The Asian bush mosquito, Aedes japonicus, is an invasive species that is well established in North America and Europe. Though it is considered a temperate species, we have observed an established population of Ae. japonicus in the subtropical climate of northwestern Florida. To evaluate the temporal patterns of Ae. japonicus abundance, mosquito larvae were collected from 15 artificial containers in Escambia County, FL, from August 2019 to July 2020, with the prediction that Ae. japonicus abundance would peak in the winter months and decline with increasing ambient temperatures. Aedes japonicus larvae were collected in low abundances during each month except for February (n = 51), with no clear temporal patterns of abundance. Larval contemporaries belonging to other species were considered in sampling of containers and were also cataloged. We demonstrate monthly observance of this temperate species at a single site in the Florida panhandle, exemplifying the persistence of Ae. japonicus through all seasons in a subtropical climate.
Aedes japonicus (Theobald), commonly known as the Asian bush mosquito, is an invasive container-inhabiting species that has become established in parts of Europe and Canada and much of the USA, including most states east of the Mississippi River and some western states (Kaufman and Fonseca 2014). The species is a potential vector of several arboviruses within its invasive range (Schaffner et al. 2011, Abbo et al. 2020). The species was first recognized in the USA in 1998 with the used tire trade implicated as the source of the introduction though distinct genetic populations have since been identified within the country, suggesting multiple introductions (Egizi et al. 2016). Aedes japonicus is considered a temperate species, with a relatively low maximum developmental temperature and a preference for immature development in temperate habitats (Reuss et al. 2018, Byrd et al. 2019), but its establishment in the subtropical Florida panhandle suggests that the species is capable of finding habitats even in apparently unsuitable climates (Riles et al. 2017).
The invasive success of Ae. japonicus is thought to be due in part to its ability to colonize and develop in larval habitats early and late in the mosquito season, when temperatures are more favorable to Ae. japonicus than other native and established species (Kaufman and Fonseca 2014). Predictive models suggest that the distribution of Ae. japonicus will shift in the coming years due to warming climate, with the amount of suitable habitat possibly declining in the southern reaches of its invasive range, though the species may persist in warmer regions (Peach et al. 2019). The subtropical Florida panhandle, where Ae. japonicus was first identified in 2014 and remains established to date, may represent the southernmost expansion of this invasive species in the USA (Riles et al. 2017, Riles and Connelly 2020, Smith et al. 2020). Containers under dense tree cover receiving vegetative detrital inputs within its expanded range may allow for successful immature development and adult harborage for the temperate species in Florida, a possible example of Ae. japonicus using environmental niches to persist in hot climates (Bartlett-Healy et al. 2012). The temporal abundance patterns of the species in Florida thus remain uncertain and are deserving of further study, as they may lead to a better understanding of how the species responds to subtropical climates throughout the year.
A site where Ae. japonicus displays monthly activity has been located in Escambia County, FL (Riles and Connelly 2020), a 5-acre (2.27 ha) residential lot containing a botanical nursery surrounded by deciduous forest (Cantonment, FL: 30.32′46″N, 87.18′39″W) where rare varietals and cultivars are collected and propagated. To examine temporal patterns of Ae. japonicus occurrence and abundance, 15 cyclic (7.57 liters) black plastic containers were placed throughout the nursery and filled with 5.6 liters of strained water from an on-site rain barrel in July 2019. The containers were sampled 1 time each month for 12 consecutive months (August 2019 to July 2020), with collections occurring during the 1st week of each month, for a total of 180 container observations. Each container observation was made by taking 2 samples with a standard dipper (catalog no. 1132BHQ; BioQuip Products, Rancho Dominguez, CA). An initial surface “skimming” sample was obtained, and after 120 sec, a second deeper sample was taken. Larvae were identified as 4th instars using a dichotomous key (Darsie and Ward 2005) and preserved in 80% ethanol. Broken or otherwise damaged containers were replaced and refilled to the 5.67 liters mark as needed.
On each day of collections, ambient temperatures (12-month range: Tmin = 16.27°C, Tmax = 32.83°C) were measured at a single-fixed location (30.32′46″, N 87.18′39″W). The water temperature was taken 1 cm and 10.5 cm below the surface (Checktemp® Digital Thermometer; Hanna Instruments HI 98501, Woonsocket, RI). Mean monthly ambient temperatures were obtained from a local weather station in Pensacola, FL (Fig. 1) (https://www.wunderground.com/weather/us/fl/pensacola). Aedes japonicus larvae were observed during 11 of the 12 monthly observations, with the only absence occurring in February 2020. Of the 3 Aedes spp. sampled overall, Ae. japonicus was the least abundant species (n = 51), with more Ae. albopictus (Skuse) (n = 109) and Ae. triseriatus (Say) (n = 57) collected in total (Table 1). Aedes japonicus larvae were collected from 12 of the 15 sampled containers during the study, but the species was present in only 23 (12.8%) of the 180 total container observations. Overall, there were no clear seasonal trends for the observed Ae. japonicus abundance. It was the most abundant Aedes species on average during the months of November, January, and April, which may be a result of its propensity to develop in temperate habitats. However, Ae. japonicus was also the most abundant in August 2019 and June 2020 when ambient temperatures were high (Fig. 1). Aedes japonicus females can develop to adulthood in water as hot as 31°C (Reuss et al. 2018), and at our study site, the water remained cool enough to support Ae. japonicus immature development even in summer months (surface water max = 30.0°C; water column max = 30.3°C). We do not know if this study site represents an uncommon ecological habitat in Florida, or if these conditions can be commonly found throughout the northern region of the state.
Because of its potential as a vector, the distribution of Ae. japonicus in the USA is of concern to arbovirus surveillance professionals. We do not know how the species may adapt to warmer climates or how effectively it finds environmental niches within those habitats (Peach et al. 2019), but we have demonstrated a single site in a subtropical climate that is conducive to immature development all year round. The possibility of finding suitable ecological habitats outside of its established range raises questions regarding its ability to adapt in subtropical climates. This observation may have important implications for how climate change will impact the distribution of Ae. japonicus, where there are many unknowns regarding its spread in Florida, including the extent and continuity of its distribution outside of its current range. Without further in-depth studies and intensive surveillance efforts, the role of Ae. japonicus as a disease vector, its geographic range, and its impacts on established mosquito populations will remain ambiguous. Mosquito control agencies in Florida and regions neighboring the current range of Ae. japonicus should maintain a heightened awareness of its potential encroachment into their jurisdiction and the possible role the species may have as a vector of endemic diseases.
We thank Brian Byrd for reviewing an earlier version of this manuscript, and we also acknowledge Roy and Joyce Smith for the extended liberal access to their property. This research was self-funded due to the authors' curiosity.
Beach Mosquito Control District, 509 Griffin Boulevard, Panama City, FL 32413.
Department of Entomology and Plant Pathology, University of Tennessee, Knoxville, TN 37996.
Environmental Security Pest and Lawn, 3182 Gateway Lane, Cantonment, FL 32533.