A study on Anopheles gambiae s.l. biting activities in wonji sugar estate, Eastern Oromia Ethiopia: a challenge to the current intervention tools

Although the behavioral flexibility of Anopheles mosquitoes poses a 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, monthly density, and weather variability were evaluated in Wonji Sugar Estate. 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 from July 2018 to June 2020. Mosquitoes were identified to species using morphological keys. ANOVA was used to compare mean monthly mosquito densities. A total of 3,504 Anopheles comprising: Anopheles gambiae s.l., An. pharoensis, and An. coustani complex were collected during the study period. 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 shows that as the number of household members going to bed increases by one, there is a decrease in the 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. A 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). The peak biting time of Anopheles gambiae s.l. coincides with the active working time of the Estate’s workers which might increase risk of residual malaria transmission. The year-round availability of malaria vector in the study might causes perennial transmissions of malaria. This calls for interventions on malaria and its vectors across all months of the year. Moreover, alternative and complementary outdoor based mosquito control measures have to be sought.