1. Resistance of some populations of the Simulium damnosum complex to temephos (100-fold at the LC50 level), with degrees of cross-resistance to chlorphoxim (14-fold) and other organophosphate insecticides, follows intensive larvicidal control of S. damnosum s.l. in West African river systems since 1975 by the WHO Onchocerciasis Control Programme. 2. Larvae of at least three sibling species of the S. damnosum complex have become organophosphate-resistant: these are the forest species S. sanctipauli Vajime & Dunbar and the savanna species S. sirbanum V. & D. and S. damnosum Theobald sensu stricto. 3. Organophosphate-resistant S. damnosum s.l. larvae show increased susceptibility to some organochlorine and pyrethroid insecticides, especially to permethrin (up to 11-fold) and OMS 3002 (up to 17-fold), as compared with organophosphate-susceptible populations. 4. This differential susceptibility is reflected by increased pyrethroid efficacy in operational use for river treatments against organophosphate-resistant field populations of S. damnosum s.l. larvae. Treatment of 100 km of the lower Bandama River in 1985 showed that permethrin at the highly selective dosage of 10 min exposure to 0.01 mg/l caused reversion towards organophosphate-susceptibility of the target population of S. sanctipauli. This effect was less pronounced when the Comoe River was treated at the lower dosage of 0.005 mg/l for 10 min. 5. To overcome temephos-resistance, it is proposed that the most rational usage of currently available larvicides would involve the following annual sequence of treatments: Bacillus thuringiensis serotype H-14 when river discharge is below 75 m3/s; chlorphoxim for about eight weekly treatment cycles after river discharge rises; permethrin (or alternative pyrethroid) for up to six treatment cycles--this should eliminate any incipient selection for chlorphoxim-resistance; resume chlorphoxim (or perhaps carbosulfan) treatments until river discharge falls below 75 m3/s permitting resumed use of B.t. H-14.

The effect of toxic compounds are variable among living organisms in different habitats. Toxicity tests are conducted to measure the effects of different pollutants on one or more species of organisms and in the form of LC 50s , assist in the development and application of water quality criteria for the protection of the aquatic environment. Active tilapia, Oreochromis niloticus, samples with size ranging between 4 and 6 cm standard length were exposed to different concentrations (between 0 and 30 µg/l) of Deltamethrin for 96 h in 50 l glass aquaria. This was to assess the dose-response of O. niloticus to different concentrations of the pesticide, and to estimate the LC 50 using Probit analysis. The very low LC 50 value of 15.47 µg/l obtained indicates that Deltamethrin is highly toxic to O. niloticus. This LC 50 value should serve as a guide for the application of Deltamethrin in agriculture. Further study into the processes and mechanism by which Deltamethrin adversely affects the physiology of fishes have been recommended.

River water conditions that might influence the efficacy of VectoBac, a formulation of the microbial insecticide Bacillus thuringiensis H-14 Berliner against Simulium damnosum sensu lato Theobald (Diptera: Simuliidae) larvae were investigated. A standard formulation was assayed 130 times over 15 months using a mini-gutter system at a field station beside the River Pra in Ghana. The lethal concentration (LC) values, river temperature, conductivity, turbidity and pH were analysed using univariate and multivariate statistics to identify which of these parameters influenced its performance. River temperature, conductivity and turbidity (in that order) were identified as having direct effects on the potency of VectoBac. Water temperature and conductivity were negatively correlated, whereas turbidity and pH were positively correlated with LC values. Analyses of river water samples revealed that despite observed differences in total solids, sodium and potassium cations and chloride concentrations, all the parameters measured did not differ significantly between wet and dry seasons. A simple method for rearing S. damnosum s.l. in the laboratory was then adopted to study the effect of conductivity on potency of VectoBac under controlled conditions. Increasing the conductivity of the water medium up to 3,000 microS enhanced potency by about 42%, whereas increasing that of the insecticide alone raised it by 37%. The results obtained suggest that for effective use of VectoBac for blackfly control in West Africa, river temperature, conductivity and turbidity should be taken into consideration, perhaps by only selecting rivers with optimal conditions for treatment. The laboratory-based system developed for assaying the product overcomes the vagaries associated with field conditions and also the demand for huge logistic requirements of the mini-gutter system, which has to be sited near rivers.

Dissection data from a Simulium damnosum s.l. population in northern Ghana, heavily parasitized by the mermithid Isomermis lairdi, are presented. A simple calculation gives reason to suppose that this parasitism may significantly reduce the transmission of onchocerciasis at certain seasons.

The Onchocerciasis Control Program of the World Health Organization is car,rying out an extensive screening program in a search foi new larvicides to be used for control of Simulium damrwsum s.l. Ernphaiii his been given to finding a pyrethroid an4 a carbamate to-supplement.the organophosphates currently in use. fhese chemicalg with differiqg ryodes of action, together wlth lrcilhts tti,triniicnsis H-14, are being used in an attempt to cope with the development and spread of resistance to the organophosphates temephos and chlorphoxim.