Abstract
BackgroundThe entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana have demonstrated effectiveness against anopheline larvae in the laboratory. However, utilising these fungi for the control of anopheline larvae under field conditions, relies on development of effective means of application as well as reducing their sensitivity to UV radiation, high temperatures and the inevitable contact with water. This study was conducted to develop formulations that facilitate the application of Metarhizium anisopliae and Beauveria bassiana spores for the control of anopheline larvae, and also improve their persistence under field conditions.MethodsLaboratory bioassays were conducted to test the ability of aqueous (0.1% Tween 80), dry (organic and inorganic) and oil (mineral and synthetic) formulations to facilitate the spread of fungal spores over the water surface and improve the efficacy of formulated spores against anopheline larvae as well as improve spore survival after application. Field bioassays were then carried out to test the efficacy of the most promising formulation under field conditions in western Kenya.ResultsWhen formulated in a synthetic oil (ShellSol T), fungal spores of both Metarhizium anisopliae and Beauveria bassiana were easy to mix and apply to the water surface. This formulation was more effective against anopheline larvae than 0.1% Tween 80, dry powders or mineral oil formulations. ShellSol T also improved the persistence of fungal spores after application to the water. Under field conditions in Kenya, the percentage pupation of An. gambiae was significantly reduced by 39 - 50% by the ShellSol T-formulated Metarhizium anisopliae and Beauveria bassiana spores as compared to the effects of the application of unformulated spores.ConclusionsShellSol T is an effective carrier for fungal spores when targeting anopheline larvae under both laboratory and field conditions. Entomopathogenic fungi formulated with a suitable carrier are a promising tool for control of larval populations of malaria mosquitoes. Additional studies are required to identify the best delivery method (where, when and how) to make use of the entomopathogenic potential of these fungi against anopheline larvae.
Highlights
The entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana have demonstrated effectiveness against anopheline larvae in the laboratory
Our previous work showed the efficacy of Metarhizium anisopliae (ICIPE-30) and Beauveria bassiana (IMI- 391510) spores in infecting and killing larvae of Anopheles stephensi and An. gambiae under laboratory conditions [12]
The quantity of white pepper and WaterSavr (30 mg and 130 mg respectively) required for covering the water surface (441 cm2) was enough to form a consistent mixture with 10 mg of fungal spores (Table 1)
Summary
The entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana have demonstrated effectiveness against anopheline larvae in the laboratory. Utilising these fungi for the control of anopheline larvae under field conditions, relies on development of effective means of application as well as reducing their sensitivity to UV radiation, high temperatures and the inevitable contact with water. The main infection sites were the feeding and respiratory apparatus [16] Most of these studies had been carried out in the laboratory and proved the application of dry fungal spores to be more effective than the application of formulated spores [13,14,18]. Dry unformulated fungal spores are more exposed to UV radiation and high temperatures, which are known to negatively affect spore persistence and germination rate [22,23]
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