Environmental impact of tsetse eradication in Senegal

Abstract

The sterile insect technique is an environment friendly control tactic and is very species specific. It is not a stand-alone technique and has been used mostly in combination with other control tactics within an area-wide integrated pest management strategy. For a period of eight years, the direct impact of a campaign to eradicate a population of the tsetse fly Glossina palpalis gambiensis in Senegal was monitored using a set of fruit-feeding insect species (Cetoniinae and Nymphalidae) that served as ecological indicators of the health of the ecosystem. Here we show that the eradication campaign had very limited impacts on the apparent densities of the most frequent species as well as three diversity indexes during the reduction phase involving insecticides but reverted to pre-intervention levels as soon as the release of the sterile male insects started. These results greatly expand our understanding of the impact of vector eradication campaigns on non-target species.

Figure 1

Illustration of traps and some ecological indicators used for the environmental monitoring in Senegal: banana trap for fruit feeding butterflies (a), banana trap for fruit feeding beetles (b), larval stages of Charaxes sp. (c) that feed on trees and of Cetoniinae (d) that are detritivorous and live in plant litter. Adult Charaxes epijasius feeding on a mango (e), Charaxes achemenes resting on a trunk (f), Chondrorrhina abbreviata feeding on a plant wound together with a bee (g) and Pachnoda marginata on the ground (h).

Figure 2

Location of the five sampling locations. The zone with the grids (each cell is 5*5 km) corresponds to the tsetse eradication area. Blocks were tackled sequentially in two phases, i.e. a suppression phase using insecticides and an eradication phase with the sterile insect technique (see text for details). The monitoring of ecological indicators thus followed a stepped wedge approach as shown in the table.

Figure 3

Temporal dynamics of the apparent densities of the most frequent ecological indicators in the eradication area of the Niayes in Senegal (Kayar = Block 1, Pout = Block 2, Dakar and Thiès = block 3, Mbour = control area outside the tsetse-infested zone). The vertical red, blue and green lines correspond to the start of the suppression phase, the start of the eradication phase and the start of the post-eradication monitoring phase, respectively (block 1 only). Cetoniinae species: Pachnoda interrupta (Pint), Diplognatha gagates (Digag) and Pachnoda marginata (Pm); Nymphalidae species: Charaxes varanes (Cvar), Charaxes epijasius (Cepi) and Melanitis leda (Mleda). Blue line is a smooth carried out by Locally Estimated Scatterplot Smoothing Loess method which is a non-parametric regression method adapted point distribution.

Figure 4

First plan of the triadic analyses of the apparent densities of the most abundant ecological indicators in the study area. The trajectories present the temporal dynamics during eight years in the four sampled locations (Kayar in block 1, Pout in block 2, Thiès and Dakar Hann in block 3 and Mbour centre IRD as a negative control site). The apparent densities of the 3 most abundant species were analyzed and Nymphalidae (left panel) and for Cetoniinae (right panel). Each location is presented with a different color and the letters (M, monitoring, S, suppression and E, eradication) present the three successive phases in each location.

Figure 5

Evolution of three diversity indexes in each location from 2010 to 2017. Green cells present Monitoring periods, grey cells Suppression periods and yellow cells Eradication periods.