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

Abstract

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.

Keywords: Age season annual effects; Eradication using targets; Stationary and mobile baits; Tsetse Glossina; Wing length.

Fig. 1

Photograph of a tsetse wing. The right wing of a female G. m. morsitans showing the endpoints a and b used as a measure of wing length in present studies. The hatchet cell length used in classical studies is measured between points c and d

Fig. 2

Tsetse wing lengths and centroid sizes. Centroid sizes plotted against wing lengths for 40 female G. pallidipes female flies sampled in January 1992. Fit by linear regression (Student’s t-test, n = 40, df = 38, P < 0.001)

Fig. 3

Catches of tsetse from trap and VET. Male and female G. m. morsitans and G. pallidipes caught using stationary traps and the VET. For each month, stationary-trap catches are expressed as a percentage of the total captured by the stationary trap and the VET for that particular sex-species combination. Both devices were run in the same area at Rekomitjie during approximately the last two hours of daylight on 59 days between January and December 1992

Fig. 4

Wing lengths and numbers of female G. pallidipes captured using a stationary trap or VET. a Mean wing length for female G. pallidipes caught in the trap or VET at Rekomitjie, during approximately the last two hours of daylight on 59 days between January and December 1992, when both sampling systems were used on the same day in the same area. Catches were pooled by month. Means and confidence intervals are calculated for the pooled data and for the upper and lower halves of the distributions. b Monthly estimates for the numbers of female G. pallidipes caught in odour-baited traps as a proportion of the total captured using a stationary trap and the VET

Fig. 5

Wing length as a function of month and method of capture, for the following years: a 1989; b 1990; c 1991; d 1992; e 1993; f 1994. Monthly means of wing lengths for female G. pallidipes captured using odour-baited traps or a vehicle-mounted electric net (VET). Data for each capture method pooled over all sampling sites at Rekomitjie

Fig. 6

Tsetse wing lengths as a function of age and month of capture: a January – April; b May – July; c August – December. Mean wing lengths of female G. pallidipes as a function of ovarian age and month of capture. Flies caught in odour-baited traps at Rekomitjie from January 1989 to December 1993

Fig. 7

Mean wing lengths for flies in different ovarian age categories (OC) caught using odour-baited traps or the VET: a OC = 0 and 4; b OC = 1 and 5; c OC = 2 and 6; d OC = 3 and 7. Mean wing lengths for flies in different ovarian age categories (C) caught using odour-baited traps or the VET. Observed values (O) are the monthly means for all flies captured using the two sampling systems. Predicted values (P) are calculated using the results in Additional file 1: Table S1. Rekomitjie; January 1989 to December 1993