Wing length and host location in tsetse (Glossina spp.): implications for control using stationary baits

Background: It has been suggested that attempts to eradicate populations of tsetse (Glossina spp.) using stationary

targets might fail because smaller, less mobile individuals are unlikely to be killed by the targets. If true, tsetse

caught in stationary traps should be larger than those from mobile baits, which require less mobility on the part

of the flies.

Results: Sampling tsetse in the Zambezi Valley of Zimbabwe, we found that the number of tsetse caught from

stationary traps, as a percent of total numbers from traps plus a mobile vehicle, was ~5% for male G. morsitans

morsitans (mean wing length 5.830 mm; 95% CI: 5.800–5.859 mm) and ~10% for females (6.334 mm; 95% CI: 6.329–

6.338 mm); for G. pallidipes the figures were ~50% for males (6.830 mm; 95% CI: 6.821–6.838 mm) and ~75% for

females (7.303 mm, 95% CI: 7.302–7.305 mm). As expected, flies of the smaller species (and the smaller sex) were

less likely to be captured using stationary, rather than mobile sampling devices. For flies of a given sex and species

the situation was more complex. Multivariable analysis showed that, for females of both species, wing lengths

changed with ovarian age and the month, year and method of capture. For G. pallidipes, there were statistically

significant interactions between ovarian age and capture month, year and method. For G. m. morsitans, there was

only a significant interaction between ovarian age and capture month. The effect of capture method was, however,

small in absolute terms: for G. pallidipes and G. m. morsitans flies caught on the mobile vehicle had wings only 0.24

and 0.48% shorter, respectively, than flies caught in stationary traps. In summary, wing length in field samples of

tsetse varies with ovarian age, capture month and year and, weakly, with capture method. Suggestions that a

target-based operation against G. f. fuscipes in Kenya caused a shift towards a smaller, less mobile population of

tsetse, unavailable to the targets, failed to account for factors other than capture method.

Conclusions: The results are consistent with the successful use of targets to eradicate populations of tsetse in

Zimbabwe. Until further, more nuanced, studies are conducted, it is premature to conclude that targets alone could

not, similarly, be used to eradicate G. f. fuscipes populations in Kenya.