Disease prevalence and transmission of Microsporidium phytoseiuli infecting the predatory mite, Phytoseiulus persimilis (Acari : Phytoseiidae)

Isolated colonies of the predatory mite, Phytoseiulus persimilis, were used to gain information regarding prevalence and transmission of Microsporidiu m phytoseiuli. Two colonies of P. persimilis were reared on spider mite (Te tranychus urticae)-infested bean plants in isolated cages. Disease prevalen ce of predators from Colony 1 remained relatively low (between 0 and 15%) o ver 57 weeks of observation whereas disease prevalence of predators from Co lony 2 increased over 3 months (from 12 to 100%). Disease prevalence among predators from Colony 1 had increased to 100% 2 months after weekly samplin g had ceased for this colony and periodic sampling confirmed that disease p revalence among individuals of both colonies remained at 100%. Microsporidi an spores were not detected in randomly chosen samples of T. urticae prey m ites that were removed and examined biweekly during this period. Although n umerous microsporidian spores were observed in smear preparations of fecal pellets examined by light microscopy, spores were not observed on leaf surf aces or predator feces when examined by SEM, The latter appeared as intact aggregates composed of numerous dumbbell-shaped crystals and it is unlikely that spores are liberated from intact fecal pellets onto leaf surfaces. Ve rtical transmission of M. phytoseiuli was 100%; horizontal transmission was low (14.3%) and occurred only when immature P. persimilis were permitted t o develop in contact with infected immature and adult predators. The mean n umber of eggs produced per mated pair was highest when uninfected females w ere mated with uninfected males (63.2 eggs per mated pair). Although mean e gg production decreased when one or both parents were infected, not all dif ferences were significant. Male predatory mites did not contribute to infec tion of their progeny. Results suggest that routine examination of P. persi milis for microsporidian spores is essential for the management of M. phyto seiuli within P. persimilis colonies. Low disease prevalence and lack of ob vious disease signs or symptoms, as in the case of M. phytoseiuli, increase the probability that these pathogens will escape notice unless individuals are routinely examined for pathogens. (C) 2001 Academic Press.