Pheidole megacephala: An invasive ant that raids colonies of the red imported fire ant, Solenopsis invicta

Abstract

The establishment and spread of many imported ant species are facilitated by a high level of reproduction, the future queens mating without nuptial flight so that the colonies reproduce through budding. The colonies are polygynous (i.e., multiple queens) and form unicolonial societies that spread over large areas and reach high densities, often outcompeting native ants through ecological dominance (Suarez et al., 2010; Wong et al., 2023).

These characteristics are present in the well-known invasive species Pheidole megacephala, or the big-headed ant, native to tropical Africa. Like most Pheidole species, its worker caste is dimorphic with small minors and big-headed majors or “soldiers,” both having small stingers that serve only to lay odor trails (Wilson, 2003). In its introduced range, this species forms large, unicolonial societies to the point that no intraspecific aggressiveness between workers was noted in different geographical areas of Australia over 3000 km (Fournier et al., 2009). Consequently, P. megacephala outcompetes native ants through interference competition and especially by raiding their colonies (Figure 1), a characteristic inherited from its native range that even permits it to resist the assault of army ants (Dejean et al., 2008, 2014; Dejean, Kenne, et al., 2007; Dejean, Moreau, et al., 2007; Sarnat et al., 2015; see Kamaru et al., 2024 for an indirect effect).

Unlike most other invasive ants, the red imported fire ant, Solenopsis invicta, is monogynous. Native to South America, it was imported into the United States in the 1930s around Mobile, Alabama. However, polygynous populations were found in Mississippi in 1973 that differed by the “social chromosome” regulating the social form; the smaller queens of the polygynous form can reproduce and spread through budding. Although the monogynous form is dominant in the United States, both forms can participate in mating flights (Arsenault et al., 2020; Kjeldgaard et al., 2022; Tschinkel, 2013). Colonies are omnivorous like other invasive ants and consume sugary food, mostly the honeydew of hemipterans. They also scavenge rather than being predatory and displace other ants through competition. Finally, it is considered one of the most costly invasive species worldwide, impacting natural ecosystems, agriculture, man-made structures, and human health (Chen et al., 2020; Menchetti et al., 2023; Tschinkel, 2013).

In Florida, USA, the prevalence of both P. megacephala and S. invicta populations in certain invaded habitats makes it likely that these two dominant and invasive species encounter each other regularly (see Booher et al., 2023), but the nature of their interactions has not been clarified. Knowledge about such interactions could elucidate how dominant invasive species co-exist and partition an environment.

The area studied is in a highly disturbed suburban environment located in the town center of a gated community in Poinciana, Florida. This area and its surroundings are overrun by several invasive species, including P. megacephala, Pheidole sp. cf. parva, and S. invicta. Ant specimens were placed in 91% isopropyl alcohol for later identification by Douglas B. Booher (USDA Forest Service, Southern Research Station, Athens, GA 30602, USA). DNA was extracted for the identification of Pheidole sp. cf. parva, which is new to the area.

To determine the precise territory occupied by P. megacephala, we first located typical nest mounds or the semi-permanent trail networks of the species (see fig. 10 in Warner & Scheffrahn, 2007). Then, we placed baits (cookie crumbs, honey, and pieces of sausage) every 1–5 m to have an idea of the possible dimensions of the colony's territory. Finally, we transferred P. megacephala workers from the farthest points of the territory to confirm that they belonged to the same colony. Indeed, all transferred workers were accepted by resident individuals and generally followed the same paths and/or entered new nests unharmed. This territory covered an irregular area of two hectares, with the majority of the nest entrances situated along the town center's Village Drive (Appendix S1: Figure S1 on which we also noted the location of S. invicta mounds).

Aggressive interactions involving the P. megacephala colony and other ant species were noted during daily observations extending from June to November 2024. Special attention was paid to the periphery of the colony where contact between P. megacephala workers and other ant species was most common. Details of these interactions are described below.

In Florida, P. megacephala populations increased between 1965 and 2018 (see fig. S2 in Booher et al., 2023), but concerning the size of the colonies' territory, we can only assert that there is no intraspecific aggressiveness between workers over 2 ha. Because we know that in Australia no aggressiveness was noted over 3000 km (Fournier et al., 2009), complementary studies are needed to determine more precisely the size of their territories. However, we noted here that the colony studied is large enough to raid small ant colonies, confirming previous results (Dejean et al., 2008; Dejean, Kenne, et al., 2007), as well as the large S. invicta colonies, themselves invasive. We cannot exclude the possibility that, due to their large size, S. invicta colonies can limit the impact of these raids underground by sealing galleries, a method employed by the invasive subterranean Formosan termite, which is able to limit P. megacephala's preying activity (Chouvenc et al., 2015). Thus, further studies are needed to verify whether P. megacephala is able to destroy entire S. invicta colonies by raiding them.

In conclusion, for a predatory ant that raids other ant species, the larger the prey colony, the more profitable the raid. If by doing so the predator also extends its territory, as noted here, both predation and territoriality are involved. Rather than “interference competition,” this most closely resembles “intraguild predation” because the predator gains an energetic reward while reducing competition for space. Indeed, the two main species involved in this study belong to the same guild (large, omnivorous, ground-dwelling territorial colonies). S. invicta is a poor predator, while P. megacephala is one of the most effective ant predators as it is able to prey on other ants whatever their size and their venom composition, especially when its colonies reach a certain size.

The authors declare no conflicts of interest.