Effects of vector control on the population structure of tsetse ( Glossina fuscipes fuscipes ) in western Kenya

Abstract

Displacement rates of tsetse affect performance of targets during vector control. Fly size, one of the indicators of population structure usually obtained from wing measurement, is among the determinants of displacement rates. Although recovery of tsetse in previous intervention areas has been widely reported, the population structure of tsetse that recover is rarely evaluated despite being associated with displacements rates. Previously, intervention trials had reduced tsetse densities by over 90% from >3 flies/trap/day to <1fly/trap/day on Big Chamaunga and Manga islands of Lake Victoria in western Kenya. In this study, we assessed the recovery in densities of Glossina fuscipes fuscipes on the two islands and evaluated the effects vector control might have on the population structure. A before and after intervention study was undertaken on four islands of Lake Victoria in western Kenya; Small and Big Chamaunga, Manga and Rusinga Islands, two of which tsetse control intervention had previously been undertaken. Three years after intervention average G. f. fuscipes catches in biconical traps were estimated on each island. Wing centroid size (CS) (a measurement of fly size) and shape, indicators of the population structure of flies from the four islands were compared using geometric morphometric analyses. CS and shape of available female but not male tsetse wings obtained before the intervention trial on Big and Small Chamaunga islands were compared with those from the same islands after the intervention trial. G. f. fuscipes apparent density on the previous intervention islands were>9 flies/trap/day. Irrespective of sex, wing shape did not isolate tsetse based on their islands of origin. The fly size from Big and Small Chamaunga did not differ significantly before intervention trials (P = 0.728). However, three years after the intervention flies from Big Chamaunga were significantly smaller than those from Small Chamaunga (P < 0.003). Further, there was an increase in the divergence of wing morphology between flies collected from Big Chamaunga and those from Small Chamaunga after tsetse control. In conclusion, even though populations are not isolated, vector control could influence the population structure of tsetse by exerting size and wing morphology differential selection pressures. Therefore, we recommend further studies to understand the mechanism behind this as it may guide future vector control strategies.

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

“…Our results for G. pallidipes and G. m. morsitans thus provide no support for the idea that campaigns using (stationary) targets could cause any meaningful shift towards tsetse that are relatively small, immobile and unavailable to targets [11]. The ability to eradicate these species thus seems unimpeded by any size-bias in the availability of tsetse to these baits.…”

“…This implies that G. f. fuscipes need not travel so far as G. pallidipes in order to find a target, so reducing any selection against large and relatively mobile individuals. Various considerations suggest, anyway, that more data need to be produced before one could unequivocally attribute the size changes recorded in the recent study on G. f. fuscipes in Kenya to the use of targets in the control campaign [11]. First, the possibility could not be excluded that increases in fly numbers following control operations in the study area were due to invasion from neighbouring areas [11].…”

“…It should be noted that Mbewe et al [11] used, as a measure of fly size, the “centroid size” (CS) defined by Dujardin & Slice [29] as the square root of the sum of the squared distances to the centroid of each of a number of pre-defined landmarks on a wing. We have data for these landmarks for a subset of 40 wings collected in January 1994 and we compared the CS values obtained for these data with our measure of the wing lengths.…”

“…Vale et al [5] did not consider whether, within a given sex and species, host-orientated behaviour patterns might also be a function of fly size. Such an effect has, however, been suggested for Glossina fuscipes fuscipes Newstead in Kenya [11]. The core evidence was that, prior to a 19-month control campaign using stationary insecticide-treated targets on Big Chamaunga Island [12], the wing sizes of sampled females were 1% larger than they were four years later.…”

Sticky small target: an effective sampling tool for tsetse fly Glossina fuscipes fuscipes Newstead, 1910

“…On these islands, G. f. fuscipes is the only tsetse fly species found and mainly inhabits the area along the lake shore [ 4 , 9 , 19 , 20 ]. Study site selection was based on the fact that G. f. fuscipes populations on these islands were well documented and high numbers of flies are present for meaningful experimentation [ 4 , 21 ]. These islands are not inhabited by humans and vegetation on the lake shore consists of a mixture of fresh water mangroves ( Aeschynomene eraphyroxylon ), tropical hydrangea ( Dombeya spp . )…”