1 | INTRODUCTION
Invasive ants often reach extremely high densities, outcompete and prey
upon native species, disrupt mutualisms, and lower ecosystem
biodiversity (Berman, Andersen, Hély, & Gaucherel, 2013; Holway, 1998;
Holway, Lach, Suarez, Tsutsui, & Case, 2002; LeBrun, Abbott, &
Gilbert, 2013; McGlynn, 1999; Porter & Savignano, 1990). Understanding
the factors that promote the success of invasive ants is critical to
discerning and managing their ecological impacts. Characteristics that
are thought to play a role in the success of many invasive ant species
include reduced nestmate recognition and an absence of boundaries
between unrelated nests (Eyer & Vargo, 2021; Holway et al. 2002;
Passera 1994). By avoiding the costs of colony defense and competition
against neighboring conspecifics, these invasive ant species can reach
higher densities (Giraud, Pedersen, & Keller, 2002; Porter, Fowler, &
Mackay, 1992) and achieve greater ecological dominance by more
effectively outcompeting other species (Holway et al., 2002; Holway &
Suarez, 2004; LeBrun et al., 2013). For example, the number of Argentine
ant workers (Linepithema humile ) was approximately 50-fold higher
in sites where nests were interconnected compared with sites where nests
defended distinct boundaries and competed with conspecifics (Holway &
Suarez, 2004). Consequently, interconnected nests of Argentine ants more
effectively outcompeted native ants, as native ant species richness was
reduced by over 50% compared with sites where nests competed with each
other (Holway & Suarez, 2004). Reduced intraspecific competition has
also been implicated in the success and ecological impacts of other
invasive and non-invasive social insects (Hanna et al., 2014; Korb &
Foster, 2010; Perdereau et al., 2015, Wilson, Mullen, & Holway, 2009).
Colony boundaries and nestmate recognition are typically delimited using
worker aggression assays, but there is increasing evidence that workers
can discriminate nestmates from non-nestmates without an aggressive
response (Breed 2003). For example, although Argentine ant workers do
not aggressively attack non-nestmates from within the same supercolony
(Giraud et al., 2002; Tsutsui, Suarez, Holway, & Case, 2000), they
spend more time antennating non-nestmates than nestmates
(Björkman-Chiswell, Van Wilgenburg, Thomas, Swearer, & Elgar, 2008),
indicating nestmate recognition despite a lack of aggression. Perhaps as
a consequence of nestmate recognition, sharing between Argentine ant
nests was consistently limited to distinct clusters of nests within a
single supercolony over a three-year-period (Heller, Ingram, & Gordon,
2008). By preferentially sharing food resources with nestmates over
non-nestmates, workers may increase their inclusive fitness,
particularly if nestmates are more related to them (Hamilton, 1964;
Helanterä et al., 2009). Because aggression bioassays do not always
reliably indicate nestmate recognition and colony boundaries, it is
important to use alternative methods to assess intraspecific
interactions in the field, such as genetic analyses and direct
quantification of resource exchange between nests (Ellis, Procter,
Buckham-Bonnett, & Robinson, 2017). Assessing some of the more subtle
interactions between and within nests improves our understanding of the
factors enhancing ant invasions and has important implications for
invasive ant management.
We examined worker interactions between nestmates and non-nestmates in
red imported fire ants (Solenopsis invicta ; hereafter fire ants).
Fire ants occur in two social forms: the polygyne form (i.e., colonies
with multiple egg-laying queens) and the monogyne form (i.e., colonies
with only a single egg-laying queen; Gotzek, Shoemaker, & Ross, 2007;
Ross, 1993; Ross, Vargo, & Keller, 1996; Tschinkel, 2006). These two
social forms are under the control of an inversion-based supergene,
which spans over 13 Mb of a “social chromosome” (Muers 2013; Wang et
al., 2013). This social chromosome contains over 400 protein-coding
genes (including Gp-9, which has been used as a marker to estimate the
social form of the colonies), and experiences greatly reduced
recombination (Arsenault et al., 2020; Yan et al., 2020). Interestingly,
the fire ant supergene (and all the complex traits associated with it)
exhibits two haplotypes (SB and Sb), which are passed on
via Mendelian inheritance (Arsenault et al., 2020; Keller & Ross, 1998;
Ross & Shoemaker, 2018; Wang et al., 2018). Colonies bearing theSb supergene haplotype express the polygyne phenotype (i.e.,
colonies accept multiple SB/Sb queens and reject any SB/SBqueens); whereas colonies bearing exclusively the SB haplotype
express the monogyne phenotype (i.e., colonies accept only oneSB/SB queen and reject all SB/Sb queens; Arsenault et al.,
2020; Gotzek & Ross, 2008, 2009; Ross & Keller, 2002). Moreover,
supergene control appears to be complete, as social organization is
independent of genetic diversity within the colony (Gotzek & Ross,
2008), as well as non-genetic factors such as environmental odors, queen
reproductive status, and prior social experience of the workers (Gotzek
& Ross, 2007; Ross & Keller, 2002).
Because colonies require only a small number (10-15%) of workers of theSb haplotype to express the polygyne phenotype, workers appear to
regulate social organization (Gotzek & Ross, 2008). One possible
mechanism for worker control includes habituating the colony to an odor
unique to b -carrying adults (Gotzek & Ross, 2008). This
hypothesis is supported by a lack of nestmate recognition between
polygyne workers in the field (Vander Meer, Obin, & Morel, 1990) and a
supposed absence of colony boundaries within polygyne populations
throughout their invaded range in the USA (Bhatkar & Vinson, 1987). As
a consequence, North American polygyne fire ant populations are often
referred to as unicolonial (e.g., Greenberg, Vinson, & Ellison, 1992;
Holway et al., 2002; Morel, Vander Meer, & Lofgren, 1990; Plowes, Dunn,
& Gilbert, 2007; Porter et al., 1992; Vander Meer, Obin, & Morel,
1990). The exchange of workers and resources between nests in polygyne
fire ants is thought to correspond with a greater abundance compared
with the monogyne form due to reduced intraspecific competition (Porter,
Bhatkar, Mulder, Vinson, & Clair, 1991). For example, polygyne mounds
were over twice as abundant on average compared with monogyne mounds in
Texas (mean ± SE: 680 ± 475 polygyne mounds/ha vs. 295 ± 240 monogyne
mounds/ha; Porter et al., 1991). The greater abundance of the polygyne
form may increase the likelihood of ants interacting with and preying
upon native species, thereby increasing their ecological impact (Allen,
Epperson, & Garmestani, 2004; Porter & Savignano, 1990).
Despite the assumption that polygyne nests are highly interconnected
(see Bhatkar & Vinson, 1987), the physical exchange of workers and
resources between polygyne nests in the field is poorly documented.
Moreover, although polygyne workers from different nests do not
aggressively attack each other in bioassays (Vander Meer et al., 1990),
their interactions within the nest are relatively unknown, particularly
in the case of worker-brood interactions. Prior research on
within-colony interactions has focused almost exclusively on
worker-queen interactions (DeHeer & Ross, 1997; Gotzek & Ross, 2008;
Ross & Keller, 2002), but worker-brood interactions are also critical
to colony dynamics and can differ from worker-queen interactions. For
example, although Formica argentea workers in polygyne colonies
show no preference towards related or unrelated queens, they
preferentially care for brood that are more closely related to them
(Snyder, 1993). Within-colony relatedness between polygyne fire ant
workers is often near zero throughout their invaded range in the USA
(DeHeer & Ross, 1997; Goodisman, Sankovich, & Kovacs, 2007; Ross,
1993; Ross & Fletcher, 1985; Ross et al., 1996), but workers may
increase their inclusive fitness by preferentially caring for more
related brood.
Our study tests fundamental assumptions about inter- and intracolonial
interactions in introduced populations of fire ants. First, we compared
colony boundaries between the two social forms in the field. To delimit
boundaries between colonies, we quantified the exchange of a15N-glycine tracer dissolved in a sucrose solution and
correlated this exchange with colony genetic structure. Using a labeled
resource in combination with genetic data allows for two different ways
to define colony boundaries (Ellis et al., 2017). We also examined
polygyne brood-tending behaviors towards nestmates and non-nestmates in
the laboratory. By studying interactions between and within colonies ofS. invicta , we further elucidate the primary factors influencing
the ecology and success of this invasive species.