Anopheles gambiae s.l. exhibits overnight biting activities in Wonji Sugar Estate, Eastern Oromia Ethiopia: a challenge to the current intervention tools
Authors: Lelisa Kidane , Golassa Lemu , Woldehawariat Yitbarek , Dugassa Sisay 
Published: January 2024

Background
Although the behavioral flexibility of Anopheles mosquitoes poses challenge to the indoor-based vector control strategies in Ethiopia, this was not well studied in irrigated areas for sugar cane plantations. Hence, the local Anopheles species composition, biting behaviors, feeding site preference, monthly density, and weather variability were evaluated in Wonji Sugar Estate.
Methods
Adult Anopheles mosquitoes were sampled using Centers for Disease Control and Prevention light traps (CDC LT), Pyrethrum spray catches (PSC), handheld mouth aspirators and artificial pit shelters. Mosquitoes were identified to species using morphological keys. ANOVA was used to compare mean monthly mosquito densities. Correlation was used to test the relationship between hourly density of Anopheles and human activities. Effect of weather variability was tested against Anopheles density.
Results
A total of 3,504 Anopheles comprising: Anopheles gambiae s.l., An. pharoensis, and An. coustani complex were collected during the study periods. Anopheles gambiae s.l. was the dominant species (75.26%, n = 2,637). Higher number of Anopheles mosquitoes were collected using CDC LT (59.80%, n = 2,098) than those collected using PSC, Pit shelter, and Handheld mouth aspirators (mean = 1.83, CI = 1.68–1.97, P = 0.000). Anopheles gambiae s.l. exhibits overnight biting pattern with peak biting hours of 7:00 to 10:00 PM (Mean = 0.20 ± 0.02, CI = 0.16–0.24, p = 0.000) and 3:00 to 05:00 AM (Mean = 0.13 ± 0.02, CI = 0.09–0.16, p = 0.000) that has a positive correlation with occupants being on activities (r = 0.135, p = 0.00). The regression analysis reveals an increase in one sleeping householder leads to a lower hourly biting density of Anopheles (β -0.037, t= -1.7, p = 0.000). Peak density of Anopheles species was noted in July 2019 followed by June 2019. There exists a positive correlation between mean monthly minimum temperature, rainfall and relative humidity and the mean monthly density of Anopheles mosquitoes at p-value < 0.05. The overall mean densities of host seeking Anopheles mosquitoes indoors (1.97per trap /night) and outdoors (2.58per trap/night) locations (t=-2.113, p = 0.072) were not statistically different. However, greater number of Anopheles gambiae s.l. was collected indoors than outdoors (t = 1.565, p = 0.001) and significant numbers of Anopheles pharoensis were collected outdoors as compared to indoors (t= -5.962, p = 0.000) which signals the differential host seeking behaviors between the two species.
Conclusion
The peak biting time of Anopheles gambiae s.l. coincides with the active working time of the Estate’s workers (from 6:00 PM to 6:00 AM) and this ensures the year-round availability of malaria vector that might result in perennial transmissions of malaria in such ecological settings. This calls for interventions on malaria and its vectors across all months of the year. Moreover, attention on outdoor based mosquito control measures as to be sought.