Aedes aegypti and Ae. albopictus are invasive mosquitoes, capable of vectoring arboviruses such as dengue, chikungunya, yellow fever, and Zika. Recent shifts in spatial distribution indicate there is a resurgence of Ae. aegypti in certain regions of Florida. After a 26-year absence, Ae. aegypti larvae were collected in a downtown neighborhood in Gainesville, Florida, in November 2019. Subsequent surveys confirmed that Ae. albopictus was completely displaced by Ae. aegypti in this neighborhood, whereas Ae. albopictus and Ae. aegypti coexisted around this community focus, and Ae. albopictus alone has been found elsewhere in the city and county since the 1990s. Field surveys revealed that Ae. aegypti is resurging in the downtown area of Gainesville and is actively dispersing to adjacent neighborhoods. Thus, Ae. aegypti could potentially replace Ae. albopictus across more of urban Gainesville in north-central Florida, as reported recently in coastal cities of northeastern Florida.
Aedes aegypti (L.) and Ae. albopictus (Skuse) are invasive species that are both competent vectors of arboviruses such as dengue, chikungunya, yellow fever, and Zika. These mosquitoes are aggressive and persistent daytime biters of humans in urban domestic habitats, making them a serious public health concern (Peacock et al. 1988). Aedes aegypti mosquitoes are assumed to have been introduced to North America during the 15th–17th centuries and became widely distributed throughout the tropical and subtropical regions of the United Sates, occurring in every county in Florida (Hayes and Tinker 1958, Tinker and Hayes 1959, Morlan and Tinker 1965, Tabachnick 1991). Aedes aegypti remained the resident, invasive container-inhabiting mosquito in the southeastern USA until 1985, when Ae. albopictus was first recorded breeding in Houston, TX (Sprenger and Wuithiranyagool 1986). By 1986, Ae. albopictus was detected in both Duval and Escambia counties in Florida, and within 6 years it had spread to all 67 counties in the state (Peacock et al. 1988, Hornby et al. 1994, O'Meara et al. 1995). While Ae. albopictus established and expanded its range, it caused rapid population declines of Ae. aegypti, to the point of extirpation in most areas except the extreme south of Florida (Peacock et al. 1988, Nasci et al. 1989, O'Meara et al. 1992, Hornby et al. 1994).
A similar competitive displacement trend occurred in the city of Gainesville, FL. Gainesville is the county seat of Alachua County, home to the University of Florida and an active tourism industry. Aedes albopictus was first discovered in Alachua County in October 1988, and was found within the city limits of Gainesville less than 1 year later (Smith et al. 1990, Gainesville Mosquito Control Services [GMCS], unpublished data). Aedes aegypti and Ae. albopictus lived in sympatry for several years, after which Ae. albopictus became the dominant species by May of 1990 (O'Meara et al. 1992, GMCS, unpublished data). The last known record of Ae. aegypti in Gainesville was collected in May of 1993.
However, in November 2019, several 4th instars Ae. aegypti were found in a small 1 gallon bucket in a residential area, known as the Pleasant Street Historic District (PSHD), near downtown Gainesville. This prompted further monitoring of the neighborhood to determine whether the species had reestablished after a 26-year absence. Preliminary surveillance indicated that Ae. aegypti was well-established in the PSHD.
The objective of the present study was to determine the local distribution of Ae. aegypti, using a door-to-door inspection approach, along with regular monitoring using BG-Sentinel traps. The study was conducted in the PSHD and surrounding areas in Gainesville from November 2019 to November 2020.
Following the discovery of Ae. aegypti larvae in the PSHD, the GMCS set up multiple BG-Sentinel® traps (BGS) (Biogents AG, Regensburg, Germany) baited with both human odor lures and dry ice at the initial larval collection site and adjacent parcels on December 3, 2019, and February 20, 2020 onward. These BGS traps failed to collect any Ae. aegypti adults until April 8, 2020, when 8 Ae. aegypti females were collected by a single BGS trap. Subsequently, 3 BGS traps collected high numbers of adults (62 female and 46 male) on April 21, 2020. These collections prompted GMCS to begin door-to-door inspections. The protocol for door-to-door inspections was to inspect, sample, and treat properties in the immediate area of the initial Ae. aegypti larval sample, gradually expanding surveillance into the adjacent neighborhoods. After 3 rounds of grid inspections in 3 consecutive weeks, searches were conducted monthly, with employees working in teams. Teams were supplied with collection bags, dippers, and a turkey baster for larval collection. Technicians inspected properties by walking through the entire parcel in search of standing water, containers, or other evidence of mosquito activity and obtained samples when larvae were present. Each sample was labelled with the address of the parcel and returned to the lab for identification using dichotomous keys by Darsie and Ward (2005) and a Leica S6E stereo-microscope (Leica Microsystems, Buffalo Grove, IL). Ten Aedes spp. larvae from each sample were randomly chosen for identification and percentage calculations. If a sample contained fewer than 10 larvae, all larvae were identified. Treatment consisted of manually emptying and manipulating containers, or using Natular® DT tablets (Clarke, St. Charles, IL) according to the insecticide label when source reduction was not possible. Throughout the study, GMCS technicians educated residents and business owners about Ae. aegypti and its resurgence, leaving pamphlets and GCMS contact information when in-person conversation was not possible.
In addition to door-to-door larval inspections and collections, 3 separate sites from the PSHD were chosen for weekly monitoring with BGS traps: the initial collection site for Ae. aegypti, Site 1 (29.656804, −82.330035), Site 2 (29.655887, −82.328936), and Site 3 (29.654156, −82.328335). Site 1 was a large parcel with 5 small houses, and the trap was placed in a shaded, damp corner near magnolia trees and ferns. Site 2 was a vacant house, with the trap placed near ferns. Site 3 was an empty lot with 2 sheds, and the trap was placed between the sheds in a completely shaded area with arrowhead vines. Traps were baited with human odor lures and with dry ice suspended above the trap. Traps were erected for 24 h, after which the collection bags were frozen for future identification.
Roughly 1,500 acres of residences, empty lots, and businesses were inspected by GCMS technicians over the course of the study. In total, 119 larval samples collected were positive for Ae. aegypti and/or Ae. albopictus, 36 of which were from the PSHD (Fig. 1). Of these positive samples, 53 contained only Ae. aegypti, 51 samples contained only Ae. albopictus, and 15 samples were a mix of both species. Mixed sample species ratios varied greatly, with Ae. aegypti ranging from 10% to 90%. There did not appear to be a directional trend of mixed samples, with different ratios found in northern, western, or southern neighborhoods.
Aedes aegypti larvae were collected up to 1.56 km away from the initial collection site. Aedes aegypti larvae were found inhabiting many types of containers, including waste tires, boat, tarp, kiddie pools, trash, dishes, planters, wheelbarrows, buckets, bird baths, and coolers. Only 1 Ae. albopictus larva was found within the PSHD neighborhood, and Ae. aegypti collections were highest in the western portion of the PSHD neighborhood compared with the rest of PSHD. Larvae of other mosquito species, such as Culex quinquefasciatus Say and Toxorhynchites rutilus (Coquillett), were also obtained, but detailed collection information is omitted from this paper.
In total, 1,431 male and female Ae. aegypti were collected in BGS traps during this study: 36.9% were male, and 63.1% female (Fig. 2). Per site, 55.0% of the total male and female Ae. aegypti were collected at Site 1, 27.7% at Site 3, and 17.3% at Site 2. Within each site, Site 1 collected 44.2% males and 55.8% females, Site 2 collected 20.3% males and 79.7% females, and Site 3 collected 33.9% males and 66.1% females. Only 2 Ae. albopictus adults were collected from the PSHD BGS traps. In 2 consecutive weeks in June, no collections were carried out due to operational conflicts. Again, other mosquito species, including Cx. erraticus (Dyar and Knab), Cx. quinquefasciatus, Ae. infirmatus (Dyar and Knab), Ae. atlanticus (Dyar and Knab), and Mansonia titillans (Walker), were collected but data are not reported here.
The distribution of Ae. aegypti and Ae. albopictus within Florida has been dynamic over the past 30 years (O'Meara et al. 1995, Britch et al. 2008, Lounibos et al. 2016). These shifts in spatial distribution are relevant to the vector potential of local mosquito populations. Thus, monitoring populations of Ae. aegypti and Ae. albopictus continues to be an area of great concern for Florida. In 2019 alone, outbreaks of dengue resulted in 18 locally acquired cases and 413 travel-related cases. In 2020, Florida had 71 locally transmitted and 41 imported cases of dengue. From 2014 to 2019, over 600 travel-related cases and 12 locally transmitted cases of chikungunya had occurred. The Florida Zika outbreaks since 2016 resulted in 300 local and over 1,500 travel or undetermined-origin cases (FDOH 2020).
According to the Centers for Disease Control and Prevention (CDC), it is “very likely” that both Ae. aegypti and Ae. albopictus occur throughout Florida, and several recent studies addressed the distributional shifts of these mosquitoes within the state, reporting Ae. aegypti as resurging (Reiskind and Lounibos 2013, Hopperstad and Reiskind 2016, Lounibos et al. 2016, CDC 2017). Wright et al. (2015) found that Ae. aegypti populations were larger and more widespread than anticipated in Jacksonville, FL, and determined that Ae. aegypti actually predominated over Ae. albopictus in most sample sites. Eastern coastal towns also experienced a resurgence of Ae. aegypti, with St. Augustine, FL, detecting Ae. aegypti after a 12-year absence (Smith et al. 2018, Dixon et al. 2020). The northern limit of Ae. aegypti was previously Apopka, located approximately 80 miles south of Gainesville (Lounibos et al. 2016). However, Parker et al. (2019) found Ae. aegypti in Marion county, roughly 40 miles closer to Gainesville, and populations are increasing in the noncoastal regions of the state.
Results of this investigation suggest that Ae. aegypti has outcompeted Ae. albopictus within the PSHD and is actively dispersing to adjacent neighborhoods. There appears to be a westward movement of Ae. aegypti from the PSHD, likely due to the density of houses and people within those neighborhoods. The neighborhoods directly east of the PSHD are high-income areas with larger parcels and lower density of people. This directional expansion is of great concern, since the University of Florida campus is less than 1 mile southwest of the PSHD.
The origin of Ae. aegypti in Gainesville after a 26-year absence is unknown. Possible avenues of reintroduction include vehicles, the used tire trade, or other containers contaminated with dormant eggs, but it is also possible that Ae. aegypti persisted in small enough populations to avoid detection. The Ae. aegypti adults collected in BGS traps were found to exhibit resistance to permethrin, whereas local, established populations of Ae. albopictus show no resistance to permethrin (Alden Estep, USDA, personal communication, GCMS, unpublished data). Such differences in pesticide resistance present a great challenge in controlling these mosquito species, as a reduction of 1 species may allow the other species to flourish.
The mechanism of the displacement of Ae. albopictus by Ae. aegypti in Gainesville remains unknown. In the late 1980s to early 1990s, when Ae. aegypti was replaced by Ae. albopictus in the southeastern United States, several mechanisms were suggested to explain the displacement, including mating interference, larval resource competition and differential effects of a larval parasite (Nasci et al. 1989, Craig 1993, Juliano 1998). However, none offered a sufficient explanation given the rapidity of displacement. Recently, satyrization has been established to be the most probable cause of competitive displacement of Ae. aegypti by Ae. albopictus (Bargielowski and Lounibos 2016, Juliano 2010, Lounibos 2007, Tripet et al. 2011). However, laboratory studies showed that allopatric populations of Ae. aegypti exposed to satyrization quickly evolve resistance to interspecific mating, which is thought to counteract reproductive interference from Ae. albopictus and promote Ae. aegypti reestablishment or persistence in areas where it was previously displaced (Bargielowski et al. 2013, Bargielowski and Lounibos 2016). Additionally, abiotic environmental factors, such as temperature, humidity, and drought may affect the outcome of this competitive interaction. For example, Ae. aegypti eggs are more tolerant of desiccation than Ae. albopictus; therefore, they survive longer at lower relative humidity and higher temperatures than Ae. albopictus (Lounibos et al. 2010).
During the study, GCMS technicians began a treatment plan for the PSHD using the A1 Mister Cobra (A1 Mist Sprayers, Ponca, Nebraska) to apply Vectobac® water-dispersible granule (WDG) containing Bacillus thurigiensis israelensis de Barjac (Valent BioSciences, LLC, Libertyville, IL). Applications were performed approximately every 2 wk. Results for this treatment were inconsistent, with some applications giving a good reduction in Ae. aegypti numbers, and some not. Caveats for this treatment method include: 1) dependency on wind conditions, which were usually insufficient at night to ensure good coverage of the area (1–2 mph); 2) people being outdoors; 3) traffic; and 4) expense per application (approximately $1,200 including insecticide and labor/equipment costs for each treatment).
Our preliminary results indicate the need for further monitoring, plus the study of the population dynamics and competitive interactions of these mosquitoes, especially within the inland regions of the state. Understanding these mechanisms would provide valuable information in developing treatment plans and reducing the risk for arbovirus transmission, while creating an opportunity for targeted mosquito education and control in areas affected by Ae. aegypti.
We thank Alden Estep, USDA ARS Center for Medical, Agricultural, and Veterinary Entomology (USDA CMAVE), Gainesville, FL, for testing the resistance profile of Ae. aegypti. The authors also thank Graham White, Department of Entomology and Nematology, University of Florida, Gainesville (ret.); Daniel Kline, USDA CMAVE; and David Dame, USDA CMAVE (ret.) for reviewing this manuscript, and Justin Scott, Gainesville Mosquito Control, for his technical and field support during this study.