Impact of Aerial Spraying of Pyrethrin Insecticide onCulex pipiensandCulex tarsalis(Diptera: Culicidae) Abundance and West Nile Virus Infection Rates in an Urban/Suburban Area of Sacramento County, California

Abstract

In response to an epidemic amplification of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), the Sacramento and Yolo Mosquito and Vector Control District (SYMVCD) sprayed ultralow-volume (ULV) formulations of pyrethrin insecticide (Evergreen EC 60-6: 6% pyrethrin insecticide, 60% piperonylbutoxide; MGK, Minneapolis, MN, applied as 0.003 kg/ha [0.0025 lb/acre] ) over 218 km2 in north Sacramento and 243.5 km2 in south Sacramento on three consecutive evenings in August 2005. We evaluated the impact of this intervention in north Sacramento on the abundance and WNV infection rates of Culex pipiens L. and Culex tarsalis Coquillett. Mortality rates of caged Cx. tarsalis sentinels ranged from 0% under dense canopy to 100% in open fields. A comparison of weekly geometric mean mosquito abundance in CO2-baited traps in sprayed and unsprayed areas before and after treatment indicated a 75.0 and 48.7% reduction in the abundance of Cx. pipiens and Cx. tarsalis, respectively. This reduction was statistically significant for Cx. pipiens, the primary vector of WNV, with highest abundance in this urban area, but not for Cx. tarsalis, which is more associated with rural areas. The infection rates of WNV in Cx. pipiens and Cx. tarsalis collected from the spray zone were 8.2 and 4.3 per 1,000 female mosquitoes in the 2 wk before and the 2 wk after applications of insecticide, respectively. In comparison, WNV infection rates in Cx. pipiens and Cx. tarsalis collected at same time interval in the unsprayed zone were 2.0 and 8.7 per 1,000, respectively. Based on the reduction in vector abundance and its effects on number of infective bites received by human population, we concluded that the aerial application ofpyrethrin insecticide reduced the transmission intensity of WNV and decreased the risk of human infection.

Gravid Culex pipiens Exhibit A Reduced Susceptibility to Ultra–Low Volume Adult Control Treatments Under Field Conditions

“…Despite this, many authors have suggested that more frequent ULV adulticiding may be necessary to alter viral transmission dynamics (Andis et al 1987, Pawelek et al 2014. It is interesting to note that some of the only papers that have demonstrated a clear reduction in viral prevalence from ULV treatments in vector mosquitoes conducted aerial treatments over 3 sequential nights (Elnaiem et al 2008, Macedo et al 2010. The work presented here further supports the notion that multiple ULV treatments, timed to account for cohorts of mosquitoes transitioning into and out of susceptibility, may yield a greater reduction in viral transmission and vector populations than isolated ULV treatments (Andis et al 1987).…”

“…Finally, trapping mosquitoes over a large area for an extended time with conventional trapping technologies is extremely labor intensive and expensive. For this reason, evaluations of the effectiveness of ULV adulticides on natural adult mosquito populations do exist but are limited (Mitchell et al 1970, Chadeeq 1985, Andis et al 1987, Fox and Specht 1988, Elnaiem et al 2008, Sudsom et al 2015, Stoddard 2018. Other strategies for assessing the effect of ULV treatments on mosquito populations have relied on egg counts or immature mosquito counts to detect changes in the population after a treatment with notably mixed results (Andis et al 1987, Chua et al 2005, Reddy et al 2006.…”

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“…That the populations at the seven colonies where we recorded productivity in 2020 were higher than elsewhere in Sacramento could be an indication of better habitat quality or lower rates of infection. Dead magpies infected with West Nile virus, however, were reported near our study areas only from mid-June through mid-September (n = 27, https://www.fightthebite.net/media/west-nile-virusactivity/), after the nesting season, when mosquitoes are more abundant (Elnaiem et al 2008, Macedo et al 2010…”

Quantitative reverse transcription PCR assay to detect pyrethroid resistance in Culex mosquitoes

“…Susceptible Cx. tarsalis utilized in insecticide CDC bottle bioassays described below were from the Kern National Wildlife Refuge (KNWR) colony [16, 17] and resistant Cx. tarsalis maintained in an insectary that were originally collected during 2019 in Woodland, California USA (Conaway strain; GPS coordinates: 38.647287, -121.668173).…”