| In strategies of environmetally sound and sustainable crop protection, biological control of herbivores plays an indispensable role. The predatory persimilis has been proven to be very effective in controlling spider mites ( Tetranychus urticae) in several field and greenhouse crops. Commercial mass production and sale of this predaotry mite has been undertaken for more than 25 years, the largest producer being situated in the Netherlands (van Lenteren 1995). However, reports of quality loss of commercially reared P. persimilis have been published during recent years ( Steiner 1993, Schutte et al. 1995). In both cases an unknown disease was suspected to induce the quality loss. There is no doubt that massproduction and frequent transport of arthropods may contribute to the development of new, unknown diseases (Sikorowski & Lawrence 1994). However, until now research in invertebrate pathology relevant to agriculture has concentrated on plant-eating species. Studies on pathogens of commercially used natural enemies are rather scarce. Since 1994 extensive research has been carried out examining factors responsible for profound quality reduction in several predatory mite populations at the Department of Entomology of the Agricultural University of Wageningen. "Low quality mites" are small, do not produce offspringm, die early, eat fewer spider mites, are less effective in finding prey, donot stay at places where prey is available and carry excretory crystals in their legs (Schutte et. al. 1995, Schutte in prep.). Classical microscopic studies of several researchers in mite pathology did not lead to conclusive results concerning the identity of the pathogen until now. However, we showed that the quality loss is a contagious phenomenon. By now the main transmission route of the unknown pathogen has been identified and an effective bioassay to monitor infectivity has been developed. With this crucial bioassay we may now: 1) identify the pathogen, 2) develop a feasible test for pathogen presence in predatory mites based on molecualar techniques, 3) develop efficientstrategies for preventing and/or curing the disease in mass production, 4) test the susceptibility of other commercially used natural enemies for the pathogen. |
