Ecological Entomology (2003) 28, 247±251

SHORT COMMUNICATION Contrasting exotic Solenopsis invicta and native pruinosus as mutualists with bignonioides, a native plant

J. H. NESS Institute of Ecology, University of Georgia, U. S. A

Abstract. 1. The suitability of the red imported fire Solenopsis invicta Buren and a native ant (Roger) as participants in a food-for-protection mutualism with a native nectaried tree Walter was compared. 2. The mean mortality of folivore larvae of catalpae Boisduval was similar for S. invicta and F. pruinosus although S. invicta attacked fewer caterpillar aggregations and was a devastating pupal predator. Solenopsis invicta also differed from the native ant in that it attacked the parasitoid Cotesia congregata Say, another plant mutualist, and visited extrafloral nectaries less frequently. 3. Habitats invaded by S. invicta are characterised by a scarcity of both herbi- vores and of beneficial that visit extrafloral nectaries. The plants do not require protection, and extrafloral nectaries are visited rarely. Although plants are defended incidentally by S. invicta, the -plant mutualism therein is greatly simplified or defunct. Key words. Exotic, extrafloral nectar, mutualism, parasitoids, pupa, tri-trophic interactions.

Introduction important to either participant (Willmer & Stone, 1997). Insofar as the displacement of native insect assemblages It is well known that exotic ants can severely depress the by exotic ants alters these currencies (plants' need for diversity and abundance of native ants and other arthro- protection and procurement thereof, collection of food by pods (Vinson, 1994; Hollway et al., 2002), however less is bodyguards, and interactions with other mutualists), the known about the impact of ant invasions on interactions in food-for-protection mutualism may be disrupted. which native participated previously, for The red imported fire ant Solenopsis invicta Buren and instance mutualisms between ants and plants (but see a widespread native ant Forelius pruinosus (Roger) were Bond & Slingsby, 1984; Christian, 2001, regarding myrme- compared in terms of these three criteria, specifically as cochory). The production of extrafloral nectar to attract mutualists with a native tree Catalpa bignonioides Walter insect bodyguards is a geographically and taxonomically (Bignoniaceae). Catalpa bignonioides attracts ants and para- widespread example of a food-for-protection mutualism sitic wasps to extrafloral nectaries, and these visitors can act typically involving ants and plants (Koptur, 1992). That as plant mutualists when the C. bignonioides requires pro- the interaction functions as a reciprocally beneficial tection against herbivores. Field observations and experi- mutualism is contingent on at least three characteristics. ments illustrated that the two ants differed in their leaf (1) Visitors must defend vulnerable plants. This requires patrolling rates, visitation at extrafloral nectaries, and visitors capable of attacking herbivores successfully (Janzen, predation of caterpillars. The ants also differed in their 1966), and herbivores in sufficient abundance to necessitate importance as pupal predators of both herbivores and plant protection (Barton, 1986). (2) Plant protectors must other plant mutualists (parasitic wasps). utilise the food offered by the plant (Sagers et al., 2000). (3) The interaction should not endanger other mutualisms

Study organisms and sites Correspondence: J. H. Ness, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, Solenopsis invicta has displaced many native arthropods in U.S.A. E-mail: [email protected] the southern U.S.A. (Porter & Savignano, 1990; Gotelli &

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Arnett, 2000; but see Morrison, 2002). Forelius pruinosus is produces new leaves with active nectaries from May to capable of co-existing at low S. invicta densities but is August (Ness, 2001), although nectar collection was limited absent from heavily invaded sites (Camilo & Phillips, 1990). to these spring-flushing leaves. The sunlit, mesic, disturbed habitats readily dominated by To contrast the two ants' visitation of extrafloral nec- S. invicta are frequently occupied by C. bignonioides, a native taries and/or leaf patrolling, the number of ants per leaf was tree with five to 10 extrafloral nectaries per leaf. These surveyed at the two sites from June to November 2000. Six nectaries typically attract a variety of visitors, including trees with large, active fire ant mounds at their bases were F. pruinosus and Cotesia congregata Say (: surveyed at Sandy Creek, and seven at the Horticulture Braconidae), that attack the gregarious caterpillar Cerato- Farm S. invicta-exclusion plot. During each survey, all the mia catalpae (Boisduval) (Stephenson, 1982; Ness, 2001). ants were counted on 15 mature leaves selected randomly Ceratomia catalpae caterpillars are the only significant defo- from the north, south, east, and west sides of each tree. liator of C. bignonioides (Baerg, 1935); dry mass per fruit Surveys were made between 16.00 and 19.00 hours on and per seed decreases with defoliation (Stephenson, 1980) sunny days to avoid morning dew and peak midday tem- and caterpillars feed on fruits of defoliated plants (J. H. peratures. Forelius pruinosus is a diurnal forager, and Ness, pers. obs.). Folivory induces an increase in the sugar S. invicta is active at this time (Porter & Tschinkel, 1987). excretion rates of damaged leaves within 36 h, thereby To test the effect of ants on caterpillar survival rates and attracting and retaining nectary visitors at leaves requiring contrast this effect between the two ant species, workers protection (Ness, 2000). Ceratomia catalpae pupates in the were excluded from caterpillar aggregations at both sites. litter layer beneath the host tree, and emerging adults com- In July 1999, 56 isolated leaves were chosen randomly on monly oviposit on these same trees (Baerg, 1935). Three trees at each site, and Tanglefoot1 was applied to the generations are typically observed from June to October. petiole of half of these leaves to exclude ants. Six second- Research was conducted using mature C. bignonioides instar C. catalpae were added to each leaf. The 112 leaves trees 4±6 m tall at the Sandy Creek Nature Center and the (28 for each treatment  ant combination) were surveyed University of Georgia Horticulture Farm, Athens, U.S.A. for caterpillars and ants 2 days later. To compare ant (33520N, 83150W), from May to November 1999±2000. recruitment at damaged and undamaged leaves, 58 ran- Trees were adjacent to a stream and pond respectively. domly selected undamaged leaves within the same trees The Sandy Creek Nature Center site includes a grove of were also surveyed for ants at that time. This experiment 75 C. bignonioides trees extending beneath an electrical was repeated in July±August 2000 with larger caterpillar powerline to mixed pine and hardwood forest on either aggregations (10 second instar), a greater duration of expo- side. The powerline cut contains a high density of S. invicta sure (3 days), and branches rather than leaves (n ˆ 20 colony mounds; colonies were not found in the forest inter- per treatment  ant combination). In both experiments, ior (J. H. Ness, pers. obs.). The Horticulture Farm includes isolated leaves and branches were chosen to decrease the six adult C. bignonioides trees and 50 2±4-year-old seedlings likelihood of ants recolonising experimental leaves through surrounded by mixed agricultural plots dominated by contact with neighbouring foliage, and to limit the destina- S. invicta. To facilitate the growth of native ant populations tions available to dispersing caterpillars (although these on and around the trees, fire ant mounds were excluded instars are largely sedentary). Neighbouring leaves were from a 325-m2 rectangular plot around the trees with tar- surveyed to include locally dispersed caterpillars, and reten- geted pesticide bait applications (Amdro1) and manual tion rates were assumed to approximate survival rate. destruction of mounds with shovels and water in 1999 and General linear models were used to examine the effect of 2000. Forelius pruinosus (and rarely Formica schaufussi) treatment, ant identity, and ant  treatment interactions on foraged on the trees and ground at low levels prior to the caterpillar survival rates. exclusion of fire ants and were more abundant thereafter. A natural experiment compared survival rates of C. catalpae pupae in four habitats that differed in succes- sional stage and ant composition: the powerline field and Methods farm plots (both S. invicta invaded) and the S. invicta- exclusion plot and secondary forest (both including native To correlate the phenology of extrafloral nectar production ants). Laboratory-reared fifth-instar C. catalpae larvae were with leaf visitation by ants, sugar excretion was measured placed underneath 473 ml plastic cups anchored top-down over the lifetime of leaves selected randomly at Sandy Creek in each habitat. This technique limits caterpillar dispersal, in May 1999. Twenty leaves on four trees were encircled and the pupae can be collected readily while construction of with Tanglefoot1 (Tanglefoot Company, Cedar Rapids, a pupal chamber within the adjacent soil remains unim- Michigan) that excluded ants (the most common visitors), peded (Lee et al., 1990). Thirty-eight to 45 caterpillars and each leaf was rinsed with water 24 h prior to nectar were deposited in each habitat. The soil underneath and collection to avoid nectar accumulation. Nectar was adjacent to the cups was excavated 18 days later, and sorted collected from the largest nectary on each leaf using a using a sieve to ascertain the fate of the pupae. Individuals microcapillary pipette, and sugar production (mg sucrose were assigned to one of four categories: living pupae, ant equivalents produced per nectary in 24 h) was estimated prey, pupae removed by small mammals, and pupae killed on site using a pocket refractometer. Catalpa bignonioides by indistinguishable causes (e.g. parasitic nematodes, fungi).

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Ant predation was distinguished by characteristically small entry holes in the pupae, only the head capsule remaining from the caterpillar, or the absence of remains beneath a securely anchored cup. Anchored cups lacking remains frequently contained entrances to ant foraging tunnels, and S. invicta generally transports prey fragments to the nest (Wilson & Oliver, 1969). Small mammal attack was indi- cated by overturned cups and excavated burial chambers. To test whether the ants influenced survival rates of C. congregata, a potentialplant mutualist, individual cocoons were placed on 112 leaves on five trees at each site and ants were excluded from half of those leaves using Tanglefoot1. Cotesia congregata pupate on their host's integument or, less frequently, adjoining vegetation (Kester & Jackson, 1996). Cocoons were stuck on these leaves using a single drop of Elmers1 white glue. This glue dries quickly, and cocoons were handled using forceps to limit the deposition of additional semiochemicals. Three days later, each cocoon Fig. 1. Phenology of extrafloral sugar production and ant occu- was identified as eaten (cocoons shredded or missing) or pancy on Catalpa bignonioides leaves (mean and SE bars are surviving (cocoons present and intact). The experiment was shown). Sugar production per nectary (open squares) was performed simultaneously at both sites between 22 July and estimated from 20 spring-flushing leaves at Sandy Creek Nature 10 September 2000 when cocoons were available. Center in 1999. Later leaves are not included. Visitation of leaves by Forelius pruinosus (open circles) and Solenopsis invicta (closed circles) was measured in 2000. Surveys included 60 leaves per tree Results on six S. invicta-dominated trees at the Sandy Creek Nature Center and seven F. pruinosus-dominated trees at the University of The May cohort of C. bignonioides leaves produced extra- Georgia Horticulture Farm. Note: in order to distinguish lines better, y axes do not start at zero. floral nectar at the greatest rate in mid-summer 1999 (Fig. 1). Leaf patrolling by F. pruinosus peaked in mid to late summer 2000, a pattern not observed for S. invicta The proportion of C. catalpae pupae surviving differed 2 (Fig. 1). The mean number of F. pruinosus ants per leaf in among the four sites (X ˆ 31.4, P < 0.001). Moth emer- 2000 (mean Æ SE: 0.63 Æ 0.13) was four-fold greater than gence was decreased approximately eight-fold in S. invicta- the mean of S. invicta (0.16 Æ 0.05). invaded habitats relative to those controlled by native ants Caterpillar survival rates were increased significantly on (Table 1). Most of this mortality was attributable to ant Tanglefoot1-encircled leaves in 1999 (approximately 1.5- predation although mammals killed 27% of the pupae in fold; F ˆ 15.35, P < 0.001) and 2000 (approximately four- an uninvaded forest. fold; F ˆ 55.07, P < 0.001), and fewer ants were found on The percentage of C. congregata pupae remaining intact these leaves (Mann±Whitney, U ˆ 4.24, P < 0.001). Mean after exposure to S. invicta (15%) was one-fifth of that in caterpillar survival rates were similar between the two ant the presence of F. pruinosus (83%) or where either ant was assemblages in both years (ant identity main effect, 1999: excluded (86 and 93% respectively). That proportion 1 F ˆ 0.53; 2000: F ˆ 1.57, both P ˆ NS), and treatment  ant differed among these four (ant  Tanglefoot treatment) 2 interactions were not significant in either year (1999: F ˆ 0.0; groups (X ˆ 102.25, P < 0.001). 2000: F ˆ 0.68, both P ˆ NS). The patterns of ant occu- pation and caterpillar survival within trees, however, dif- fered between the ant species. Forelius pruinosus workers Discussion were more than twice as common on caterpillar-damaged leaves (mean Æ SE: 1.61 Æ 0.46 ants per leaf) as on ran- This field study demonstrates that the exotic S. invicta and domly selected undamaged leaves (0.67 Æ 0.12) (Mann± native F. pruinosus ants differ in their capacities to act as Whitney, U ˆ 2.02, P < 0.05), and killed >60% of the reciprocally benefitting mutualists with C. bignonioides,a caterpillars in a majority of the aggregations. Solenopsis native nectaried plant. In the following section, the two ants invicta numbers at damaged (0.72 Æ 0.33) and undamaged are contrasted with regard to on-plant nectar collection and (1.16 Æ 0.57) leaves were indistinguishable (U ˆ 0.37, caterpillar predation, effects on herbivore recruitment (an P ˆ NS). Predation by S. invicta was inconsistent among indirect measure of plant vulnerability), and their inter- branches; all the caterpillars were killed in 42% of the actions with C. congregata, an additional plant mutualist. aggregations, although the mortality rates in the remaining Forelius pruinosus and S. invicta workers foraged on aggregations were indistinguishable from that on ant- plants differently. Forelius pruinosus was more abundant on excluded leaves (1999: F ˆ 0.83; 2000: F ˆ 0.41, both leaves, and killed caterpillars in most aggregations. Forelius P ˆ NS). pruinosus also appeared to be attuned better to reward

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Table 1. Mortality rate of Ceratomia catalpae pupae at four sites varying in ant composition. Powerline field ˆ Sandy Creek Nature Center powerline field, Forest ˆ Sandy Creek Nature Center forest, Farm ˆ University of Georgia Horticulture Farm, Exclusion plot ˆ S. invicta- exclusion plot at the University of Georgia Horticulture Farm. Different letters indicate significant differences in the proportion of total pupae killed by ants (multiple comparisons by site, P < 0.01in all cases).

Per cent mortality of Ceratomia catalpae pupae

Habitat Ants Total By ants By mammals Powerline field S. invicta 97.6 95.2 a 2.4 Forest Native ants 58.5 31.7 b 26.8 Farm S. invicta 92.3 87.2 a 5.1 Eclusion plot Mostly F. pruinosus 55.3 50.0 b 2.6

production by C. bignonioides. Linking ant attendance with eight-fold decrease in C. catalpae moth emergence in nectar production at the whole-plant scale is difficult, as the S. invicta-invaded habitats benefits C. bignonioides indir- present study tracked extrafloral nectar production for a ectly, although two points should be noted. (1) Pupal pre- cohort of leaves rather than for entire trees. Estimates from dators that do not ingest plant rewards (e.g. small mammals spring leaves in 1999 give little indication of when plant- and S. invicta) are plant commensalists rather than mutual- scale peaks in nectar production occur, although rates cer- ists. (2) The plant's investment in bodyguard attractants is tainly increase from spring to summer (also see Stephenson, beneficial only when protection from herbivores is required. 1982). Forelius pruinosus attendance was greatest in mid to The absence of folivores obviates the need for (and benefits late summer 2000, and the ants recruited to herbivore- of) on-plant protection, and subsequent reward production damaged leaves that can excrete more sugar (Ness, 2003). probably incurs a net cost to the plant. In contrast, the phenology of S. invicta leaf patrolling Solenopsis invicta also differed from F. pruinosus in and extrafloral nectar production were not correlated, attacking C. congregata pupae. This braconid parasitoid and attendance at damaged and undamaged leaves did not visits Catalpa extrafloral nectaries and attacks C. catalpae differ. These results are consistent with this ant's disinterest caterpillars (Stephenson, 1982; Ness, 2001). Because in carbohydrate-rich baits during warmer months (Stein C. congregata can parasitise >50% of late instars on a tree et al., 1990) and lack of a preference for nectars with higher (Ness, 2001) and these larvae are sufficiently large to sugar content (Lanza et al., 1993). The disparity in the avoid ant predation, attracting this wasp may benefit number of ants observed on the plants is particularly strik- C. bignonioides even when other bodyguards are present. ing given Porter and Savignano's (1990) finding that Whether the interaction between this plant and parasitoid S. invicta was 10±30 times more abundant than native ants constitutes a mutualism important to either population is in adjacent uninvaded habitats. Perhaps as a consequence uncertain. Other Cotesia species can increase host plant seed of the poor match between S. invicta dietary preference and set by parasitising folivores (Hoballah & Turlings, 2001) the rewards offered by C. bignonioides, most caterpillar and live longer when sugar nectar is available (Siekmann aggregations showed no evidence of ant predation. The et al., 2001). Plants in invaded sites may require protection lower prey encounter rates of S. invicta appeared to be from herbivores less frequently than plants in uninvaded balanced by greater aggressiveness on locating caterpillar sites, although they also have fewer bodyguards available to aggregations, as mean caterpillar mortality rates did not attract. differ between trees dominated by the two ants. Differences An abundant and aggressive carnivore, S. invicta provides among the ants nonetheless remain important. (1) If con- indirect protection to nectaried and unnectaried plants centrated defoliation has a greater detrimental effect on alike (Agnew et al., 1982; Stiles & Jones, 2001). Nectar plants than similar amounts of damage dispersed across rewards are poorer attractants in these invaded sites as the entire plant (e.g. Lowman, 1982; Edwards et al., 1992; the dietary preferences and foraging strategies characteristic Marquis, 1992; Mauricio et al., 1993), F. pruinosus may be a of species in the more diverse native communities better plant bodyguard than S. invicta. (2) To the extent are lost. The most familiar threats to food-for-service mutu- that S. invicta does not collect extrafloral nectar, it receives alisms between plants and insects are visitors that collect no reciprocal benefit from C. bignonioides. This ant±plant food but fail to provide reciprocal benefit to the plant interaction may well be commensalistic but it is not mutual- (nectar thieves). By contrast, S. invicta disrupts the mutual- istic. (3) There was no evidence that C. bignonioides ceased ism between C. bignonioides and native arthropods such as to produce sugar at unvisited nectaries in invaded habitats, F. pruinosus by its indifference towards nectar, local extir- thereby avoiding the costs of the mutualism. pation of herbivores, and predation/exclusion of other plant Solenopsis invicta was a devastating pupal predator in bodyguards. Any resulting plant protection by S. invicta is comparison with F. pruinosus and other predators. The incidental to, rather than a consequence of, the mutualism.

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