Temperature and time of host-seeking activity impact the efficacy of chemical control interventions targeting the West Nile virus vector, Culex tarsalis

Abstract

West Nile virus (WNV) is the leading mosquito-borne disease causing-pathogen in the United States. Concerningly, there are no prophylactics or drug treatments for WNV and public health programs rely heavily on vector control efforts to lessen disease incidence. Insecticides can be effective in reducing vector numbers if implemented strategically, but can diminish in efficacy and promote insecticide resistance otherwise. Vector control programs which employ mass-fogging applications of insecticides, often conduct these methods during the late-night hours, when diel temperatures are coldest, and without a-priori knowledge on daily mosquito activity patterns. This study’s aims were to 1) quantify the effect of temperature on the toxicity of two conventional insecticides used in fogging applications (malathion and deltamethrin) to Culex tarsalis, an important WNV vector, and 2) quantify the time of host-seeking of Cx. tarsalis and other local mosquito species in Maricopa County, Arizona. The temperature-toxicity relationship of insecticides was assessed using the WHO tube bioassay, and adult Cx. tarsalis, collected as larvae, were exposed to three different insecticide doses at three temperature regimes (15, 25, and 35°C; 80% RH). Time of host-seeking was assessed using collection bottle rotators with encephalitis vector survey traps baited with dry ice, first at 3h intervals during a full day, followed by 1h intervals during the night-time. Malathion became less toxic at cooler temperatures at all doses, while deltamethrin was less toxic at cooler temperatures at the low dose. Regarding time of host-seeking, Cx. tarsalis, Aedes vexans, and Culex quinquefasciatus were the most abundant vectors captured. During the 3-hour interval surveillance over a full day, Cx. tarsalis were most-active during post-midnight biting (00:00–06:00), accounting for 69.0% of all Cx. tarsalis, while pre-midnight biting (18:00–24:00) accounted for 30.0% of Cx. tarsalis. During the 1-hour interval surveillance overnight, Cx. tarsalis were most-active during pre-midnight hours (18:00–24:00), accounting for 50.2% of Cx. tarsalis captures, while post-midnight biting (00:00–06:00) accounted for 49.8% of Cx. tarsalis. Our results suggest that programs employing large-scale applications of insecticidal fogging should consider temperature-toxicity relationships coupled with time of host-seeking data to maximize the efficacy of vector control interventions in reducing mosquito-borne disease burden. Author summary: Mosquito-borne pathogens such as West Nile virus (WNV) are expected to increase due to climate change, and public health programs rely on insecticides to reduce mosquito vector populations. Insecticidal fogging is often conducted during post-midnight hours when temperatures are coldest. Temperature has been shown to impact the toxicity of insecticides, but this effect is understudied, particularly for WNV vectors. The researchers aimed to test the effect of temperature on the toxicity of two common insecticides to Culex tarsalis, a primary mosquito vector of WNV, and to identify when local vectors were actively seeking a host to bite. The research location was a productive riparian (wetland) area adjacent to many urban centers in Arizona. The researchers found that both insecticides became less toxic at colder temperatures and that early evening accounted for a significant portion of captures, yet is outside of normal fogging hours. Human outdoor activities also increase during these hours, which may result in elevated risk of disease transmission. This research serves to improve our understanding and usage of insecticides to reduce disease burden. As insecticides are used globally, the findings of this research may relate to all mosquito-borne diseases, including malaria, and are relevant for agricultural pest management.

Publication information

Originally published in PLOS Neglected Tropical Diseases on 2024-08-30. Original DOI: 10.1371/journal.pntd.0012460.

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