TIMING OF LARVICIDE TREATMENTS FOR THE CONTROL OF PEST BLACK FLIES (DIPTERA, SIMULIIDAE) IN A SEMIARID ENVIRONMENT IN SOUTH-AFRICA

Seasonal changes in the abundance of larvae, the rate of larval develo pment, and the survival adults of the pest black fly Simulium chutteri Lewis were used to determine the timing of larvicide treatments in th e middle and lower reaches of the Orange River, South Africa. Larval a bundance was highest in late winter (July/August), low in late summer (January/February), and dropped sharply during blue-green algal blooms in autumn (March/April). The time taken for the first cohort of larva e to pupate, following larvicide treatment, ranged from seven days in midsummer (water temperature 25-29 degrees C) to 37 days in midwinter (10-16 degrees C). Adult abundance was consistently high in spring (Se ptember and October) and sometimes in autumn (April and May). Abundanc e dropped sharply in summer when evaporation exceeded 12 mm/day. The e stimated number of generations per year was 11-13, although adequate c ontrol during normal (non-flood) years was obtained with six ''spring' ' and three ''autumn'' larvicide applications. It is recommended that applications to prevent a spring outbreak should start towards the end of July, when most of the population is in the larval stage, and cont inue into late October. Applications to prevent an autumn outbreak, wh en necessary, should start in mid-March and continue to the end of Apr il. Although water temperatures provided reasonably accurate informati on for the correct timing of treatments, single-site applications unde restimated the appropriate timing of subsequent large-scale control tr eatments. Treatment intervals should be based on water temperature acc ording to the function y=exp(0.065t-4.13), where y=larval development rate (in days(-1)), and t=water temperature (in degrees C).