New-World Spread of the Old-World Robust Crazy Ant , Nylanderia bourbonica ( Forel ) ( Hymenoptera : Formicidae )

Forel (1911) listed 15 tramp ant species, spread by human commerce, which had achieved or were in the process of achieving broad cosmopolitan distributions. Over the past 100 years, many additional ant species, not on Forel’s (1911) original list, have attained broad distributions in both the Old World and New World (Wetterer, 2015). Here, I examine the geographic distribution of one of these new cosmopolitan species, Nylanderia bourbonica (Forel) (formerly Paratrechina bourbonica). This species, native to the Old-World tropics and subtropics, is now spreading through parts of North America and the West Indies. Abstract The robust crazy ant, Nylanderia bourbonica (Forel) (formerly Paratrechina bourbonica), is native to the Old-World tropics and subtropics. Its earliest known record in the New World was collected in 1924 in Miami, Florida. Here, I examine the subsequent spread of this species to other parts of North America and the West Indies. I compiled published and unpublished New World N. bourbonica specimen records from 446 sites, documenting the earliest known records for 24 geographic areas (countries, island groups, major islands, and US states), including nine for which I found no previously published records: Anguilla, Antigua, Barbuda, British Virgin Islands, Jamaica, Turks and Caicos Islands, Missouri, New York, and Washington. The vast majorityof New World site records for N. bourbonica (89%) come from Florida, where this species is now known from 37 counties. Most, if not all, of the 14 site records of N. bourbonica in North American north of 30.5°N come from indoors. Although the earliest record of N. bourbonica from Cuba dates to 1933, the spread of N. bourbonica to many West Indian islands appears to be much more recent. In Florida, N. bourbonica is a widespread, though relatively minor household and agricultural pest, and also is common in some more natural environments. It remains to be seen whether N. bourbonica will become a significant pest in the West Indies or elsewhere in the New World. Sociobiology An international journal on social insects


Introduction
In 1924, Alexander E. Wight collected the earliest known N. bourbonica specimens in the New World in Miami, Florida (Wheeler, 1932). Smith (1930) reported that N. bourbonica "workers were observed running over sand and the pavement of sidewalks at Miami. It would appear that this species is not only well established at Miami but that it is capable of living outdoors. Whether the species will prove to be a house infesting form is not known, but it would appear that there might be a strong possibility." Nylanderia bourbonica has since become a common household and agricultural pest throughout much of peninsular Florida (Klotz et al., 1995). Trager (1984) reported that N. bourbonica was "established outdoors in the US in peninsular Florida from Gainesville Florida Atlantic University, Jupiter, Florida, USA RESEARCH ARTIClE -ANTS New-World Spread of the Old-World Robust Crazy Ant, Nylanderia bourbonica (Forel) (Hymenoptera: Formicidae) south. I have also seen specimens from Mobile, Alabama which were probably collected outdoors". In addition to Florida and Alabama, Trager (1984) plotted N. bourbonica site records from Kansas, South Carolina, Texas, Cuba, and Mexico on a distribution map.
The genus is named in honor of William Nylander (1822-1899), a Finnish naturalist. The species is named after Île Bourbon (now Réunion island), where the type specimens were collected. One common name for N. bourbonica is the robust crazy ant. Robust refers to its relatively large and "husky" size compared to other species in the genus; crazy refers to the fast, erratic movements typical of Nylanderia species.
Geographic coordinates for collection sites came from published references, specimen labels, maps, or geography web sites (e.g., earth.google.com, www.tageo.com, and www. fallingrain.com). If a site record listed a geographic region rather than a "point locale," and I had no other record for this region, I used the coordinates of the largest town within the region or, in the case of small islands and natural areas, the center of the region. I mapped site records using carto.com.
The northernmost record in Florida, collected at Florida Community College North (now called Florida State College at Jacksonville North Campus) in Jacksonville, Florida (30.4°N;20-Apr-1991  Nylanderia bourbonica was once included on an online list of the ants of Georgia, but was removed when no voucher specimens could be found (J.A. MacGown, pers. comm.). Antweb once listed N. bourbonica as in Illinois, but this list has been removed. Boer (2019) listed N. bourbonica from Curaçao, but more recently Boer (2021a) did not include it, and Boer (2021b) wrote "N. bourbonica unknown from the ABC islands" (i.e., Aruba, Bonaire, and Curaçao).

Discussion
Most N. bourbonica site records in New World come from Florida (89%; Figs 1 and 2). Given how widespread N. bourbonica is in peninsular Florida, it is surprising that it has been so rarely collected elsewhere in the New World (Figs 1 and 2). Although the first record of N. bourbonica from Cuba was less than a decade after the earliest records in Florida, the spread of N. bourbonica to many other West Indian islands appears to be recent (Table 1). While this may be, in part, an artifact of limited collecting on some islands, it is striking that N. bourbonica has not yet been recorded on Puerto Rico and Hispaniola, where there has been much ant research in the past few decades (e.g., see Torres & Snelling, 1997;Lubertazzi, 2019).
Since 1972, when N. bourbonica was first collected in Gainesville (S.D. Porter, pers. comm.), there have been only four Florida records of this species north of Gainesville (>29 .7°N; Fig 1; see Results). This suggests that N. bourbonica populations have shown little northward spread in Florida over the past 48+ years, and that the northern most outdoor populations in Florida may be near their climatic limits in terms of cold weather. Deyrup (2016), in fact, noted that N. bourbonica "records from Mobile, Alabama (Trager, 1984), and from Jacksonville, Florida, might represent populations in urban sites protected from cold weather." In the New World, there are only 12 records of N. bourbonica from sites north of 30.5°N, and at least nine of these were collected indoors (see results).
In Florida, N. bourbonica is now a widespread household and agricultural pest (Klotz et al., 1995). Wilson (1964) reported that N. bourbonica "workers were commonly seen foraging during the day at several locations on the streets of residential sections of Key West." Although Vail et al. (1994) reported that N. bourbonica is a major structure-invading pest in South Florida, Deyrup (2016) wrote "its chief crime seems to be that of traipsing about on the patio, where it seems to be a regular but not strikingly abundant visitor." Deyrup (2016) wrote: "In natural habitats in Florida, bourbonica is most often found in wet areas that are naturally disturbed, such as the edges of marshes and upper zones of beaches." In fact, Wetterer et al. (2007) found N. bourbonica was the second most common ant (after Solenopsis invicta Buren) collected at tuna bait on sea turtle nests on beaches in Palm Beach County, Florida.
Several other Nylanderia species have been spread around the world through human commerce (Wetterer, 2011;Williams et al., 2020). One species, Nylanderia fulva (Mayr), originally from South America, has become a significant pest in parts of the southeastern US and in the West Indies, where it can have enormous, localized population explosions followed by population crashes (Wetterer et al., 2014). It remains to be seen whether N. bourbonica will become a significant pest in Florida, the West Indies, or elsewhere in the New World.