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.
Temephos resistance in savanna cytospecies of Simulium damnosum Theobald s.l. and the forest cytospecies S. sanctipauli Vajime & Dunbar, from West Africa is correlated with an increase in general esterase activity. Metabolism studies indicated that esterase products were the major metabolites in both forest and savanna resistant strains of S. damnosum s.l. compared to the susceptibles. In the presence of an esterase synergist, a large amount of the oxon analogue of temephos was produced by the resistant forest cytospecies S. sanctipauli. This strain is also resistant to chlorphoxim, and it is likely that the increase in oxidative activity observed is connected with the chlorphoxim rather than the temephos resistance. There was no evidence of glutathione transferase-, oxidase-or acetylcholinesterase-based temephos resistance mechanisms in the savanna species of 5. damnosum s.l.
Chlorphoxim resistance is found in forest strains of Simulium sanctipauliVajime & Dunbar, in some areas of the Onchocerciasis Control Programme in West Africa. There is an increase in esterase activity in both adults and larvae in the resistant compared to the susceptible strain with the general esterase substrate 1-naphthyl acetate. This appears to be associated with the temephos and chlorphoxim resistance which are found in the same resistant strain. It is possible that one or more esterases are qualitatively and/or quantitatively changed in the resistant strain compared to the susceptible. Preliminary studies indicated that esteratic hydrolysis and oxidative cleavage were both involved in chlorphoxim metabolism in S. damnosum Theobald sensu latu. There was evidence of an increase in detoxication by both these routes in the resistant compared to the susceptible strain of the cytospecies S. sanctipauli. The increase in oxidative detoxication seen in the resistant strain was associated with a slight, but non-significant increase in cytochrome P450 as measured by the sodium dithionite reduced carbon monoxide difference spectra.
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.
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