Population dynamics of early larval instars, late instars and pupae of Anopheles spp . in open field trials exposed to water-dispersible granules (WDG) of Bti . Arrows indicate the date of treatment. Trial period from October 8th to November 5th 2012. White horizontal bars indicate no significant difference between treatment and control tubs, blue bars do (dark blue: α = 0,05; light blue α = 0,10).