One of the best-known symbioses in the Neotropics is the association between ant-acacias and Pseudomyrmex ants that live in the acacia’s hollow thorns. We surveyed ants on two species of ant-acacia, Acacia cornigera (L.) and Acacia sphaerocephala Schlechtendal & Chamisso, growing outside their native range at five sites in Florida. We found eleven ant species: five native Florida ants (Brachymyrmex sp. nr. obscurior, Camponotus floridanus (Buckley), Pseudomyrmex cubaensis (Forel), Pseudomyrmex ejectus (Smith), and Pseudomyrmex elongatus (Mayr)), two Neotropical exotics (Camponotus sexguttatus (Fabr.) and Pseudomyrmex gracilis (Fabr.)), and four Old World exotics (Monomorium floricola (Jerdon), Paratrechina longicornis (Latreille), Pheidole megacephala (Fabr.), and Technomyrmex albipes (Smith)). Only the two Neotropical exotics, Ps. gracilis and C. sexguttatus, inhabited thorns with holes that appeared to have been perforated by ants as entrances. For Ps. gracilis, and perhaps also for C. sexguttatus, their association with ant-acacias in Florida represents the reconstitution in an exotic locale of a facultative symbiosis evolved in the Neotropics.
One of the best-known Neotropical symbioses is the association between Acacia trees and Pseudomyrmex ants (Belt 1874; Janzen 1966; Janzen 1967; Hölldobler & Wilson 1990). Thirteen Neotropical Acacia species are “ant-acacias,” specialized myrmecophytes that house ants in their thorns and provide ants with extrafloral nectaries and nutritious Beltian bodies (Seigler & Ebinger 1995). Thirteen Neotropical Pseudomyrmex ant species obligately live in ant-acacias. Nine of these species vigorously defend the Acacia from herbivory and overgrowth by vines, whereas the other four provide little or no defense (Ward 1993). In addition, numerous other ant species live opportunistically in ant-acacias, but also nest elsewhere, typically in hollow twigs (Wheeler 1913). Only two of these facultative symbionts, Pseudomyrmex gracilis (Fabr.) and Camponotus planatus Roger, are known to show specialized behaviors in exploiting ant-acacia thorns.
The present study was motivated by our observation in September 1999 of Pseudomyrmex ants living in an ant-acacia tree growing at Mounts Botanical Garden in West Palm Beach, Florida. We noticed a thorn on an Acacia cornigera (L.) tree that had a round hole indicative of an ant entrance. Breaking open the thorn, we found it full of adult Pseudomyrmex gracilis ants and brood. This unexpected discovery of ants inhabiting domatia of a non-native myrmecophyte contrasted with an earlier finding concerning another well-known Neotropical myrmecophyte, Cecropia obtusifolia Bertol. In Hawaii, where neither Cecropia nor ants are native, Wetterer (1997) found no ants inhabiting the hollow trunks and branches of the exotic Cecropia obtusifolia trees that grow abundantly in the disturbed lowlands.
In the present study, we wished to determine what ant species live on and in exotic ant-acacias growing in Florida, a region devoid of both native ant-acacias and obligate acacia-ants.
Methods and Results
We contacted and visited numerous botanical gardens around Florida looking for live ant-acacia specimens, and found ant-acacias growing at four locations in addition to Mounts Botanical Garden: Fairchild Tropical Garden in Miami, University of South Florida Botanical Garden in Tampa, Walt Disney World in Orlando, and on the property of G. Joyner in West Palm Beach.
On 10 May 2000, at Fairchild Tropical Garden, we collected ants on three ant-acacias: one bull-horn acacia tree, Acacia cornigera; and two bee wattles, Acacia sphaerocephala Schlechtendal & Chamisso. We found five ant species: Brachymyrmex cf. obscurior, Camponotus floridanus (Buckley), Monomorium floricola (Jerdon), Pseudomyrmex cubaensis (Forel), and Technomyrmex albipes (Smith) (Table 1). We found no ant entrance holes on any thorns.
On 13 May 2000, at Mounts Botanical Garden, we collected ants on one A. cornigera, grown from seed. We found four ant species: Paratrechina longicornis (Latreille), Pseudomyrmex cubaensis, Pseudomyrmex ejectus (Smith), and Pseudomyrmex gracilis (Table 1). As in December 1999, we again noted smooth, round ant entrance holes on thorns of this tree. The ants inhabiting the hollowed thorns were Ps. gracilis.
On 19 May 2000, at University of South Florida Botanical Garden, we collected ants on one small A. cornigera. We found a single Ps. gracilis worker (Table 1) but did not observe any ant entrance holes.
On 21 May 2000, at Animal Kingdom, a part of Walt Disney World, we collected ants on one A. cornigera growing in a planter in the Africa section. The tree was purchased in Miami; the store had received it from a customer (J. Thompson, pers. comm.). On this tree we found Brachymyrmex cf. obscurior workers on the trunk (Table 1). We found one thorn with a smooth, round ant entrance hole. We broke open this one thorn and found Camponotus sexguttatus(Fabr.) workers and brood inside (Table 1).
On 12 June 2000, on Alexander Street in West Palm Beach, we collected ants on an A. cornigera grown from seed by G. Joyner. This tree differed from the well-pruned trees we had examined in botanical gardens because it had many dead branches and twigs, which we were free to break open. We also broke open many dead and live thorns. We found four ant species: Brachymyrmex cf. obscurior, Pheidole megacephala (Fabr.), Pseudomyrmex elongatus (Mayr), and Pseudomyrmex ejectus (Table 1). Inside many dead twigs, we found Ps. ejectus colonies with brood, including alates. In a few dead thorns we found Pheidole megacephala, Pseudomyrmex elongatus, and Pseudomyrmex ejectus, in some cases with brood. None of the entrance holes to these thorns were smoothly rounded, suggesting that none were created by ants.
We found eleven ant species living on or in exotic ant-acacia trees in Florida. Five of these ant species are native to Florida (Brachymyrmex cf. obscurior, Camponotus floridanus, Pseudomyrmex cubaensis, Pseudomyrmex ejectus, Pseudomyrmex elongatus), though the last three are also found in the Neotropics (native/exotic designations from Deyrup et al. 1988, Deyrup 1989). Two ant species we found are New World exotics (Camponotus sexguttatus and Pseudomyrmex gracilis) and four are Old World exotics (Monomorium floricola, Paratrechina longicornis, Pheidole megacephala, and Technomyrmex albipes). Of greatest interest were Ps. gracilis and C. sexguttatus, the only ants we found inhabiting Acacia cornigera thorns that appeared to have been perforated by ants.
Pseudomyrmex gracilis ranges from Argentina to Texas and the Caribbean (Kempf 1972; Jaffe & Lattke 1994) and has invaded Hawaii and Florida (Beardsley 1979; McGlynn 1999). The earliest Ps. gracilis records in Florida were from Miami ca. 1960 (Whitcomb et al. 1972). By 1970, Ps. gracilis was common throughout southeastern Florida, as far north as West Palm Beach (Whitcomb et al. 1972), and by 1988, Ps. gracilis was found from the Florida Keys north to Duval County, near the Georgia border (Johnson 1986; Deyrup et al. 1988, Deyrup 1989). Pseudomyrmex gracilis opportunistically nest in acacias, providing little or no defense for the tree, but also commonly nests in hollow branches, twigs, and stems, as well as building crevices (Buren & Whitcomb 1977; Cassani 1986; Ward 1993; Klotz et al. 1995). Wheeler (1942) found that Ps. gracilis, “though a very frequent tenant of dead twigs and Cordia domatia in regions where there are no Acacias, nevertheless exhibits a strong proclivity not only to inhabit the spines of these plants [ant-acacias], wherever they are available, but also to perforate them at the same point, to visit the foliar nectaries and to collect food-bodies.”
Camponotus sexguttatus ranges from Argentina to Nicaragua and the Caribbean (Kempf 1972) and has invaded Florida and Hawaii (McGlynn 1999). The earliest known Florida specimens date to 1993 (Deyrup et al. 2000). Our observation is the first record of C. sexguttatus inside the thorns of an ant-acacia. It is unclear whether the C. sexguttatus simply occupy previously prepared thorns or if they perforate and hollow out the thorns themselves, as do another species of carpenter ant, C. planatus. Camponotus planatus ranges from Columbia to Texas and the Caribbean (Kempf 1972) and has invaded Florida, Hawaii, and the Galapagos Islands (McGlynn 1999). Like Ps. gracilis, Wheeler (1942) considered C. planatus“of special interest” because of its specialized behaviors in exploiting ant-acacias. Wheeler (1913) observed C. planatus workers perforating a new thorn, indicating that this species does not merely take possession of thorns excavated and abandoned by other ants, but actually opens and excavates its own acacia thorns. Wheeler (1913) considered it “extraordinary that C. planatus, which throughout tropical America so constantly lives in hollow twigs, should be able in widely separated localities to utilize the acacia thorns as perfectly and in precisely the same manner as the regular Pseudomyrmas.” Camponotus planatus occurs in southernmost Florida (Deyrup et al. 1988), though we did not find any on the ant-acacias we examined.
The native range of A. cornigera trees in Central America and Mexico (Seigler & Ebinger 1995) overlaps with the native ranges of both Ps. gracilis and C. sexguttatus ants. Thus, for Ps. gracilis, and perhaps also C. sexguttatus, their association with A. cornigera in Florida represents the reconstitution in an exotic locale of a facultative symbiosis evolved in the Neotropics. Seigler and Ebinger (1995) report that naturalized populations of A. cornigera occur in the Caribbean and in southern Florida. In the future, we hope to study the ant fauna of these and other non-native ant-acacias populations.
We thank S. Cover for ant identification; M. Wetterer for comments on this manuscript; S. Wetterer for field assistance; R. Wunderlin at the Institute for Systematic Botany at the University of South Florida and K. Perkins at the University of Florida Herbarium for information on ant-acacia specimens collected in Florida; J. Thompson for information on the acacia at Walt Disney World; G. Joyner for information on and access to his acacia; and Florida Atlantic University for financial support.