Distribution of oviposition sites and characteristics of egg development in the 'Blandford fly' Simulium posticatum (Diptera: Simuliidae).
The distribution of oviposition sites of Simulium posticatum on the banks of the River Stour, UK, was studied in relation to a local medical problem caused by the bites of the females. The characteristics of the river bank were noted. Sites with a vertical bank profile and loamy soil in the shade were favoured by the females. Aspect and water velocity were not related to oviposition site. Four weeks of exposure to low temperature was required to break the diapause of the eggs; the minimum total time taken to hatching was 8 weeks. High temperatures, after the diapause had been broken, inhibited hatching (4% hatch after 9 days at 14°C, compared with 48% at 5°C). Larvae were seen to hatch on damp soil but hatching success was greater among eggs immersed in water. Mortality of 1st-instar larvae was low at 5°C (6% after 7 days, compared with >70% at 9°C and 14°C). This may be an adaptation to the time required to find a suitable feeding site at the ambient temperature of the river. Eggs were not resistant to desiccation even though they normally remained in soil from early summer to the following February. At relative humidities of ≤87%, more than 70% of eggs were distorted after 3 days, although the proportion of eggs distorted by this treatment was much greater (>97%). It was concluded that high relative humidity and the presence of capillary water in soil produced favourable conditions for egg development and survival.