An incomplete ring of electrified nets was placed round a trap or round an electrified net and visual target in woodland infested with Glossina morsitans morsitans Westw. and G. pallidipes Aust. in Rhodesia. The distribution of catches in these systems was used to estimate the minimum efficiencies with which the trap or net and target captured flies that were initially attracted by odours derived from a herd of cattle hidden below ground. With mature tsetse, the estimates for the trap were 23 and 21% for male and female G. morsitans, respectively, and 70 and 49% for male and female G. pallidipes. For the net and target, the corresponding figures were 29, 30, 53 and 43. The coefficient of variation of such estimates (about 10%) was low enough to suggest that the technique affords a quick and reliable means of screening the efficiency of many trap designs. Factors responsible for the efficiency of the trap were elucidated. Data for other insects were obtained.
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