DOWNSTREAM CARRY OF LARVICIDES USED IN THE CONTROL OF PEST BLACK FLIES (DIPTERA, SIMULIIDAE) IN THE ORANGE RIVER, SOUTH-AFRICA

The downstream carry of larvicides used in the control of the pest bla ck fly Simulium chutteri(Diptera: Simuliidae) was assessed during 55 f ield trials in the middle reaches of the Orange River, South Africa. L arvicides were applied in the vicinity of Upington by means of a boat, a helicopter, and from bridges. Larvicidal carry was determined by as sessing the abundance of black fly larvae on 10 to 30 substrates (ston es or trailing vegetation) at various sites before and after each appl ication. Downstream carry was highly variable, but the main factors wh ich affected carry were flow, the presence of pools and vegetation, al gal blooms, high concentrations of suspended solids, dosage, and formu lation. At median flows (100 m(3)/s) the bacterium Bacillus thuringien sis var. israelensis (B.t.i.) was usually effective (>80% larval morta lity) for 6-9 km, and up to 20 km, downstream of the point of applicat ion. By contrast, the organophosphate temephos was Usually effective f or 15-50 km. Carry was further following multiple-site (control) appli cations compared to single-site (experimental) applications. We conclu de that considerable savings can be made by spacing treatment interval s as far apart as possible.