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Microsporidan parasites of Simulium ornatum Mg. in South England

(1) Simulium ornatum larvae were regularly collected from the River Lea (Hertfordshire) and from Tilling Bourne (Surrey) during 1967–9. Preliminary surveys revealed no parasitism in pupal and adult stages of S. ornatum but several parasites, some common, occurred in the Simulium larvae. These were studied by dissection, smears and histologically. (2) Microsporida were well represented by two genera - Thelohania and Plistophora. Infection rates never exceeded 9% of larvae. Spore measurements statistically evaluated from different larval infections justified recognition of five species of Thelohania (T. minuta sp.nov., T. bertrami sp.nov., T. canningi sp.nov., T. simulii sp.nov., and T. avacuolata sp.nov.), and two species of Plistophora (P. tillingbournei sp.nov. and P. leasei sp.nov.). (3) Intra-epithelial organisms of the larval gut wall, referred to provisionally as Microsporidan-like bodies, caused light infections. (4) With the exception of the slight Microsporidan-like infections, all species of Microsporida were found parasitizing the fat body of the larva, causing its disintegration. Delay in larval growth and pupation occurred, followed by death. (5) Seasonal cycles of abundance of infection were related to summer and overwintering populations of Simulium larvae. (6) Transmission of infection was attempted by exposing apparently uninfected wild-caught Simulium larvae to spores of Thelohania but no evidence of active infection being established in the exposed larvae was obtained. (7) Although the pathogenic Microsporida taken in this study must have exerted control on natural larval densities, too little is known of their biology, mode of transmission or methods of culture to envisage their development, for biological control of simuliids, at present.