Motile zoospores, the only infective stage of Pythium species, attached, encysted and infected 100% second instar larvae of Aedes aegypti, Aedes africanus, Aedes simpsoni, and Anopheles gambiae. Motile zoospores variably attached and infected Culex spp. and Eretmopodites but did not attack field collected Toxorhynchites sp. We observed mycelial development within the exposed larvae after exposure indicating that mortality was due to the fungus. The fungus infected 75 ± 10% (SD) second instar larvae of Aedes aegypti, under aseptically reared conditions. It infected 65 ± 12.8% (SD) second instar larvae of Culex quinquefasciatus in aseptic conditions in the laboratory. All field collected larvae were heavily infested with vorticella but after treatment with 5% sodium hypochlorite, control mortalities ranged from 0 to 8.6% (mean = 4.3%).

The fungus was most infective between 20–27.5°C; at 30°C many zoospores were attached to the cuticle but did not infect. Mortality rates were as low as 51.5% at 30°C, with no infection at all at 35°C. pH range from 6–8 was optimum for infection, but pH above 8 inhibited infection. Infection level of 100% was obtained in aseptically reared larvae at higher densities of 1600 larvae of second instar larvae of Aedes aegypti and 97% infection for 1600 second instar larvae of C. quinquefasciatus when exposed to fungal infection. Phosphate, borate and acetate buffers inhibited zoospore production.