The effects of different drying methods and sun exposure on the concentrations of deltamethrin in nets treated with...

The method used to dry bednets after they have been treated with an insecticide solution may affect the levels of insecticide and the uniformity of the insecticide deposits on the dry nets. In an attempt to see how the drying method may affect the insecticide deposits on the dry net, and to select the best drying method, laboratory and field studies have recently been carried out (in the U.K. and Iran, respectively). Conventional polyester nets were each treated with a deltamethrin solution (made with one K-O Tab tablet in 500 ml water) and then dried, either while hanging vertically or laid horizontally on the floor, in the sun or shade. The concentrations of deltamethrin in 25-cm2 samples cut from the dry nets (from the inner folds, surface folds, and top and bottom of each net dried vertically, and from the upper and lower surfaces of each net dried horizontally) were then determined using high-performance liquid chromatography (HPLC). Drying the treated nets in the sun or the shade did not make a significant difference to deltamethrin concentrations on the nets. Mean deltamethrin concentrations were, however, higher on the lower parts of the nets that had been hung to dry vertically than on the upper parts of these nets, and greater on the upper surfaces of nets dried horizontally than on the lower surfaces of such nets. In general, the layers and folds of the nets that had been on the outside of the drying nets contained more deltamethrin than the inner folds. These patterns are consistent with the hypothesis that deltamethrin tends to accumulate at the points where the water from the insecticide solution evaporates from the drying nets and also, in the case of nets dried vertically, at the lower points of the drying nets (as the result of gravity). In order to obtain an even and adequate distribution of insecticide, it is therefore not necessary to dry the net in the shade. To achieve a uniform deposit of deltamethrin, the drying net should be folded as little as possible and dried quickly.