Historical, anecdotal records of the state-endangered Miami blue butterfly, Cyclargus thomasi bethunebakeri (Comstock & Huntington) (Lepidoptera), have mentioned larval associations with the Florida carpenter ant, Camponotus sp. Recent population studies confirm that C. t. bethunebakeri larvae associate with Camponotus floridanus (Buckley) as well as another member of the genus, Camponotus planatus (Roger). Additionally, caterpillars have been observed tended by Crematogaster ashmeadi (Emery), Forelius pruinosus (Roger), and Tapinoma melanocephalum (Fab.). Field surveys of remaining Miami blue habitat and recent butterfly reintroduction sites reveal other potential ant associates, Paratrechina longicornis (Latreille) and Paratrechina bourbonica (Forel), and a host of possible predaceous ant species. The corresponding conservation implications are discussed. Detailed information is also presented about larval ant-associated organs and their mediation of this facultative symbiosis.
The Miami blue, Cyclargus thomasi bethunebakeri (Comstock & Huntington) (Lycaenidae: Polyommatinae), represents one of Florida's rarest endemic butterflies and is currently listed as state-endangered. Once commonly found in tropical coastal hammocks, beachside scrub, and tropical pine rocklands from the southern Florida mainland south through the Florida Keys to Key West and the Dry Tortugas, the species' overall distribution and numerical abundance has been reduced to a single remaining metapopulation within the boundaries of Bahia Honda State Park in the Lower Keys (Klots 1964; Kimball 1965; Lenczewski 1980; Minno & Emmel 1993; Ruffin & Glassberg 2000; Calhoun et al. 2002). Developing larvae of C. t. bethunebakeri have been shown to be tended by ants in the genus Camponotus but the extent of the relationship remains poorly understood (Minno & Emmel 1993). Recent population studies of the butterfly at Bahia Honda State Park and additional reintroduction sites within Everglades National Park confirm a continued association.
Over 75 percent of lycaenid larvae with known life histories associate with ants (Pierce et al. 2002). Such myrmecophilous relationships may be mutualistic to varying degrees or even parasitic whereby larvae are predatory in ant nests (Pierce & Mead 1981; Fiedler & Maschwitz 1988; New 1993). The resulting communication between larvae and ants is mediated by a complex array of tactile, chemical, and often audible signals (DeVries 1990). Specifically, larvae possess highly specialized organs that can extrude alarm, reward, or appeasement chemicals. In response, tending ants often protect the surrounding larvae from a variety of natural predators and parasitoids, and thus can potentially provide a benefit for survival (Thomas 1980; Webster & Nielson 1984; Pierce & Easteal 1986; Savignano 1994). Cushman & Murphy (1993) suggest that ant associations also may play an important role in the persistence of lycaenid populations. They additionally propose that species with a dependence on ants, whether facultative or obligatory, display an increased sensitivity to environmental change, and thus are more susceptible to endangerment than species that lack ant associations. Here, we identify additional ant associates and potential predatory ant species and discuss the corresponding implications for the conservation and recovery of the Miami Blue butterfly, a critically imperiled butterfly.
Materials and Methods
Field surveys of ant species were conducted at Bahia Honda State Park and the Flamingo Campground, Rowdy Bend Trail, and Bear Lake Road sites in southern portions of Everglades National Park during daylight hours on 24-27 May, 2004 and 31 July-2 August, 2004. These areas contain low numbers of Cyclargus thomasi bethunebakeri, either as part of a remaining natural metapopulation or as reintroduced individuals. Hand-collecting and baiting were used to survey ants on and around patches of the butterfly's larval host, Caesalpinia bonduc (L.) Roxb. (Fabaceae). Sugar baits consisting of index cards with approximately 10 g of crushed pecan cookie were placed along transects at the base of C. bonduc plants. Baits were left in the field for one hour, at which time all cards were collected in Ziploc-style plastic bags. Additionally, when C. t. bethunebakeri larvae were found in association with ants, 1-2 ant specimens were collected from the tended larvae.
Finally, to provide additional detail on the structure of the larval ant organs, three C. t. bethunebakeri larvae from a captive colony maintained at the University of Florida were preserved and used for SEM and Auto-Montage photographic analysis. Larvae were placed in near boiling water for 60 seconds, transferred to 25% ethanol for two hours, 50% ethanol for another two hours, and stored in 75% ethanol before being photographed. No additional preparation or gold coating was done to prepare specimens.
Eighteen ant species were collected in Everglades National Park and Bahia Honda State Park (Table 1). Of these, Camponotus floridanus, Camponotus planatus, Crematogaster ashmeadi, Forelius pruinosus, and Tapinoma melanocephalum were confirmed to tend larvae of Cyclargus thomasi bethunebakeri. Late instars were always found in association with ants but early instars, prepupae, and pupae were frequently found without ants present. Camponotus floridanus tended larvae for the majority of the observations and all other ants were encountered 1-2 times, with no two species tending larvae simultaneously. Two ants typically tended a larva at a time, with the exception of Crematogaster ashmeadi which often tended in higher numbers (Fig. 1).
We name two additional species, Paratrechina longicornis and Paratrechina bourbonica, as potential ant associates. The former species was found in proximity to C. t. bethunebakeri larvae and appeared to tend them although encounters were brief. The latter species was observed tending larvae of another lycaenid, Strymon martialis (Herrich-Schäffer), on Caesalpinia bonduc at Bahia Honda State Park. No predation by these ants was observed.
Details of the ant organs of C. t. bethunebakeri are shown in Fig. 2. Second through fifth instars possess a dorsal nectary organ (=honey gland) with associated perforated cupola organs on abdominal segment A7 and a pair of eversible tentacular organs on abdominal segment A8. Abdominal segments A7 and A8 are fused dorsally. Tentacular organs were observed to evert independently in the field when stimulated by attendant ants, and liquid droplets from the dorsal nectary organ were actively imbibed by all species of ants. Camponotus floridanus became excited and agitated, evidenced by increased body and antennal movements, when the tentacular organs were everted.
This study documents Camponotus floridanus to be the primary ant species attending Miami blue larvae. Camponotus floridanus is a native ant species primarily active at night throughout Florida; they are commonly found foraging on C. bonduc and tending C. t. bethunebakeri larvae in both the Everglades and Bahia Honda locations. Camponotus planatus is a diurnal species, but is not commonly encountered in association with larvae, having never been found tending larvae in the Everglades and only once in Bahia Honda. It is possible that in higher densities C. planatus may more regularly tend larvae and could potentially be important at protecting larvae during the day. Buckley & Gullan (1991) have shown that more aggressive ants provide better protection for soft scales and mealybugs, and as a corollary the large and potentially aggressive Camponotus species may prove effective in deterring predators and parasitoids (Axén 2000).
Crematogaster ashmeadi were observed tending individual larvae in Bahia Honda but have not yet been observed with larvae in the Everglades. While not commonly found tending larvae, interactions involving C. ashmeadi were characterized by a minimum of five individuals. This behavior demonstrates the quality of their trailing and recruitment signals. Other Crematogaster species have been found worldwide to tend lycaenid larvae and this genus seems predisposed to lycaenid symbioses (Atsatt 1981; Fiedler 1991; Pierce et al. 2002; Saarinen 2005). These ants are equipped with a flexible abdomen and attached sting; despite their small size they are potentially capable of defending larvae from other ants or harmful invertebrates.
Both Forelius pruinosus and Tapinoma melanocephalum may be opportunistically imbibing food rewards from C. t. bethunebakeri larvae. Field observations suggest that their behavior offers little or no protection for the larvae they tend; however merely their presence may deter predators. Both Paratrechina longicornis and P. bourbonica, along with T. melanocephalum, have been referred to as “tramp ants” (Passera 1994). While such species may not provide demonstrative protection for larvae, at the very least they tolerate nearby larvae and coincidentally tend instars feeding on C. bonduc flowers and buds adjacent to where the ants are also gathering nectar.
The facultative ant associations of C. t. bethunebakeri encompass four genera (five including Paratrechina) and three subfamilies; Formicinae, Myrmicinae, and Dolichoderinae. These lycaenid larvae may secrete “non-specific” ant semiochemicals as attractants to various ant species, as proposed by Henning (1983). These chemicals, primarily from the tentacular organs and potentially from the perforated cupola organs, may serve to alarm, excite, or appease ants. Further study into the chemical secretions of all ant organs may clarify the “intentions” of the larvae in their emissions. Further comparisons of each ant species' alarm and attractant pheromones with those isolated from lycaenid volatiles may further elucidate ant-larval relationships, including if certain ants are chemically targeted and if others are simply opportunistic tenders.
No interactions between other identified ant species and C. t. bethunebakeri larvae were observed. Several of these ants, however, may be predating larvae at other times. All three Pseudomyrmex species may be predators, possibly excepting P. simplex (Smith) due to its small size. Miami blue larvae are always found in proximity to abundant colonies of Camponotus floridanus and further field observations, especially at night when C. floridanus are most active, need to be carried out to assess interactions within the ant mosaic of symbionts and predators.
This study also shows the persistence of Wasmannia auropunctata (Roger) on Bahia Honda State Park (first recognized there by Deyrup et al. 1988). This invasive tramp ant is native to the New World tropics and its presence in the Florida Keys may be a cause for concern. W. auropunctata, also known as the little fire ant is an opportunistic feeder that forages day and night and bears a painful sting. Both W. auropunctata and the red imported fire ant, Solenopsis invicta Buren, have been implicated in the displacement of endemic species, resulting in a loss of biodiversity (Meier 1994; Wojcik 1994). Neither ant has been found near C. t. bethunebakeri in the Everglades, nor have they been observed harvesting immatures or predating adults in Bahia Honda. Solenopsis invicta mound density is perhaps not high enough to impact the C. t. bethunebakeri metapopulation on Bahia Honda because they do not appear as large or extensive in area as those found in more disturbed habitats (personal observation). Further field work will need to examine predation rates by ants and the specific impact that these invasive ants may have on endemic butterfly species, especially species of special concern.
Ant attendance may be critical to the long-term survival of lycaenid taxa by impacting larval development time, larval weight gain, and other developmental responses (Robbins 1991; Wagner 1993). The presence of an ant guard has led to larger, more fecund adults in the related butterfly Hemiargus isola (Reakirt) (Wagner 1993). However in the Australian species Jalmenus evagoras (Donovan), ant-tended larvae pupate at a smaller size, pupate for a shorter duration, and develop into smaller adults (Pierce et al. 1987). In an assessment of potential ant partners, it was shown that Tapinoma sessile (Say) is a “neutral partner” for the widely distributed North American lycaenid Glaucopsyche lygdamus (Doubleday), providing no significant cost or benefit (Fraser et al. 2001). Researchers of the critically imperiled European lycaenid butterfly Maculinea rebeli (Hirschke) have repeatedly emphasized “the importance of identifying local host ant species prior to further management conservation strategies in order to avoid failure of management programs or even damage to populations on the edge of extinction” (Steiner et al. 2003). Ant attendance, obligate or facultative, is not trivial; it can have profound effects on the length of time individuals spend in vulnerable immature stages as well as the resulting fecundity of adults. Both symbiotic ant partnerships and the negative impacts of predaceous ants should be addressed in management plans for the conservation of endangered lycaenid taxa.
This research was conducted under permits EVER-2004-SCI-0038 and 5-04-58 and we thank Sue Perry of Everglades National Park and Bob Rundle of Bahia Honda State Park for assistance with permitting. We thank Mark Deyrup and Lyle Buss for assistance with the ant identifications and ant life histories, Paul Skelley for assistance with the SEM at the Division of Plant Industry in Gainesville, Florida, and Tom Emmel for useful comments on the manuscript.