Linking oviposition site choice to offspring fitness in Aedes aegypti: consequences for targeted larval control of dengue vectors

Abstract

Background: Current Aedes aegypti larval control methods are often insufficient for preventing dengue epidemics. To improve control efficiency and cost-effectiveness, some advocate eliminating or treating only highly productive containers. The population-level outcome of this strategy, however, will depend on details of Ae. aegypti oviposition behavior.

Methodology/principal findings: We simultaneously monitored female oviposition and juvenile development in 80 experimental containers located across 20 houses in Iquitos, Peru, to test the hypothesis that Ae. aegypti oviposit preferentially in sites with the greatest potential for maximizing offspring fitness. Females consistently laid more eggs in large vs. small containers (β = 9.18, p<0.001), and in unmanaged vs. manually filled containers (β = 5.33, p<0.001). Using microsatellites to track the development of immature Ae. aegypti, we found a negative correlation between oviposition preference and pupation probability (β = -3.37, p<0.001). Body size of emerging adults was also negatively associated with the preferred oviposition site characteristics of large size (females: β = -0.19, p<0.001; males: β = -0.11, p = 0.002) and non-management (females: β = -0.17, p<0.001; males: β = -0.11, p<0.001). Inside a semi-field enclosure, we simulated a container elimination campaign targeting the most productive oviposition sites. Compared to the two post-intervention trials, egg batches were more clumped during the first pre-intervention trial (β = -0.17, P<0.001), but not the second (β = 0.01, p = 0.900). Overall, when preferred containers were unavailable, the probability that any given container received eggs increased (β = 1.36, p<0.001).

Conclusions/significance: Ae. aegypti oviposition site choice can contribute to population regulation by limiting the production and size of adults. Targeted larval control strategies may unintentionally lead to dispersion of eggs among suitable, but previously unoccupied or under-utilized containers. We recommend integrating targeted larval control measures with other strategies that leverage selective oviposition behavior, such as luring ovipositing females to gravid traps or egg sinks.

Figure 1. Experimental oviposition containers.

Four blue containers made of rigid plastic were set out per house. Two containers were large trash cans (40 cm diameter×70 cm height) and two containers were small buckets (21 cm diameter×23 cm height). Container size was crossed with fill method (manually filled vs. unmanaged) to create four different treatments.

Figure 2. Flowchart of preference-performance field experimental design.

Four container treatments were set out in each of 20 houses in Iquitos, Peru (80 containers total). Larvae originating from field-collected eggs (for the purpose of monitoring oviposition) were re-introduced into the same container to simulate colonization. Twenty-five F₁ larvae from laboratory families were introduced into each container on day 8 to compare juvenile developmental success.

Figure 3. Diagram of oviposition sites within semi-field enclosure.

Trials 1 and 3 were pre-intervention trials during which females were presented with two large unmanaged containers (grey circles) and six small manually filled containers (white circles). Trials 2 and 4 were post-intervention trials during which females had access to eight small manually filled containers. Containers 1–4 were located outside in the yard and containers 5–8 were inside the house. The windows were left open to allow free movement of mosquitoes indoors and outdoors. Containers are not drawn to scale with the house.

Figure 4. Oviposition by container type.

Mean (± SE) number of eggs laid by Ae. aegypti in four container treatments in Iquitos, Peru (80 containers located in 20 houses). Daily egg counts for each container were summed over each week and divided by container circumference.

Figure 5. Standardized pupation probability by container type.

Mean (± SE) proportion of larvae from laboratory families that pupated during the experiment. Proportions were calculated by genotyping all collected pupae to identify those originating from laboratory families (25 F₁ first instars introduced into each container). Data came from eight houses during the first trial.

Figure 6. Adult size by container type.

Mean (± SE) wing length of females developing in four container treatments.

Figure 7. Concentration of eggs within a single container.

Within the enclosure, the frequency distribution for the maximum proportion of each egg batch concentrated in any single container is shown for: A) trial 1 (pre-intervention), B) trial 2 (post-intervention), C) trial 3 (pre-intervention), and D) trial 4 (post-intervention). Preferred (large unmanaged) containers were available during pre-intervention trials (denoted by grey bars) but not during post-intervention trials (denoted by black bars).

Figure 8. Egg dispersion by females inside enclosure.

Mean values (± SE) of the Shannon equitability index for each semi-field trial. This index takes into account both the number of eggs laid and their relative distribution among containers. This index reaches the maximum value (1) when eggs are evenly distributed among all containers and the minimum value (0) when eggs are concentrated within a single container.