Ovarian ages were obtained for tsetse flies Glossina morsitans morsitans Westwood and G. pallidipes Austen caught in Zimbabwe using a stationary bait ox, an odour-baited trap in the presence or absence of humans, a mobile ox fly-round and a vehicle-mounted electric net. The biggest daily catches were obtained from the electric net and the unacccompanied trap, but while for G. m. morsitans the ratio of the catches from these two methods was 7:1, for G. pallidipes it was 0.7:1. Evening catches were 1.3–5.5 and 2.9–18.8 times higher than morning catches for the two species, respectively; the differences were greater for stationary than for mobile baits. The sample age structures differed significantly (P < 0.05: x2) between methods in six of the ten possible contrasts between pairs of methods in G. m. morsitans and for all ten in G. pallidipes. The differences in age distribution and mean catch levels are attributed to increases in flight activity with age; at each age G. pallidipes is more active than G. m. morsitans. Man's presence in the trapping system reduced catches significantly (P < 0.05: t test) but this effect was small in the youngest and oldest flies. Odour-baited traps are biased in favour of G. pallidipes and, for both species, in favour of older flies; mortalities calculated from age distributions of trap samples therefore underestimate the true value.
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