Challenging tramps
Published 1 November 2023
As our world became increasingly interconnected over the past several decades, biosecurity became an increasingly significant concern. The identification of newly arrived exotic pests and the potential mitigation of their impact has become a global hot topic. Ants have been a particular focus in this regard, both because of ease with which they may spread and establish, and the potential severity of their economic and environmental impact when they do. Which is all just to say: beware Nylanderia.
Nylanderia is one of several forms of ant (not all directly related) commonly referred to as ‘crazy ants’ in reference to their seemingly directionless foraging behaviour. Until recently, Nylanderia was treated as a synonym of the related genus Paratrechina but recognition that the latter was polyphyletic lead to its resurrection, taking the larger part of the ‘Paratrechina’ species with it. As redefined, Nylanderia species are small to medium-sized ants, typically not strongly sculpted, with long antennae. Defining features of the genus include mandibles with six (rarely seven) teeth, and paired macrosetae on the pronotum and mesonotum but very rarely on the propodeum (LaPolla et al. 2011).
Well over 100 species of Nylanderia are known to date, with representatives found almost worldwide and among the most commonly collected ants in leaf litter. The fast-moving Nylanderia are often among the first ants to make their way to sources of food, but tend to give way to later-arriving but more aggressive ant species. A number of Nylanderia species are regarded as ‘tramps’, readily spreading alongside humans into new areas where they may become significant pests. Many Nylanderia species are known to be polydomous—that is, a colony occupies multiple nests. At least one species, N. flavipes, is known to be both polydomous and monogynous—despite a colony occupying multiple nests, only one queen is present in the colony (LaPolla et al. 2011). Other species may be polygynous with multiple queens per colony. An extreme example may be provided by invasive populations of the tawny crazy ant N. fulva (a South American native) in the southern United States. Studies of the colony structure of this species by Eyer et al. (2018) found that its entire US range may represent a single supercolony extending over more than 2000 km. Each nest of this supercolony was home to several queens, sometimes more than hundreds of them, each of which was mated with a single male.
Nylanderia species are notoriously difficult to distinguish; workers within a species are often variable whereas features distinguishing species are often obscure. Again, a notable example of this is provided by N. fulva. This species is all but indistinguishable from another exotic species that has made inroads in the US, N. pubens (like N. fulva, N. pubens is a native of South America though its native range is more northerly and also includes Central America and the Caribbean). The two can only be distinguished morphologically by features of the male genitalia, something that requires actually having males on hand to examine. They also seem to differ in nesting habits. Nylanderia fulva is far more invasive than N. pubens, sometimes reaching plague numbers. Wetterer et al. (2014) describe various examples of infestations, including a house on the island of St Croix surrounded by ‘mats’ of dead ants and with the walls covered ‘inside and out’, and a house in Florida with ants nesting in the plumbing: ‘when JKW turned on the bath-tub faucet, thousands of N. fulva poured out’. Eventually, however, these plague infestations tend to collapse of their own accord. Nylanderia pubens appears not to exhibit such extreme booms and busts of population, rather staying at lower numbers throughout.
Obviously, if different lineages may be so different in their impact despite being so difficult to distinguish, it would be useful to know just which you are dealing with. Williams et al. (2022) examined the possibility of using molecular data to distinguish tramp species within their respective complexes. In some cases, molecular data matched recognized species boundaries, in other case, they did not. Notably, molecular data did not clearly distinguish US samples of N. fulva and N. pubens. Are the two not truly distinct? Are they distinct in their native ranges but blurred by hybridisation in their invasive ranges? The implications of either are worth considering.
References
Eyer, P.-A., B. McDowell, L. N. L. Johnson, L. A. Calcaterra, M. B. Fernandez, D. Shoemaker, R. T. Puckett & E. L. Vargo. 2018. Supercolonial structure of invasive populations of the tawny crazy ant Nylanderia fulva in the US. BMC Evolutionary Biology 18: 209.
LaPolla, J. S., S. G. Brady & S. O. Shattuck. 2011. Monograph of Nylanderia (Hymenoptera: Formicidae) of the world: an introduction to the systematics and biology of the genus. Zootaxa 3110: 1–9.
Wetterer, J. K., O. Davis & J. R. Williamson. 2014. Boom and bust of the tawny crazy ant, Nylanderia fulva (Hymenoptera: Formicidae), on St. Croix, US Virgin Islands. Florida Entomologist 97 (3): 1099–1103.
Williams, J. L., Y. M. Zhang, J. S. LaPolla, T. R. Schultz & A. Lucky. 2022. Phylogenomic delimitation of morphologically cryptic species in globetrotting Nylanderia (Hymenoptera: Formicidae) species complexes. Insect Systematics and Diversity 6 (1): 10.