Pyrethroid insecticides in urban catch basins: A potential secondary contamination source for urban aquatic systems.

Pesticide contamination is a threat to many aquatic habitats, and runoff from residential homes is a major contributor of these chemicals in urban surface streams and estuaries. Improved understanding of their fate and transport can help identify areas of concern for monitoring and management. In many urban areas, runoff water congregates in numerous underground catch basins before draining into the open environment; however, at present essentially no information is available on pesticide presence in these systems. In this study, we collected water samples from a large number of underground urban catch basins in different regions of California during the active pest management season to determine the occurrence and profile of the widely used pyrethroid insecticides. Detectable levels of pyrethroids were found in 98% of the samples, and the detection frequency of individual pyrethroids ranged from no detection for fenpropathrin to 97% for bifenthrin. In the aqueous phase, total pyrethroid concentrations ranged from 3 to 726 ng/L, with a median value of 32 ng/L. Pyrethroids were found to be enriched on suspended solids, with total concentrations ranging from 42 to 93,600 ng/g and a median value of 2,350 ng/g. In approximately 89% of the samples, whole water concentrations of bifenthrin were predicted to have toxic units >1 for sensitive aquatic invertebrates. The high detection frequency of bifenthrin and overall pyrethroid concentrations, especially for particle-bound residues, suggest that underground urban catch basins constitute an important secondary source for extended and widespread contamination of downstream surface waters by pesticides such as pyrethroids in urban regions.

Detection and Establishment of Aedes notoscriptus (Diptera: Culicidae) Mosquitoes in Southern California, United States.

Aedes notoscriptus (Skuse), the Australian backyard mosquito, is a pestiferous daytime-biting species native to Australia and the surrounding southwestern Pacific region. It is suspected to play a role in the transmission of several arboviruses and is considered a competent vector of dog heartworm, Dirofilaria immitis (Leidy). This highly adaptable mosquito thrives in natural and artificial water-holding containers in both forested and urbanized areas, from tropical to temperate climates, and has benefitted from a close association with humans, increasing in abundance within its native range. It invaded and successfully established in New Zealand as well as in previously unoccupied temperate and arid regions of Australia. Ae. notoscriptus was discovered in Los Angeles County, CA, in 2014, marking the first time this species had been found outside the southwestern Pacific region. By the end of 2019, immature and adult mosquitoes had been collected from 364 unique locations within 44 cities spanning three southern California counties. The discovery, establishment, and rapid spread of this species in urban areas may signal the global movement and advent of a new invasive container-inhabiting species. The biting nuisance, public health, and veterinary health implications associated with the invasion of southern California by this mosquito are discussed.

Quantifying sociodemographic heterogeneities in the distribution of Aedes aegypti among California households.

The spread of Aedes aegypti in California and other regions of the U.S. has increased the need to understand the potential for local chains of Ae. aegypti-borne virus transmission, particularly in arid regions where the ecology of these mosquitoes is less understood. For public health and vector control programs, it is helpful to know whether variation in risk of local transmission can be attributed to socio-demographic factors that could help to target surveillance and control programs. Socio-demographic factors have been shown to influence transmission risk of dengue virus outside the U.S. by modifying biting rates and vector abundance. In regions of the U.S. where Ae. aegypti have recently invaded and where residential areas are structured differently than those in the tropics where Ae. aegypti are endemic, it is unclear how socio-demographic factors modify the abundance of Ae. aegypti populations. Understanding heterogeneities among households in Ae. aegypti abundance will provide a better understanding of local vectorial capacity and is an important component of understanding risk of local Ae. aegypti-borne virus transmission. We conducted a cross-sectional study in Los Angeles County, California during summer 2017 to understand the causes of variation in relative abundance of Ae. aegypti among households. We surveyed 161 houses, representing a wide range of incomes. Surveys consisted of systematic adult mosquito collections, inspections of households and properties, and administration of a questionnaire in English or Spanish. Adult Ae. aegypti were detected at 72% of households overall and were found indoors at 12% of households. An average of 3.1 Ae. aegypti were collected per household. Ae. aegypti abundance outdoors was higher in lower-income neighborhoods and around older households with larger outdoor areas, greater densities of containers with standing water, less frequent yard maintenance, and greater air-conditioner use. We also found that Ae. aegypti abundance indoors was higher in households that had less window and door screening, less air-conditioner usage, more potted plants indoors, more rain-exposed containers around the home, and lower neighborhood human population densities. Our results indicate that, in the areas of southern California studied, there are behavioral and socio-demographic determinants of Ae. aegypti abundance, and that low-income households could be at higher risk for exposure to Ae. aegypti biting and potentially greater risk for Zika, dengue, and chikungunya virus transmission if a local outbreak were to occur.

Aedes aegypti Males as Vehicles for Insecticide Delivery.

Aedes aegypti continues to spread globally and remains a challenge to control, in part due to its 'cryptic behavior' in that it often deposits eggs (oviposits) in larval habitats that are difficult to find and treat using traditional methods. Auto-dissemination strategies target these cryptic breeding sites by employing mosquitoes to deliver lethal doses of insecticide. This report describes the initial field trials of an application known as Autodissemination Augmented by Males (ADAM), utilizing A. aegypti males dusted with pyriproxyfen (PPF). Findings presented here are drawn from both caged and field trial studies. Together, these trials examined for the ability of A. aegypti males to disseminate PPF and to impact field populations. PPF-dusted males were able to effectively deliver lethal doses of PPF to oviposition sites under the conditions tested. Results from field trials in Florida and California demonstrated reduced A. aegypti populations in treated areas, compared to areas where PPF-treated males were not released. These results indicate that the release of PPF-dusted A. aegypti males can impact A. aegypti populations as measured by both reduced larval survival and lower numbers of adult female A. aegypti. We propose the ADAM approach as an addition to existing mosquito control techniques targeting A. aegypti and other mosquitoes that utilize cryptic larval habitats.

Genetic analysis of invasive Aedes albopictus populations in Los Angeles County, California and its potential public health impact.

The Asian tiger mosquito, Aedes albopictus, is an anthropophilic aggressive daytime-biting nuisance and an efficient vector of certain arboviruses and filarial nematodes. Over the last 30 years, this species has spread rapidly through human travel and commerce from its native tropical forests of Asia to every continent except Antarctica. In 2011, a population of Asian tiger mosquito (Aedes albopictus) was discovered in Los Angeles (LA) County, California. To determine the probable origin of this invasive species, the genetic structure of the population was compared against 11 populations from the United States and abroad, as well as preserved specimens from a 2001 introduction into California using the mitochondrial cytochrome c oxidase 1 (CO1) gene. A total of 66 haplotypes were detected among samples and were divided into three main groups. Aedes albopictus collected in 2001 and 2011 from LA County were genetically related and similar to those from Asia but distinct from those collected in the eastern and southeastern United States. In view of the high genetic similarities between the 2001 and 2011 LA samples, it is possible that the 2011 population represents in part the descendants of the 2001 introduction. There remains an imperative need for improved surveillance and control strategies for this species.

Experimental and natural vertical transmission of West Nile virus by California Culex (Diptera: Culicidae) mosquitoes.

Culex (Diptera: Culicidae) mosquitoes, the primary summer vectors of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), also may serve as overwintering reservoir hosts. Detection of WN viral RNA from larvae hatched from eggs deposited by infected females during late summer and fall may provide evidence for the vertical passage of WNV to overwintering cohorts. To determine whether vertical transmission to the overwintering generation occurs in populations of Culex mosquitoes throughout California, larvae from naturally infected females were tested by family for WN viral RNA by real-time quantitative reverse transcription-polymerase chain reaction during August through October 2011. Viral RNA was detected in 34 of 934 Culex tarsalis Coquillett and Cx. pipiens complex females that laid viable egg rafts. From these egg rafts, first-instar larvae from nine families tested positive, yielding an overall field vertical transmission rate of 26% (n = 34). To determine whether the WNV may be lost transtadially during development to the adult stage, first-instar larvae and adult progeny from experimentally infected Cx. pipiens complex females were assessed for the presence and quantity of WN viral RNA. Most (approximately 75%) WNV infections were lost from positive families during larval development to the adult stage. In field and laboratory studies, only infected mothers with mean cycle threshold scores < or = 20 vertically transmitted WNV to larval progeny, adult progeny, or both. In summary, vertical transmission of WNV was detected repeatedly in naturally infected Culex mosquitoes collected throughout California during late summer and fall, with females having high titered infections capable of passing WNV onto their progeny destined for overwintering.

Antecedent avian immunity limits tangential transmission of West Nile virus to humans.

BACKGROUND: West Nile virus (WNV) is a mosquito-borne flavivirus maintained and amplified among birds and tangentially transmitted to humans and horses which may develop terminal neuroinvasive disease. Outbreaks typically have a three-year pattern of silent introduction, rapid amplification and subsidence, followed by intermittent recrudescence. Our hypothesis that amplification to outbreak levels is contingent upon antecedent seroprevalence within maintenance host populations was tested by tracking WNV transmission in Los Angeles, California from 2003 through 2011. METHODS: Prevalence of antibodies against WNV was monitored weekly in House Finches and House Sparrows. Tangential or spillover transmission was measured by seroconversions in sentinel chickens and by the number of West Nile neuroinvasive disease (WNND) cases reported to the Los Angeles County Department of Public Health. RESULTS: Elevated seroprevalence in these avian populations was associated with the subsidence of outbreaks and in the antecedent dampening of amplification during succeeding years. Dilution of seroprevalence by recruitment resulted in the progressive loss of herd immunity following the 2004 outbreak, leading to recrudescence during 2008 and 2011. WNV appeared to be a significant cause of death in these avian species, because the survivorship of antibody positive birds significantly exceeded that of antibody negative birds. Cross-correlation analysis showed that seroprevalence was negatively correlated prior to the onset of human cases and then positively correlated, peaking at 4-6 weeks after the onset of tangential transmission. Antecedent seroprevalence during winter (Jan - Mar) was negatively correlated with the number of WNND cases during the succeeding summer (Jul-Sep). CONCLUSIONS: Herd immunity levels within after hatching year avian maintenance host populations <10% during the antecedent late winter and spring period were followed on three occasions by outbreaks of WNND cases during the succeeding summer. Because mosquitoes feed almost exclusively on these avian species, amplification was directly related to the availability of receptive non-immune hosts.

Sentinel chicken seroconversions track tangential transmission of West Nile virus to humans in the greater Los Angeles area of California.

In Los Angeles, California, West Nile virus (WNV) has followed a pattern of emergence, amplification, subsidence, and resurgence. A time series cross-correlation analysis of human case counts and sentinel chicken seroconversions revealed temporal concordance indicating that chicken seroconversions tracked tangential transmission of WNV from the basic passeriform-Culex amplification cycle to humans rather than antecedent enzootic amplification. Sentinel seroconversions provided the location and time of transmission as opposed to human cases, which frequently were reported late and were assumed to be acquired 2-14 days before disease onset at their residence. Cox models revealed that warming degree-days were associated with the increased risk of seroconversion, whereas elevated herd immunity in peridomestic birds dampened seroconversion risk. Spatially, surveillance data collected within a 5 km radius of flock locations 15-28 days before the bleed date were most predictive of a seroconversion. In urban Los Angeles, sentinel chicken seroconversions could be used as an outcome measure in decision support for emergency intervention.

West Nile virus emergence and persistence in Los Angeles, California, 2003-2008.

West Nile virus (WNV) invaded Los Angeles in September 2003, and during the subsequent five-year period followed a pattern of amplification, subsidence, and resurgence. Enzootic transmission was tracked by abundance and infection incidence in Culex pipiens quinquefasciatus and Cx. tarsalis and by seroprevalence in peridomestic passerine birds, infection in dead birds, and seroconversions in sentinel chickens. Culex p. quinquefasciatus served as the primary vector of WNV, with gravid traps serving as the best sampling method and the most consistent indicator of viral activity. Spatial scan statistics applied to mosquito infection and positive dead bird data delimited three major clusters of WNV transmission, with introduction occurring in the Los Angeles Basin, and amplification and dispersal events carrying transmission to the San Fernando and Santa Clarita valleys. Los Angeles experienced major epidemics in 2004 and 2008, providing a unique opportunity to investigate specific patterns of enzootic amplification preceding epidemics.

An assessment of mosquito production and nonchemical control measures in structural stormwater best management practices in southern California.

A 3-year study was conducted to assess mosquito production in structural stormwater Best Management Practices (BMPs) installed by the California Department of Transportation in San Diego and Los Angeles Counties. Thirty-seven BMPs were monitored weekly for presence and relative abundance of immature mosquitoes and for conditions conducive to mosquito production. Species identified were Aedes squamiger, Anopheles franciscanus, An. hermsi, Culex quinquefasciatus, Cx. stigmatosoma, Cx. tarsalis, Culiseta incidens, and Cs. inornata. Structures designed with accessible, permanent sources of standing water in sumps, vaults, or basins were observed to support immatures all year. In BMPs intended to drain rapidly and completely, observed larval habitats resulted from design features, component failure, construction flaws, and non-stormwater runoff flows. Specific nonchemical mitigation measures to minimize or eliminate mosquito production were developed, implemented, and evaluated. This study provides a 1st assessment of mosquito production, species composition, and nonchemical control measures in structural stormwater BMPs in southern California.

Has Aedes albopictus established in California?

Significant numbers of the Asian tiger mosquito, Aedes albopictus, were detected on the west coast of the USA in mid-June 2001, in containerized oceanic shipments of "lucky bamboo" (Dracaena spp.) originating from South China. Wholesale nurseries in California importing large quantities of lucky bamboo became the focal points of infestation. Greater Los Angeles County Vector Control District immediately implemented an adulticiding protocol at the Los Angeles/Long Beach Harbors, followed by larviciding soon after the shipment was delivered to the wholesale nursery. Intensive surveys are currently being conducted above ground and in the underground storm drain systems using battery-operated CDC/CO2-baited light traps and ovitraps, both enhanced with an attractant (water rinse of tiger shrimps), to determine extent of infestation and perhaps establishment of Ae. albopictus locally.

Introduction of Aedes albopictus (Skuse) in southern California and potential for its establishment.

Isolated incidences of Aedes albopictus in the continental U.S. were reported as early as 1946 and the first incidence in California was reported in 1972. These introductions were referred to as "isolated incidences" because very few immatures were observed in used tires shipped from Southeast Asia. The first major discovery of a large population and subsequent establishment of Ae. albopictus in the U.S. was reported in 1986 from Houston, TX, in a shipment of used tires from Japan. In early June 2001, infestations of this species associated with containerized oceanic shipments of "lucky bamboo" (Dracaena spp.) packaged in standing water were introduced into southern California from mainland south China. Focal points of infestation are currently at the wholesale nurseries in southern as well as northern California. A control protocol for adulticiding and larviciding has been implemented by the Greater Los Angeles County Vector Control District. Surveys are presently being conducted by local vector control agencies in southern and northern California to determine the extent of infestation. Potential forAe. albopictus establishment is discussed.