New study sheds light on temperature's impact on mosquito control and West Nile virus risk

A new study conducted by environmental life sciences PhD candidate, Josh Kalmouni and professor Krijn Paaijmans from Arizona State University’s School of Life Sciences and Center for Evolution and Medicine and the Maricopa County Environmental Services Department reveals critical insights into how temperature and mosquito behavior influence the effectiveness of insecticides in controlling mosquitoes that transmit West Nile virus (WNV). With WNV being the leading mosquito-borne disease in the United States, the findings could significantly impact public health strategies aimed at reducing mosquito populations and disease transmission.

The paper, titled, “Temperature and time of host-seeking activity impact the efficacy of chemical control interventions targeting the West Nile virus vector, Culex tarsalis,” focuses on Culex tarsalis, a key vector for WNV, and explores how temperature affects the toxicity of two commonly used insecticides—malathion and deltamethrin. Additionally, the study looks at the time of day when mosquitoes are most actively seeking hosts, offering vital information on when mosquito control efforts should be conducted for maximum impact.

“It was a natural collaboration given the interface of this work. Maricopa County Vector Control (MCVC) provided the facilities in which this work was carried out, but more importantly, they routinely conduct critical services such as surveillance and chemical control in the area. Our results serve to inform MCVC and highlight opportunities to improve their effectiveness in controlling vector populations and reducing the burden of West Nile virus in the county.”

Key Findings

Mosquito Activity Patterns: Culex tarsalis were most active between midnight and early morning. However, a significant portion of mosquito activity was also observed in the evening hours before midnight, a period not typically targeted by current insecticide fogging operations.

Temperature Matters: The study found that both malathion and deltamethrin became less effective at cooler temperatures and more effective at higher temperatures, which is particularly concerning as insecticide fogging is often conducted during late-night hours when temperatures are at their lowest. Malathion’s toxicity was reduced across all doses in cooler conditions, while deltamethrin's efficacy dropped at lower doses. 

“Mosquito control in Maricopa County (which includes Phoenix) is carried out between 12-5 a.m. by MCVC - during the coolest part of the day/night cycle,” says Kalmouni, “and our results would suggest that using insecticides which become more toxic at cooler temperatures would likely increase the efficacy of their chemical control.”

Strategic Insecticide Use: The study recommends that public health programs take both temperature and mosquito activity patterns into account when planning large-scale insecticide applications. Using chemicals at temperatures that increase their toxicity and when mosquitoes are most active could significantly improve the control of mosquito vectors.

“We also wanted to identify the time of host-seeking for vectors in the area during the pre-monsoon season. We found that a good deal of mosquito captures fell outside of the insecticidal fogging window, which is relevant for mosquito-human interactions and disease risk. However, even within the fogging window, we found that there was a difference between hourly captures,” Kalmouni explains. “The concern is that since residual insecticides from fogging do not last indefinitely, mosquitoes that are active one or two hours after fogging may not receive lethal doses of the insecticide, despite being active within this 'fogging window'. We hope our findings encourage vector control programs to routinely investigate time of host-seeking to better optimize their time of fogging.”

Implications for Public Health: Given that there are no vaccines or treatments for WNV, mosquito control is the primary strategy for reducing the risk of infection. This study highlights the importance of refining fogging operations by considering the temperature-toxicity relationship of insecticides and mosquito behavior. By adjusting the timing of insecticide applications, vector control programs may be able to achieve better results, reducing mosquito populations and ultimately lowering the incidence of WNV.

Kalmouni says that his “hope is that this research exemplifies the need to adapt the methods of chemical control targeting mosquitoes to local conditions. This work is meant to promote collaboration and improve global public health through a more efficacious use of universally established tools, such as the insecticides already pre-qualified by the World Health Organization for public health use.”