Few historical records depict the distribution of Mansonia titillans (Walker) in South Carolina. Mosquitoes of the genus Mansonia Blanchard were first collected in South Carolina as single adult specimens of Ma. dyari Belkin, Heinemann, and Page (Darsie and Hager 1993) and Ma. titillans (Goddard and Harrison 2005) in Beaufort County in 1993 and 1995, respectively. Beaufort County is the second-most southern county in South Carolina and is located along the eastern side of the state, bordering the Atlantic Ocean. Mansonia titillans was not reported in Beaufort County again until 2015, after which sporadic collections were made. Mansonia titillans has been detected in multiple counties in Georgia since 1997, including Chatham County whose border ranges from 2 to 10 mi SW of Beaufort County (Smith and Floore 2001). New county records of Ma. titillans reported here expand the recorded northern distribution of this species in the USA. These new county records can be attributed partially to the statewide arbovirus surveillance program conducted by the South Carolina Department of Health and Environmental Control and mosquito control programs within the state.

Adult specimens were collected by using 3 trap types with 3 types of attractants: 1) Centers for Disease Control and Prevention light traps baited with dry ice or a combination of dry ice and BG-Lure (Biogents, AG, Regensburg, Germany) (Beaufort, Clarendon, and Colleton counties); 2) BG-Sentinel 2.0 (Biogents) traps baited with a combination of BG-Lure and dry ice or BG-Lure and carbon dioxide gas (Berkeley, Clarendon, and Georgetown counties); or 3) encephalitis vector survey traps baited with dry ice (Beaufort County). Surveillance for immature stages of Ma. titillans was conducted using a telescoping dipper and a 3-part mesh sieve.

Mansonia are medium sized, dark, and speckled with dark and light scales throughout the body, with pale basal bands on the tarsomeres. Although both Mansonia and Aedes adult mosquitoes have postspiracular setae, Aedes have a narrow or pointed abdominal apex while Mansonia have a blunt or rounded abdominal apex (Belkin et al. 1970, Harrison et al. 2016). Separation of Mansonia species from Coquillettidia perturbans (Walker) may be difficult the 1st time Mansonia is encountered. Coquillettidia perturbans is common in the Mid-Atlantic region, particularly in late spring and early summer (King et al. 1960), and shares similar characters with Mansonia, such as the same size, posture, large pale and dark scales on the wing, a broadly rounded abdominal apex, and pale basal bands on the abdomen. Unlike Mansonia though, Cq. perturbans are lighter in color, lack postspiracular setae, have a broad median pale band on hindtarsomere 1, and have a distinct preapical pale band on the hind tibia (King et al. 1960, Burkett-Cadena 2013, photo, p. 127).

Specimens reported here were identified using Darsie and Ward (2005), Burkett-Cadena (2013), and Harrison et al. (2016). Mansonia dyari and Ma. titillans adults can be difficult to differentiate. Three of the characters used to identify adult females of Mansonia species in the USA can be ambiguous due to individual variation or slight differences between the species. These ambiguous characters are: 1) the length of the palpi in relation to the proboscis (Burkett-Cadena 2013, Harrison et al. 2016); 2) the ventral surface of the proboscis with a patch of pale scales or being mostly dark-scaled (Darsie and Ward 2005); and 3) the arrangements of peg-like spiniform setae on the lobes of abdominal tergum VIII.

Two other characters found only on Ma. titillans, as described in Belkin et al. (1970), were found to be less ambiguous and more reliable: 1) the presence of a horizontal row of small preapical spiniform setae just before the posterior margin of abdominal tergum VII; and 2) the presence of a median patch of a few large black scales on the 1st antennal flagellomere. The presence of these 2 characters, which were found to be 100% accurate based on the examination of numerous specimens from Mississippi, Georgia, and South Carolina (Harrison et al. 2016), were used to identify specimens in this study, including badly rubbed specimens. The row of small preapical spiniform setae found on tergum VII of Ma. titillans, but not Ma. dyari, must be examined under at least 55× magnification for visibility and can be obscured by large apical scales, which normally need to be removed to reveal the spiniform setae underneath them. These tergal VII setae are illustrated in Darsie and Ward (2005, fig. 473), Burkett-Cadena (2013, illus. on p. 39), Varnado et al. (2014, fig. 6a), and Harrison et al. (2016, fig. 267). The abdominal tergum VII spiniform setae are found only on Ma. titillans and should not be confused with abdominal tergum VIII setae, which are used by females of both species during oviposition to clear detritus from the leaf surface to allow proper adhesion of eggs to the leaf (Linley 1989). Linley (1989) hypothesized that tergal VII spiniform setae are found only on Ma. titillans because the setae might be needed to stabilize the abdomen's position on the leaf to prevent tergal VIII setae, which on Ma. titillans are larger and more organized than those found on Ma. dyari, from damaging the leaf surface during oviposition. The tergal VIII setae on Ma. titillans might be larger for the purpose of clearing the more heavily detritus-laden undersides of aquatic plant leaves (Linley 1989). Mansonia titillans deposits eggs only on the underside of leaves of free-floating plants, whereas Ma. dyari will deposit eggs on the upper- or underside of leaves (Lounibos and Dewald 1989).

Similar to Cq. perturbans, Mansonia larvae do not surface to obtain oxygen, but instead obtain oxygen by using a pointed siphon to pierce roots of aquatic plants (Horsfall 1955, King et al. 1960). Water lettuce (Pistia stratiotes L.) and water hyacinth (Eichhornia crassipes Martius) are free-floating aquatic plants commonly associated with Ma. titillans oviposition (Eads 1950, Horsfall 1955, King et al. 1960). These 2 plants are included in the South Carolina Noxious Weed List (SCDNR 2010), making their possession, import, sale, and distribution illegal in the state, though they remain popular water-garden items (SCDNR 2008).

The United States Geological Survey (USGS) keeps records of reported nonindigenous aquatic species, accessible via an interactive map. Because the USGS relies on publications, state or local reporting, and citizen science to track invasive aquatic plants, some plants from smaller ponds and remote coves might not be reported (USGS 2017). According to the South Carolina Department of Natural Resources, water hyacinth existed in South Carolina prior to 1980, with the largest concentration in water bodies near Charleston (Fig. 1). Water hyacinth has since spread south to the Savannah River, north to the Waccamaw and Pee Dee rivers, and northwest to upper Lake Marion (SCDNR 2008) (Fig. 1).

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Water lettuce was first found in Georgetown County on the Waccamaw River near Brookgreen Gardens (Fig. 1) in 1991, although it was presumed to have been unable to survive the cold winter temperatures after its initial discovery. Water lettuce is now limited to the Goose Creek Reservoir in Berkeley County (SCDNR 2008) (Fig. 1). Leaves of water hyacinth and water lettuce can be killed at freezing temperatures; however, water hyacinth is able to regrow from stem tips below the water's surface, whereas water lettuce must rely on regrowth from its seeds, which can only survive for approximately 2 months in submerged water at 4°C (Jacono et al. 2018, Thayer et al. 2018). In tropical locations like Central and South America and the Florida Keys, USA, other mosquito species are also associated with water lettuce, including Culex erraticus (Dyar and Knab) (Lounibos and Escher 1985), Aedeomyia squamipennis (Lynch Arribalzaga) (Burkett-Cadena and Blosser 2017), and Cx. panocossa Dyar (Blosser and Burkett-Cadena 2017).

Mansonia titillans adults were collected from August to December 2017 at 8 sites in 5 counties in the Coastal Plain of South Carolina (Fig. 1). Twenty-one specimens of Ma. titillans adult females were collected from Cottageville (32°55′25.1″N, 80°28′50.96″W) in Colleton County on September 20. Single specimens of Ma. titillans adults were collected in South Carolina in 2017 from Manning (33°39′35.87″N, 80°10′20.32″W) and Summerton (33°30′50″N, 80°18′57″W) in Clarendon County on September 5 and October 12, respectively, as well as from Pawleys Island (33°28′22.71″N, 79°06′56.62″W) in Georgetown County on September 13 and Moncks Corner (33°05′02.9″N, 80°01′30.96″W) in Berkeley County on October 2. The locations in Berkeley, Clarendon, and Colleton counties are relatively undeveloped and mainly consist of farmland, forests, and small freshwater ponds. The site in Georgetown County is in a populated area with small, private freshwater ponds.

In addition to the 4 new county records reported here, Beaufort County Mosquito Control collected small numbers of Ma. titillans adults from 5 different sites in Beaufort County (Fig. 1). Single adult specimens were collected from Yemassee (32°39′37.38″N, 80°43′01.06″W) and Port Royal (32°23′35.51″N, 80°40′58.04″W) on September 15 and November 3, 2015, respectively. Several specimens were collected from St. Helena Island (32°17′26.33″N, 80°38′38.73″W) on July 12, 2016, August 22, 2017, and December 5, 2017. Several specimens were also collected from Bluffton (32°16′49.18″N, 80°48′20.19″W) on September 24, 2017, and Ladys Island (32°24′33.06″N, 80°39′36.66″W) on September 24 and December 12, 2017. Two of the collection sites in Beaufort County are rural with one adjacent to a freshwater swamp, and the other, a salt marsh. The 3 other sites in the county are in populated areas with small, private freshwater ponds.

The newly documented collections of Ma. titillans in northern counties confirms the presumption of Moulis et al. (2015) that Ma. titillans populations could be expanding north. Failure to detect Ma. titillans in these new, more northern areas prior to now might be due to limited surveillance. Prior to 2017, only the location in Moncks Corner (Berkeley County) had been sampled routinely in the past 10 years, but no Ma. titillans were detected. The seasonality of Ma. titillans adults in South Carolina is currently unknown, but comparisons of recent and prior collections to those in Georgia (Moulis et al. 2015) suggest that Ma. titillans adults are probably most active in summer and fall (Table 1).

Table 1. Summary of Mansonia titillans seasonality in Georgia and South Carolina.¹

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The presence of water hyacinth was visually confirmed in 3 areas within 2 mi of the collection sites in Moncks Corner and Manning, but no Ma. titillans larvae or pupae were collected. Although the immature stages of this species are more difficult to collect than most other mosquito species, more extensive larval surveillance should be conducted, considering that this species might be more prevalent in South Carolina than suspected. More research is needed to determine the habitat range, seasonality, and other possible host-plant associations of Ma. titillans in South Carolina.

We want to thank Jeff Cary of Berkeley County Mosquito Control, Randall Lowder and Matt Maxwell of South Carolina Department of Health and Environmental Control, and Kathy Tallman of the Santee Cooper Vector Management for assisting us with field collections. Funding was provided in part by the Centers for Disease Control and Prevention, Epidemiology and Laboratory Capacity (ELC) for Infectious Diseases Cooperative Agreement.