Water-based ultra-low volume (WULV) applications to cotton at fourday intervals of a 1% a.i. polyurea-based microencapsulated formulation of the pheromone inhibitor 9-dodecenyl acetate (E/Z 80:20) in a 0·2-ha field cage in Malawi resulted in an average reduction of 60·2% in nightly mating of females of Diparopsis castanea Hmps. A relatively constant level of inhibitor was maintained by spraying with 30 g/ha initially and decreasing by 10% with each successive spray. Examination of the mated status of 1712 female moths sampled on 88 occasions revealed that, in the presence of inhibitor, mating increased linearly with increasing population density from 22 to 2844 moths/ha (equal numbers of males and females). In an openfield trial, WULV applications of 30 and 60 g of 1 % a.i. microencapsulated inhibitor/ha to isolated cotton plots (0·4–1·2 ha) at seven- and 14-day intervals had no measurable disruptive effect on released populations of moths as measured by oviposition and larval infestation of the crop, although there was some reduction in egg fertility and catches of males in pheromone traps in the treated plots. Timeseries analyses by gasliquid chromatography (GLC) of the residual inhibitor on filterpaper discs and cotton leaves in the field gave similar results, and GLC measurement of inhibitor on filter papers removed at intervals from the test plots showed that inhibitor loss was very rapid, typically 60% within two days and almost 100% after six days. Loss of inhibitor was less rapid in the field cage and under laboratory conditions. Physical and chemical evaluation of microcapsule deposition on cotton plants revealed that spray droplets penetrated to all parts of the plant but that deposition was greatest on the upper laminae of the top leaves of the plant. Methodology for assessing the success of mating-disruption experiments is described and discussed, and the failure of the open-field experiments is attributed mainly to the rapid loss of inhibitor from the formulation in the field.