A novel bacterial disease of the predatory mite Phytoseiulus persimilis: disease syndrome, disease transmission and pathogen isolation

The predatory mite Phytoseiulus persimilis Athias-Henriot (Acari, Phytoseiidae) is a specialist predator of spider mites. Since more than three decades P. persimilis has been successfully applied worldwide in biological control of the two-spotted spider mite Tetranychus urticae Koch (Acari, Tetranychidae) in several greenhouse and field crops. The importance of P. persimilis and other predatory mites in integrated pest control has stimulated research, particularly on predator-prey interactions and foraging behaviour.During the past two decades studies by different research groups have consistently demonstrated that adult female predatory mites are attracted to volatiles emanating from Lima bean plants infested with their prey T. urticae . These so-called herbivore-induced plant volatiles (=HIPV) are produced by the plant after herbivore attack. However, in mid-1992 a sudden and permanent change in behavioural response to HIPV was recorded in our laboratory: adult female P. persimilis of our laboratorypopulation,subsequently designated non-responding (=NR-) population , showed a lower degree of attraction to HIPV than adult females of other P. persimilis populations designated responding (=R-) populations.Moreover, adult female predators of the NR-population show a characteristic set of symptoms, subsequently designated non-responding (=NR-) syndrome . Predators shrink aftermating,cease oviposition immediately after shrinkage and die several days later. Other characteristics of the syndrome are a tendency to leave a prey patch with ample food, moving fast between places, ceasing predation altogether, having a low excretion rate and carrying excretory crystals in the legs. The NR-syndrome was induced in non-symptomatic adult female predators after exposure to symptomatic predators and faeces and debris released by such predators. Moreover, it was shown that bacteria present in faeces and debris play a role in this process. Several bacterial species were isolated from symptomatic adult female predators, their faeces and debris. For one of these isolates the Koch's postulates were satisfied, which constitutes the final proof that the isolate in question is the causative agent of the novel disease. The isolate represents a new bacterial species in a new bacterial genus, described as Acaricomes phytoseiuli . This is the first record of a bacterial pathogen in predatory mites. The results presented in this thesis are of great importance to applied and fundamental science,as P. persimilis is a cornerstone in biological control and plays a major role in research on predator-prey interactions worldwide.