had no influence on predator performance, measured as predation rate and oviposition rate on a diet of first instar thirps larvae. Rates of predation and oviposition were the same for selected and unselected lines in both species; rates of predation and oviposition were higher for A. cucumeris 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 these predatory mites. Finally, a small-scale greenhouse experiment in a sweet pepper crop showed that the selected non-diapause line of A. cucumeris established successfully under diapause-inducing short-day conditions.