Field and laboratory studies on water conditions affecting the potency of VectoBac^® (Bacillus thuringiensis serotype H‐14) against larvae of the blackfly, Simulium damnosum

Abstract

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.

The Effect of Seston on Mortality of Simulium vittatum (Diptera: Simuliidae) From Insecticidal Proteins Produced by Bacillus thuringiensis subsp. israelensis

“…The turbidities of the various media in this experiment ranged from 0.3 to 7.7 NTU, illustrating again that a measurement of turbidity alone is insufÞcient for predicting the outcome of a treatment. This also tends to negate the possibility of kaolinite clay-binding midgut receptors or the proteins thereby reducing efÞcacy as suggested by Wilson et al (2005).…”

“…Guillet et al (1985) suggested that when the larval gut is full of particles, there may be reduced ICP ingestion. Wilson et al (2005) found that favorable performance of a commercial product in West Africa was achieved with river turbidities below 4.2 Nephelometric Turbidity Units (NTUs). They speculated that either the Þne mud particles were blocking the mid-gut receptors from the ICPs or that the ICPs were being adsorbed onto the surface of the suspended particles.…”

Assessments of the transmission of Onchocerca volvulus by Simulium sanctipauli in the Upper Denkyira District, Ghana, and the intermittent disappearance of the vector

“…ceased below a river channel into which large amounts of blasting dust settled (Walsh 1984). The concentrations of silt in 2014 were about 15 times higher than those measured by Wilson et al (2005) in samples collected during 1997-1998 in the lower Pra River at Daboase (5°4°N, 1°39°W) (27.3 mg/l in the dry season and 24.4 mg/l in the rainy season, with turbidities ranging from 4.2 to 72.0 NTU). Their mean turbidity of 23.9 NTU was similar to the turbidity of 13.6 measured in the Okumayemfuo stream in 2014.…”

Susceptibility of Simuliidae larvae (Diptera) to a new solid formulation based onBacillus thuringiensisvar.israelensis(Bti), and bioecological aspects of the breeding sites in Cascavel - Paraná, Brazil

“…Regarding water parameters, in an experiment conducted by Wilson et al (2005) the best mortality results of a Bti-based product against S. damnosum Theobald, 1903 larvae were observed as the temperature increased. According to the authors, temperature influences metabolism and, consequently, greater ingestion of toxins by larvae.…”