TARGET BARRIERS FOR TSETSE-FLIES (GLOSSINA SPP) (DIPTERA, GLOSSINIDAE) - QUICK ESTIMATES OF OPTIMAL TARGET DENSITIES AND BARRIER WIDTHS

The probability that tsetse flies (Glossina spp.) cross a barrier of o dour-baited targets is calculated for barriers of different widths and target density, and for tsetse flies with varying natural rates of su rvival, daily step lengths (d) and probabilities of being killed by an odour-baited target. If the barrier is only as wide as d, and for a s pecies which has a 2% natural daily mortality and a further 2% mortali ty due to each target per unit area, tsetse flies have probability (P) of ca. 0.1 of penetrating the barrier even if the target density is 6 4 per unit area. To ensure that P < 0.001 the barrier must be about 4d wide for target densities 32 per unit area; doubling the width to 8d means that target densities could be cut by about 75%, and total numbe rs of targets in the barrier by 50%. These biological considerations a nd the economic costs of different target barriers suggest that, for a ll tsetse fly species, a safe and relatively inexpensive barrier is ac hieved with barrier width 8d when the optimum target density is roughl y the same as for normal operational areas. This has the important pra ctical consequence that there is no need to treat barriers as a specia l case. Practical results from research and control operations in Zimb abwe are in accord with the theoretical findings, but further work is required to ascertain whether the safety margin, and hence costs, can be reduced.