Field trials reveal the complexities of deploying and evaluating the impacts of yeast-baited ovitraps on Aedes mosquito densities in Trinidad, West Indies

Abstract

The use of lure-and-kill, large-volume ovitraps to control Aedes aegypti and Aedes albopictus populations has shown promise across multiple designs that target gravid females (adulticidal) or larvae post-oviposition (larvicidal). Here we report on a pilot trial to deploy 10 L yeast-baited ovitraps at select sites in Curepe, Trinidad, West Indies during July to December, 2019. Oviposition rates among ovitraps placed in three Treatment sites were compared to a limited number of traps placed in three Control areas (no Aedes management performed), and three Vector areas (subjected to standard Ministry of Health, Insect Vector Control efforts). Our goal was to gain baseline information on efforts to saturate the Treatment sites with ovitraps within 20-25 m of each other and compare oviposition rates at these sites with background oviposition rates in Control and Vector Areas. Although yeast-baited ovitraps were highly attractive to gravid Aedes females, a primary limitation encountered within the Treatment sites was the inability to gain access to residential compounds for trap placement, primarily due to residents being absent during the day. This severely limited our intent to saturate these areas with ovitraps, indicating that future studies must include plans to account for these inaccessible zones during trap placement.

Figure 1

Map of study area in Curepe, Trinidad, West Indies from July to December 2019. Block areas identifying sites for Control, Vector, and Treatment yeast-baited 10 L ovitrap placement are highlighted. Figure was prepared with Environmental Systems Research Institute (ESRI). "Topographic basemap.” "World Topographic Map." (Sept. 28, 2021). https://www.arcgis.com/home/item.html?id=10df2279f9684e4a9f6a7f08febac2a9. (May 21, 2019).

Figure 2

Individual yeast-baited 10 L ovitrap locations and median egg counts/trap from July to December 2019. (A) Control blocks; (B) Vector blocks; (C) Treatment blocks.

Figure 3

Summary of weekly egg collections of yeast-baited 10 L ovitraps positioned across Control, Vector, and Treatment blocks. (A) Mean egg counts; (B) Maximum egg counts; (C) A. aegypti vs. A. albopictus composition from eggs reared from select weekly samples.

Figure 4

The inferred number of egg laying events in the index ovitraps in the Treatment blocks. The inferred number of egg-laying events over time for each index ovitrap in the Treatment blocks are plotted as lines. The index ovitraps were measured for 3 weeks before additional ovitraps were placed in the Treatment blocks (therefore increasing the number of removed eggs in those blocks). The red vertical line indicated the time at which new ovitraps were placed. The red and blue horizontal lines represent the loess curves for the index ovitraps in the Treatment block and the ovitraps in the combined control group (Control and Vector blocks), respectively.

Figure 5

Relative positions of the ovitraps in space with illustration of local density measure. The relative physical locations of ovitraps are shown as dots with colors indicating the values of the local density measure under the four parameter levels used in the analysis. Heterogeneity in the spatial distribution of traps allows us to examine the effect of varying the local density of traps on the number of observed mosquito eggs.

Figure 6

Number of inferred egg laying events and the local density of ovitraps in the previous week. This plot shows the number of inferred egg-laying events by the local density of ovitraps in the previous week for all ovitraps. The rows correspond to the Normal and Uniform local density models and the columns give the distance parameter for those models. The x-axis can be loosely interpreted as the number of other ovitraps that are within about the specified number of meters of the given ovitrap. The blue line shows the predicted mean of the linear model of the data.

Figure 7

Mean Number of inferred egg laying events and the mean local density of ovitraps in the previous week over the course of the study. This plot shows the mean number of inferred egg laying events by the mean local density of ovitraps in the previous week for all ovitraps. The rows correspond to the Normal and Uniform local density models and the columns give the distance parameter for those models. The x-axis can be loosely interpreted as the number of other ovitraps that are within about the specified number of meters of the given ovitrap. The blue line shows the predicted mean of the linear model of the data.