Semi-Field Evaluation of the Residual Efficacy of Natular Mini DT Larvicide against an Invasive Malaria Vector Anopheles stephensi in Ethiopia

Abstract

Introduction: The recent detection of An. stephensi, an invasive vector, in Ethiopia and other parts of the Horn of Africa has raised concerns about increased malaria transmission due to its adaptability to urban environments. Larviciding is crucial to combat this challenge, especially given the limitations on multiple-insecticide resistance by the vector and environmental contamination from chemical insecticides. Hence, this study aims to address the knowledge gap in the biological activity and residual efficacy of Natular Mini DT larvicide against An. stephensi, a critical step towards developing targeted and sustainable control strategies. Methods: Longitudinal semi-field study was conducted at the Tropical and Infectious Disease Research Center (TIDRC) Field Research Station in Awash Sebat Kilo, Afar, Ethiopia, between November 2024 and February 2025. The research evaluated prolonged larvicidal effect of Natular Mini DT in 50-litre of water under two conditions; with no water replacement and 50% weekly water replacement. Larval count data were collected every 24 hours across treatment and control groups and presented using descriptive statistics and bar graphs with 95% confidence intervals. A difference in larval mortality between treatment and controls was assessed using t- test. Larval mortality difference between treated water regimens and residual efficacy were statistically assessed using binary logistic regression. Results: Our study revealed that Natular Mini DT exhibited a rapid larvicidal effect, with mortality rates reached a peak at 72 hours, despite 100% mortality was not recorded within 72 hours for the first two weeks, regardless of water conditions. There was a significant difference in mean larval mortality between treatment and control groups. The comparison of treated water regimens revealed that the absence of water replacement resulted in a 75% lower likelihood of larval mortality rate within 24 hours. Furthermore, the larvicide showed significant residual efficacy for 12 weeks. Conclusion: The study showed that Natular Mini DT larvicide resulted in 100% larval mortality and residual efficacy for 12 weeks against An. stephensi, which showed a promising solution for controlling malaria transmission. However, the residual efficacy and non-target effects of Natular Mini DT larvicide should be evaluated under field condition in different eco-epidemiological settings and seasons