Field bioassays. (a) Forty An. gambiae larvae were placed in plastic containers (with nylon screened holes, indicated by an arrow) with a soil layer (2 cm) at the bottom and a 3 cm layer of water. The screened holes were a precautionary measure to retain larvae in the tubs in case of overflow due to heavy rain. The plastic containers were placed in larger tubs, also filled with soil and water, to prevent ants from access to the bioassays. (b) Unformulated (dry) Metarhizium anisopliae (10 mg) spores applied on the water surface. Note the two large clumps just outside the centre of the containers. (c) Shellsol T-formulated Metarhizium anisopliae (10 mg) spores applied on the water surface. Note that spores are spread more evenly over the surface by ShellSol T than dry spores (Figure b).