1.1 Sterile pressure

The sterile pressure is a calculated variable that estimates roughly the number of sterile individuals alive in the population at a given date in a logarithmic scale, based on a hypothesised daily survival probability. It takes into account the number of sterile individuals released prior to the target date, and the time elapsed since each release. Let \(\pi\) be the daily survival probability, the number of sterile individuals alive at time \(t\) is: \[\begin{equation} z_\pi(t) = \sum_{i:\; t(i) < t} R_i \exp(t \, \log\pi) \end{equation}\] where \(R_i\) are the released number of sterile individuals in release \(i\), for which the release times \(t(i)\) are prior to the target time.

The sterile pressure \(P(t) = \log(z_\pi(t))\) is then calculated at each observation time. We have used \(\pi = 0.9\), which yields the temporal estimates of sterile population and pressure shown in Figure 1.2. Other values have been tested, without improved association.

Figure 1.2: Sterile population and corresponding pressure over time estimated as the log-number of sterile individuals in the population, assuming a daily survival probability of 0.9.

2.1 Adult surveys

Figure 2.1 shows the number of adult Anopheles arabiensis captured in each trap, at each collection day, by sex and sector. Most often, the traps were empty, since they were actually resting sites and don’t accumulate captures. Only the 3% of non-zero values are represented in the figure.

Figure 2.1: Number of individuals catched in adult traps, by survey date, sex, trap and sector, in a logarithmic scale. Only non-zero catches are represented, as the vast majority of surveys are 0. Sterile males were released in sector 3, highlighted in orange, at the intervention times represented with light vertical lines.

Still, most of the raw outcomes are clustered on the lower-end of the scale, with only a few extreme observations that are actually visible. Thus, we need to summarise these data from various angles in order to describe the patterns properly.

Consider the capture rates (CR), measured as the average number of adult Anopheles captured per trap in a single survey. Figure 2.2 shows the capture rate across traps and sexes by sector and date.

There seems to be a initial drop in the average catches in the first months of the study (late 2014) at the sit sector, reaching almost zero by 2015 and staying at a low level with some sporadic peaks in 2016 and 2017.

However, the individual variability is very large, with a vast majority of zeroes and a few larger values for some specific surveys. This produces very noisy averages with large standard deviations when large values occur.

Furthermore, the peaks and drops are not obviously associated with the release times.

Figure 2.2: Capture rate by survey date and sector, across sexes and traps. The sit sector is highlighted in orange with a band of ±1 SD. Vertical lines represent release times.

Figure 2.3 summarises further the data, aggregating the observations by year and displaying capture rates by year and sector.

Here again, the initial decline in the capture rate at the sit sector is apparent. Yet, the variations are still very high, and consistent with natural variation in the control groups.

Figure 2.3: Capture rate by year and sector, across sexes, traps and surveys. The sit sector is highlighted in orange with a band of ±1 SD. Vertical lines represent release times. Numbers at the right-hand side label the control sectors with highest capture rates.

Figure 2.4: Proportion of zeroes in catches by sector (points) and sex, across traps and time. Proportions in the sit sector are highlighted in orange.

Figure 2.5: Empirical distribution of the number non-zero catches by sector and sex, across traps and time, in a logarithmic scale. Distributions from the sit sector are highlighted in orange.

From figures 2.4 and 2.5, there does not seem to be evidence of any decrease in the abundance of the wild population of Anopheles arabiensis in the sit sector, beyond the natural variation across sectors.

However, the impact of the releases of sterile males could be limited in time, and thus not noticeable when outcomes are averaged across time.

We explore this question next, by looking at results as a function of time since the last release.

Figure 2.6: Proportion of zeroes in catches by sector, and time since last release across traps and sexes. Proportions in the sit sector are highlighted in orange.

There might be a trend, but the pattern is also consistent with random variation.

Figure 2.7: Distribution of non-zero catches by role and time since last release, across traps, control sectors and sexes. Distributions from the sit sector are highlighted in orange.