Behavioural and chemical evidence for multiple colonisation of the Argentine ant, Linepithema humile, in the Western Cape, South Africa

Abstract

BackgroundThe Argentine ant, Linepithema humile, is a widespread invasive ant species that has successfully established in nearly all continents across the globe. Argentine ants are characterised by a social structure known as unicoloniality, where territorial boundaries between nests are absent and intraspecific aggression is rare. This is particularly pronounced in introduced populations and results in the formation of large and spatially expansive supercolonies. Although it is amongst the most well studied of invasive ants, very little work has been done on this ant in South Africa. In this first study, we investigate the population structure of Argentine ants in South Africa. We use behavioural (aggression tests) and chemical (CHC) approaches to investigate the population structure of Argentine ants within the Western Cape, identify the number of supercolonies and infer number of introductions.ResultsBoth the aggression assays and chemical data revealed that the Western Cape Argentine ant population can be divided into two behaviourally and chemically distinct supercolonies. Intraspecific aggression was evident between the two supercolonies of Argentine ants with ants able to discriminate among conspecific non-nestmates. This discrimination is linked to the divergence in cuticular hydrocarbon profiles of ants originating from the two supercolonies.ConclusionsThe presence of these two distinct supercolonies is suggestive of at least two independent introductions of this ant within the Western Cape. Moreover, the pattern of colonisation observed in this study, with the two colonies interspersed, is in agreement with global patterns of Argentine ant invasions. Our findings are of interest because recent studies show that Argentine ants from South Africa are different from those identified in other introduced ranges and therefore provide an opportunity to further understand factors that determine the distributional and spread patterns of Argentine ant supercolonies.

Cuticular hydrocarbons correlate with queen reproductive status in native and invasive Argentine ants (Linepithema humile, Mayr)

“…Furthermore, neither the di-methyl alkane 5,11-diMeC 31 nor the alkene C 29:1 have been observed in the CHCs of workers from the main European supercolony [ 15 ]. In addition, the alkene C 29:1 —which showed the highest degree of correlation with fertility—was completely absent in workers from other Argentine ant supercolonies, namely the Corsican, the Catalan, the Californian, the Japanese, the South African, and the Australian supercolonies [ 15 , 29 , 31 , 32 , 37 , 52 ]. This finding supports the idea that this particular CHC is characteristic of the Argentine ant queen’s CHC profile (as suggested by de Biseau and colleagues [ 42 ]) and thus plays an important role in signalling fertility.…”

Recent range expansion of the Argentine ant in Japan

“…It is possible that more haplotypes and supercolonies could be found near ports in these other regions. Further research in introduced ranges may contribute to finding new supercolonies, as was the case in South Africa (Mothapo & Wossler, ).…”

When invasive ants meet: effects of outbreeding on queen performance in the tramp ant Cardiocondyla itsukii

“…This opens a new avenue for empirical research into the consequences of multiple introductions of invasive ants. Previous studies have documented how the colonization of new areas by colonies originating from different source populations leads to the establishment of aggressively defended borders between different supercolonies (e.g., Thomas et al ., , ; Frouz et al ., ; Mothapo & Wossler, ), but the frequency of crossbreeding between sexuals from different supercolonies and its consequences have rarely been investigated (but see Sunamura et al ., ). Gene flow between colonies introduced from multiple geographical sources might help invasive species to overcome the genetic bottlenecks resulting from the accidental introduction of only few individuals into new areas (Suarez & Tsutsui, ).…”

“…For example, in Argentine ants, Linepithema humile , some populations have enormous “supercolonies” of connected, genetically similar nests among which ants can be exchanged without aggression (e.g., Giraud et al ., ; Tsutsui et al ., ). In contrast, in other introduced populations, the cooccurrence of several mutually hostile and genetically distinct “supercolonies” suggests multiple introductions (Giraud et al ., ; Tsutsui et al ., ; Thomas et al ., , ; Björkmann‐Chiswell et al ., ; Sunamura et al ., ; Mothapo & Wossler, ). In general, repeated colonization events may increase invasiveness by counteracting the reduction of genetic variability associated with founder effects (Sakai et al ., ; Lavergne & Molofsky, ; Wilson et al ., ).…”