WHO cone tests of three species of filth fly against the mechanical insecticide diatomaceous earth. Adults of the house fly, Musca domestica, the secondary screwworm, Cochliomyia macellaria, and the grey flesh fly, Sarcophaga bullata, under low and high humidity were exposed to the mechanical insecticide, and mortality was determined.

DE, Celite 610, was provided by the manufacturer, Imerys (Imerys Filtration Minerals, Inc., Roswell, GA, USA) and was stored in its original packaging at room temperature and humidity until needed. A modified World Health Organization (WHO) cone test (World Health Organization, 2013) was utilized to determine fly mortality. A glass funnel was placed, large opening down, into the bottom of a horizontally positioned glass Petri dish treated with DE (no DE for the control). The funnel had a diameter of 10 cm for the large opening and 1.6 cm for the small opening with a height of 12 cm. Flies inside of the cone (described later) were prevented from flying into the narrow neck of the funnel by an orange stopper held in place with a plastic cap fitted over the outside of the small neck of the funnel. The Petri dish inside bottom was 13.8 cm in diameter and was treated with 5 g of DE/m² (similar to the VR treatment level used as a home, residual wall spray in Africa to control mosquitoes for six months; Deguenon et al., 2020). One milliliter of the DE at 74.8 mg/mL in absolute ethanol (Fisher Scientific International, Inc., Hampton, NH, USA) was added to the dish, and the dish tilted back and forth to cover the bottom of the dish in toto. The control was treated with 1mL of ethanol only. All dishes were placed flat on the bench top in a fume hood overnight to allow for complete evaporation of the ethanol. Flies were anesthetized with carbon dioxide gas at room temperature and pressure for 30 s, and 10 flies transferred with soft forceps to each Petri dish bottom after which they were covered by the capped funnel as previously described. Funnels and dishes were placed into a Percival I-36NL incubator (Percival Scientific, Inc, Perry, Iowa, USA) at either 30˚ C and 50% relative humidity or 30˚ C and 70% relative humidity, both with a 16:8 L:D cycle. The number of dead flies were counted every half hour. Flies were counted as dead when they exhibited no movement after light tapping of the funnel with a pencil. Three replicates of three cones per replicate with corresponding controls were performed each day and repeated on two additional days.