Effectiveness of Traps in Tsetse Surveys in the Liberian Rain Forest

K. R. S. Morris The Liberian Institute of the American Foundation for Tropical Medicine, Harbel, Liberia, West Africa

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Under the conditions of tropical rain forest and secondary forest in Liberia it was found that surveys of the tsetse fly Glossina palpalis, the vector of Gambian sleeping sickness, could be more efficiently carried out by traps than by the conventional method of hand catching with fly-collectors.

Unbaited traps, 2 feet high and 2 feet long, covered with natural burlap, were adopted because they gave a uniform rate of sampling. Traps twice as large caught more tsetse but were difficult to standardize. Folding traps, more easily carried than the rigid standard type, were equally effective in a short trial.

In direct comparisons, four traps caught as well as two collectors in low densities of G. palpalis, with a marked superiority during the dry season, when their catches gave a truer index of the numbers of tsetse present. In high densities of this species, traps gave a better performance than men, since their rate of catching was unaffected by the numbers of tsetse to be caught. It is economical to employ traps, but not men, for detecting flies at very low densities.

Traps can be used to measure the factor “man-fly contact.” Comparability requires a standard procedure in visiting traps. The catches obtained with daily visits are nearly doubled with visits twice a day, and fall to 70% with visits on alternate days and to below 40% when visited twice a week. These differences are due both to the escape of flies between visits and to the attraction of tsetse to the traps by hosts each time a trap is visited.

These traps could be used also for surveys of tabanids and other insects.