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Ability of Resident Ants to Destruct Small Colonies of Solenopsis invicta (Hymenoptera: Formicidae)

(CC)
Asha Rao, S. Bradleigh Vinson
DOI: http://dx.doi.org/10.1603/0046-225X-33.3.587 587-598 First published online: 1 June 2004

Abstract

The ant Solenopsis invicta Buren has spread across the United States and is reported to have significantly reduced the diversity of the native ants. Much of this spread is occurring on land that has been repeatedly disturbed by continually changing land use practices. S. invicta seems to outcompete and eliminate other resident ant species. However, this inference may not be true because several resident native ant species are known to persist in S. invicta-infested areas. Thus, in this study, we analyze the aggressive interaction between selected resident ants and S. invicta by evaluating whether some of these ants are capable of attacking small worker-defended S. invicta colonies or are instead attacked by the S. invicta colonies. Our results suggest that the native ant species Monomorium minimum (Buckley), Pheidole dentata Mayr, and Solenopsis molesta (Say) and exotic tramp ants Tetramorium bicarinatum (Nylander) and Monomorium pharonis (L.) do interact with the S. invicta and will attack and eliminate worker-defended S. invicta colonies of varying sizes ranging from 30 to 480 workers. M. minimum, P. dentata, S. molesta, T. bicarinatum, and M. pharonis also were observed to prey upon S. invicta brood once most of the defending workers were eliminated. The native species Forelius sp. was not observed to invade the S. invicta colonies; instead, it prevented S. invicta workers from leaving their nest to forage, which may have contributed to the decline of S. invicta colonies of up to 60 workers over time.

  • Monomorium
  • Pheidole
  • S. molesta
  • Forelius
  • biological control

Introduction

SINCE ITS INTRODUCTION, Solenopsis invicta Buren has become one of the dominant species in the southeastern United States (Vinson 1997). The increasing population density of S. invicta in the United States has been attributed to the lack of coevolved natural enemies from its native South America (Jouvenaz et al. 1981, Porter et al. 1992). Numerous studies also have reported that the spread of S. invicta in the United States has had a major impact on the native ant fauna (Porter and Savignano 1990, Jusino-Atresino and Phillips 1994, Gotelli and Arnett 2000, Holway et al. 2002). For example, Whitcomb et al. (1972) reported that S. invicta reduced the native ant complex in agroecosystems. S. invicta also has largely replaced native Solenopsis species Solenopsis xyloni McCook and Solenopsis geminata Fabricius (Hung and Vinson 1978, Wojcik 1983, Porter et al. 1988). The implications of this work are that native ants have had little chance of survival against S. invicta.

Wojcik (1994) reported some native ants persist in S. invicta-infested areas, and Stein and Thorvilson (1989) reported that several native ant species coexist with the S. invicta, especially in habitats with lower S. invicta infestations. Helms and Vinson (2001) observed a number of native ant species coexisting with a small S. invicta population in an undisturbed post oak savanna in Texas. More recently, Morrison (2002) suggested that the abundance and diversity of native ants had returned to the level observed before the invasion of S. invicta in Brackenridge Field Laboratory, University of Texas, Austin. This comparative study was conducted 12 yr after the initial study by Porter and Savignano (1990) on the effect of S. invicta on the native fauna in that area. Although the mechanisms underlying the coexistence or the rebound of native ants was not examined in these studies, they do reveal the possibility that native ants can defend themselves and maintain viable populations in certain invaded areas.

Native ants are known to play a significant role in the survival of founding S. invicta colonies initiated by newly mated queens, which is a critical stage in the life cycle of fire ant societies (Hӧlldobler and Wilson 1990). Nevertheless, there have been questions on the importance of colony founding by newly mated queens, particularly in polygynous form, in the successful spread of S. invicta as single foundresses are not believed to be successful (Porter et al. 1988, Keller and Ross 1993). Also, empirical evidence from allozymes (Ross 1992) suggests that polygyne-derived female alates depend on fertile males from monogyne colonies for mating (Shoemaker and Ross 1996), thus failing to explain areas that are exclusively inhabited by polygyne populations (Porter et al. 1991, Ross and Shoemaker 1997). Polygyne populations are believed to accept reproductive queens from other colonies (Wilson 1971, Holldobler and Wilson 1990), but they are believed to spread predominantly through colony budding (Vargo and Porter 1989). In Texas, however, polygyne colonies form a mosaic and new polygyne colonies far from each other are continually found (S.B.V., unpublished data), suggesting that new polygyne population may be independently formed in addition to the mechanisms described above. The newly founded colonies of S. invicta are known to suffer high mortality before the first workers have a chance to forage (Tschinkel 1987). In areas where native ants persist, a high percentage of the newly mated S. invicta queens perish before they can even begin a nest, and native ants are often responsible. Whitcomb et al. (1973) reported that at least nine species of ants are successful predators of founding S. invicta queens and Nichols and Sites (1991) documented 12 additional ant predators of S. invicta queens. For example, no founding S. invicta queens were observed to have survived to construct a brood chamber in an area occupied by the dolichoderine ant Conomyrma insana (Buckley), a successful predator of S. invicta queens (Nickerson et al. 1975). Ants in the Solenopsis subgenus Diplorhopturm also are known to be important predators of S. invicta queens within their claustral brood chamber (Lammers 1987).

Given the successful attainment of a single S. invicta colony to gigantic size (up to 10–50,000 workers) in the field, it may be assumed that once workers eclose and begin to forage, the queen may be protected from many of these predatory ants. This being an assumption, we initiated laboratory studies to determine and document the ability of small S. invicta colonies to defend against native and other tramp ant species. For simplicity, both native and other tramp ant species used in this study will be hereafter referred to as “resident” ant species, unless specified otherwise. The resident species investigated, M. minimum, S. molesta, P. dentata, T. bicarinatum, and M. pharonis, except Forelius sp., were observed to have the ability to invade small S. invicta colonies (Rao and Vinson 2002). This article further examines whether S. invicta workers, brood, or the queens are killed by resident ant species and compares the ability of these resident ants to destroy small S. invicta colonies after invasion. We also report novel statistical applications with analysis and interpretation that extend the predictive value of the data discussed here and in Rao and Vinson (2002).

Materials and Methods

The S. invicta colonies used in experiments were reared from newly mated queens collected from several locations in Brazos County, Texas. Colonies were initiated by placing two dealated S. invicta queens together that were collected immediately after a nuptial flight and that were allowed to lay eggs. Once the workers eclosed, the colonies were maintained with honey water, and crickets or mealworm larvae and pupae ad libitum. Mature native and other exotic ant colonies used for the experiments were collected from Brazos County in Texas. The colonies were maintained with honey water, crickets, and mealworm pupae ad libitum. Native ant species used in the experiment were M. minimum, Forelius sp., S. molesta, and P. dentata. In addition, we examined the interaction between the S. invicta and two exotic species, M. pharonis and T. bicarinatum. All of the resident ant species collected occurred as polygyne colonies.

In all the experiments, size of the resident ant colonies was kept constant with 500-1000 workers and five to six queens depending on the species used. The experimental S. invicta colonies consisted of two queens, brood, and either 1) 30, 2) 60, 3) 120, 4) 240, or 5) 480 workers. Each of the resident ant colonies was provided access to each one of the S. invicta colony size (defined as colony worker numbers) categories. As a control to these experiments, other S. invicta colonies (with 1000 workers and five to six queens) were set up to interact with each of the S. invicta colony of varying colony sizes (i.e., S. invicta versus S. invicta). This was done to better understand the interactions among polygyne S. invicta colonies. Each resident ant species-S. invicta colony size combination was replicated five times.

Confrontation between the two species was initiated by placing the experimental S. invicta colony and either a native, exotic, or control S. invicta colony next to each other and providing worker access to both trays, through a 2.54-cm-wide paper bridge. A rubber cork attached at each end of the paper bridge held the bridge in place and ensured equal access by both species to each otherʼs nest. Movement of the resident ant queen and workers was unrestricted.

Preliminary experiments revealed a sequence of major events in S. invicta colonies after the invasion by opposing resident species. These events included 1) brood abandonment by all the S. invicta workers due to the overwhelming presence of other species in their nest, 2) death of all S. invicta workers, and 3) death of S. invicta queens. These events were used as standard measurements in experiments. Time required for each of these events to occur was measured because it was considered a measure of the ability of the ant colony in question to defend itself. Based on the results from preliminary experiments, observations were recorded every hour for the first 12 h, followed by once every 24 h. Queen mortality was monitored daily. The experiments were terminated after 2 mo.

Statistical Analyses

Two methods of survival analysis, Kaplan–Meier technique and Cox proportional hazards regression model (CPHR), were used to analyze the time until brood abandonment by the S. invicta, worker, and queen death of S. invicta (Cox and Oakes 1984, Le 1997). In addition, data on the time taken by resident ant species to invade S. invicta nests of varying colony sizes (Rao and Vinson (2002) were analyzed using these statistical procedures. Thus, the terminal events analyzed were 1) invasion of the S. invicta colonies by a species, 2) brood abandonment by S. invicta in presence of a particular species, 3) death of S. invicta workers in presence of a particular species, and 4) death of S. invicta queens in presence of a particular species. When evaluating time-until-an-event data in a time-limited experiment such as this, some species do not exhibit the events described above within time set for the experiments. Such observations are termed “censored” or truncated observations (Cox and Oakes 1984). In our experiments, data were considered censored if any of the aforementioned events did not occur by the time the experiments were terminated. Survival curves for all the species and colony sizes were estimated with the Kaplan–Meier technique for each event. These survival–estimate curves for each species and colony size were compared using a log rank test with a χ2 approximation.

The independent variables species and S. invicta colony size affecting the events described above also were analyzed by using the CPHR model. This method yields the estimates of the hazard ratios for each of the variables considered in the model. In the current study, the hazard ratio for species (a categorical variable) was interpreted as the expected change in the hazard or risk of an event when the variable changes from 0 to 1, with 0 being a dummy variable (Le 1997). In all of these regressions, the S. invicta in the control experiments (S. invicta versus S. invicta) was used as a dummy variable. Thus, the hazard ratio could also be interpreted as the expected change in the risk of an event when the intraspecific interactions (S. invicta versus S. invicta) change to interspecific interactions (e.g., S. invicta versus M. minimum, S. invicta versus S. molesta). Similarly, the hazard ratio for S. invicta colony size (acategorical variable) was interpreted as the expected change in the hazard or risk of a terminal event when the S. invicta colony size changes from one size to another (e.g., 30–60, 60–120). Effect modification by key variables species and colony size was evaluated by including interaction terms in the model. Data analyses were performed using SPSS Statistical Software version 10.1 for Windows.

Results

Invasion

There were a total of 60 observation days in this study. In all experiments, resident ant species, including S. invicta, with the exception of Forelius sp., invaded S. invicta colonies regardless of colony size. Because Forelius sp. did not invade, this species was not included in the model.

In CPHR model, both species and colony size were strong predictors of invasion of S. invicta colonies (species Wald χ2 =123.21, df =5, P <0.0001; colony size Wald χ2 =110.26, df =4, P <0.0001) (Table 1). The interaction term between species and the colony size also was significant (Wald χ2 =79.09, df =20, P <0.0001), indicating that each resident species responded differently to increasing S. invicta colony sizes, thus differing in invasion time, and sometimes not invading at all beyond a certain colony size (Fig. 1). Because the interaction term was significant, the survival–estimate curve for the event invasion were created and compared for all species within each S. invicta colony size separately. In general, M. pharonis, T. bicarinatum, M. minimum, and S. invicta had markedly shortened invasion time compared with P. dentata and S. molesta (Fig. 1). The former two species with the exception of M. minimum also pose high invasion risk for S. invicta colony with a very high factor of 270 and a coefficient of 5.59, and a factor of 1.16 with a coefficient of 0.151 (Table 1). Conversely, for M. minimum with a hazard ratio of 0.16 and a coefficient of -1.86 means that the risk by M. minimum is reduced by a factor of 0.16 compared with the risk of being invaded by S. invicta themselves, i.e., the risk of invasion by M. minimum is one-sixth the risk posed by S. invicta. Similarly, the risk of invasion by P. dentata is reduced by a factor of 0.03 and S. molesta by a factor of 0.25. Significant differences also exist between species within each colony size. Within a colony size of 30, except for M. pharonis and S. molesta, there was a significant difference in the invasion time between all other species (Fig. 1A). With the increase in the S. invicta colony size to 60 workers, no significant differences were observed between M. minimum and P. dentata, and these two species took longer to invade the S. invicta colonies compared with the other species (Fig. 1B). However, when the colony size increased to 120 and 240, S. molesta and P. dentata took longer to invade and significantly differed from other species (Fig. 1C and D). When the S. invicta colony size reached 480 workers, only M. pharonis significantly differed from others (Fig. 1E).

View this table:
Table 1.
Fig. 1.

(A–E) Probability of invasion of the S. invicta nest of varying sizes by resident ant species. Each cumulative curve for a species represented in each colony size category was based on N =5. A species followed by the same letter are not significantly different at P <0.05, and the abbreviations of each species are defined in box 1. Note: The x-axis values differ between the graphs above and below the divider line. The horizontal scale in the graph represents the times marked at uncensored observations, and the vertical scale represents the probability to invade.

Figure 1 also suggests that the invasion times by resident species increased with the increase in the colony size of S. invicta colonies. Within 3 h of encounter, there is a 50% chance of all species, including S. invicta themselves, invading the S. invicta colonies with 30 or 60 workers (Fig. 1A and B), whereas it takes up to 10 h after encounter to have the same 50% chance of invading the colony sizes of 120,240, and 480 workers (Fig. 1C–E). The expected change in the risk of invasion by a resident species, when S. invicta colony size changes by a unit expressed as hazard ratio Exp (b) is indicated in Table 1. Here a unit change in colony size refers to the doubling of colony size, for example, increase from a colony size of 30–60 is one unit, from 60 to 120 is another unit. The hazard ratio of each colony size except the first colony size of 30 is compared with the size preceding it. For example, the hazard ratio of 0.27 for a colony size 60, with a coefficient of -1.29 means that when the S. invicta colony size increases from 30 to 60, the risk of S. invictabeing invaded decreases by afactor of 0.27. Similarly, the risk of invasion with the colony size of 120 workers is 1/20 the risk with a colony size of 60. This indicates that the risk of invasion is significantly reduced when the colony size changes from 30 to 60 and from 120 to 240. The risk is not significantly reduced when the colony changes from 240 to 480, suggesting the risk remains the same once the S. invicta colony size exceeds the 240 worker size. The colony size of 480, in which the two species T. bicarinatum and S. molesta did not invade unlike other species, is reflected by a high standard error of 60.88 of the coefficient.

Brood Abandonment byS. invicta.

The CPHR model for brood abandonment shows that both species and colony size are significant predictors of the S. invicta brood abandonment after being invaded (Wald χ2 =109.88, df =6, P <0.0001; Wald χ2 =112.72, df =4, P <0.0001) (Table 2). The interaction term between species and the colony size did add a significant effect to the model (Wald χ2 =128.08, df =20, P <0.0001). Due to significant interaction term, the survival estimate curves for species within each S. invicta colony size was generated separately and is presented in Fig. 2. The result indicate that the probability of brood abandonment by S. invicta after 3–200 h of invasion was 0.5 after being invaded by M. pharonis, M. minimum, and P. dentata (Fig. 2A–E). The risk of brood abandonment was highest with M. pharonis compared with control colonies of S. invicta, followed by P. dentata (Table 2). The least risk was in the presence of T. bicarinatum when compared with S. invicta itself, followed by S. molesta. The former species caused brood abandonment by the S. invicta rapidly at a colony size of 30 with a 50% chance of causing the S. invicta to abandon their brood within 30 min of encounter, whereas it was 8 h after the encounter for the latter species (Fig. 2A). However, when the S. invicta colony size increased to >120, both T. bicarinatum and S. molesta were last ones to cause brood abandonment by S. invicta compared with other species (Fig. 2C–E). In contrast, M. pharonis and M minimum were the fastest to bring about brood abandonment by S. invicta when the colony sizes increased beyond 60 workers (Fig. 2C–E). Brood abandonment by small S. invicta colonies was not observed when invaded by its own species or when exposed to Forelius sp.

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Table 2.
Fig. 2.

(A–E) Probability estimates of brood abandonment by S. invicta after being invaded, as a function of time (hours). Each cumulative curve for a species represented in each colony size category was based on N =5. The species followed by different letters are significantly different at P <0.05, defined in box 1 in Fig. 1. Note: The x-axis values differ between the graphs above and below the divider line. The horizontal scale in the graph represents the times marked at uncensored observations, and the vertical scale represents the probability of brood abandonment.

The hazard ratios in Table 2 indicate the expected change in the risk of brood abandonment by a competitor species, when S. invicta colony size changes by a unit. The risk is significantly reduced when the colony size changes from 30 to 60 and from 120 to 240. The risk, however, is not significantly reduced when the colony changes from 240 to 480, suggesting the risk of S. invicta abandoning the brood in the presence of resident species remains the same once the S. invicta colony size exceeds the 240 worker size as observed with invasion data.

S. invicta Worker Survival.

The CPHR model for the S. invicta worker survival indicates that as with other events mentioned above, both species and colony size are significant predictors of the S. invicta worker survival after being invaded (Wald χ2 =146.78, df =6, P <0.0001; Wald χ2 =131.37, df =4,

P <0.0001) (Table 3). The interaction term between species and the colony size also was significant in the model (Wald χ2 =126.38, df =24, P <0.0001). Compared with control (S. invicta itself), the highest risk for S. invicta worker survival was with M. pharonis, followed by M. minimum (Table 3). Worker survival time also was the least in presence of M. minimum and M. pharonis, irrespective of colony sizes of S. invicta (Fig. 3A–E). After 800 h of encounter, there was 50% chance for S. invicta to survive in presence of M minimum, M. pharonis, and also P. dentata, irrespective of the S. invicta colony size. Within 200 h, both M minimum and M. pharonis were able to cause S. invicta worker mortality for colony sizes of up to 240 (Fig. 3A–D), where as P. dentata caused mortality after 600 h. Compared with intraspecific interaction, the risk of S. invicta workers dying increased with every interspecific interaction irrespective of the species. The risk to S. invicta workers in the presence of S. molesta is more than P. dentata (Table 3) because the former caused S. invicta worker mortality within 250 h up to colony size of 120 compared with the latter that took up to 500 h (Fig. 3A–C). However, it took more time (800 h) for S. molesta to cause worker mortality once the colony increased to 240 compared with 625 h for P. dentata (Fig. 3D). As observed in brood abandonment data, S. molesta and T. bicarinatum did not cause S. invicta worker mortality in the colony size of 480. These two species significantly differed in their survival times with the exception of S. invicta colony size of 60. Although Forelius sp. did not invade S. invicta colonies or cause brood abandonment (Figs. 1 and 2), there was death of the S. invicta workers and queens in certain S. invicta colony sizes. This is evident in Fig. 3D and E, which displays that at 280 and 390 h, the probability of S. invicta surviving in presence of Forelius sp. in a colony size of 30 and 60 was 0.5, thus increasing the risk of S. invicta survival by a factor of 43.7. Control S. invicta colonies did not have any risk on the worker survival of S. invicta colonies of various sizes.

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Table 3.
Fig. 3.

(A–E) Probability estimates of survival times of S. invicta workers after being invaded by resident ant species, as a function of time (hours). Each cumulative curve for a species represented was based on N =5. The species followed by different letters are significantly different at P <0.05, defined in box 1 in Fig. 1. Note: The horizontal scale in the graph represents the times marked at uncensored observations, and the vertical scale represents the probability of worker survival.

There also was an increase in the S. invicta worker survival times with increase in colony sizes of S. invicta (Fig. 3A–E). Significant differences were observed between the survival times of all species when colony size reached 480. With colony size of 240, S. invicta worker survival was similar in the presence of S. molesta, P. dentata, M. pharonis, and M. minimum. Survival curves were significantly different between all the species when the colony size of S. invicta was 60, except T. bicarinatum and Forelius sp. Generally, the S. invicta colony size predicted the risk of S. invicta worker death. With the increase in the colony size from 30 to 60, the risk of S. invicta worker death decreases by a factor of 0.12. However, the risk decreases by a factor of 0.60 when the colony size changed from 120 to 240.

S. invicta Queen Survival.

The CPHR model for S. invicta queen survival was very similar to S. invicta worker survival (Wald x2 =137.45, df =6, P <0.0001; Wald x2 =117.31, df =4, P <0.0001) (Table 4). With the increase in the colony size, the survival times for S. invicta queens increased (Fig. 4). Compared with the control, every interspecific interaction had an increased risk for S. invicta queens (Table 4). Similar to the S. invicta workers, the risk of the S. invicta queen death was highest in interspecific interactions with M pharonis and M. minimum with least survival time for S. invicta queens in presence of the latter within colony sizes of 120, 240, and 480 (Fig. 4C–E). S. invicta queens had the same risk of being killed by S. molesta and P. dentata. S. molesta, however, took less time to cause S. invicta queen death in the colony sizes of 30 and 60 (Fig. 4A and B) and took more time to kill S. invicta queen with increase in the colony size to 240 compared with P. dentata (Fig. 4C and D). T. bicarinatum projected less risk because it took the longer for this species to cause S. invicta queen death irrespective of the S. invicta colony size (Fig. 4A–E). The least risk was to S. invicta worker survival by Forelius sp. (Table 4). No risk was found with control S. invicta colonies because they did not cause any queen mortality in the S. invicta colonies of varying sizes. Significant differences also were detected between species within each colony size. Differences existed between all the species within the colony size of 480, whereas within the colony size of 240, every species differed from each other except S. molesta and P. dentata, or M. minimum or M. pharonis. In the colony size of 120, differences were not detected between M. minimum and M. pharonis, and S. molesta and T. bicarinatum. However, differences were significant between all other species. For the colony size of 30, there were no significant differences between T. bicarinatum and Forelius sp. The same was true within the colony size of 30 workers.

View this table:
Table 4.
Fig. 4.

(A–E) Probability estimates of survival times of S. invicta queens after being invaded as a function of time (hours) generated by Kaplan–Meier survival technique. Each cumulative curve for a species in each S. invicta colony size was based on N =5. The species followed by different letters are significantly different at P <0.05, defined in box 1 in Fig. 1. Note: The horizontal scale in the graph represents the times marked at uncensored observations, and the vertical scale represents the probability of queen survival.

The increase in the colony size of 30– 60 and 60–120, indicated similar risks for S. invicta queens (Table 4). Significant differences were observed between the colony sizes except 30 and 60, and 120 and 240 (Table 4). Risk to S. invicta queens generally decreased with the doubling of colony size.

Discussion

The study demonstrates that, even after the emergence of workers, S. invicta colonies under 480 workers are susceptible to destruction by all the resident ant species studied except Forelius sp., which did not even attempt to invade the S. invicta colonies that had the colony size beyond 60 workers. These findings provide evidence that small S. invicta colonies can be destroyed by native ant species. The two exotic ant species M. pharonis and T. bicarinatum sp. also successfully invaded nests and killed S. invicta colonies of 480 and 240 workers, respectively (Figs. 1–4). However, the former species seems to be ecologically restricted to buildings and is less likely to come in contact with the S. invicta (Hedges 1992).

The study also demonstrates that both resident species and S. invicta colony size had a significant effect on the time to invade, brood abandonment by S. invicta, and death of S. invicta workers and queens (Tables 1, 2, 3, and 4). Furthermore, the time to the above-mentioned events differed between species with an increase in S. invicta colony size (Figs. 1–4). This may be attributed to differences in resident strategies used by different species. The risk of small S. invicta colonies being invaded by M. pharonis was the highest followed by T. bicarinatum compared with control colonies with a S. invicta colony invading another S. invicta colony (Table 1). Among the native species, the risk of being invaded by M. minimum, P. dentata, and S. molesta is 1/6, 1/30, and 1/24 the risk of being invaded by another S. invicta colony, respectively (Table 1). However, invading ability was not a good predictor of S. invicta colony destruction. For example, Forelius sp. did not invade the S. invicta colonies but did result in death of S. invicta colonies with up to 60 workers. On the contrary, larger S. invicta colonies invaded smaller S. invicta colonies with up to 480 workers but did not kill them, instead both colonies merged. Invasion was time dependent and also dependent on the size of S. invicta colonies and the resident ant species involved. For example, S. molesta invaded S. invicta colonies under 60 workers within 20–30 min but took nearly 10 h when the colony size increased beyond 60 workers (Fig. 2). This suggests that S. molesta can stealthily invade small S. invicta colonies without being readily detected but are readily detected and attacked when the S. invicta worker number increases to >160.

The risk of worker and queen death also was influenced by the species invading and the size of S. invicta colony being invaded. Two species, M. pharonis and M minimum, caused the S. invicta worker and queen death at afaster pace compared with the other species, thus resulting in the greatest risk for S. invicta colonies (Figs. 2–4; Table 2). Within 200 h, the entire S. invicta colony of sizes up to 480 was killed by these two species (Figs. 3 and 4). Conversely, the risk for S. invicta being killed was the lowest in presence of Forelius sp. (Table 2). The risk of S. invicta colonies being killed by P. dentata and S. molesta is almost the same (Tables 3 and 4). The risk of death to S. invicta was lowest by T. bicarinatum, although the risk of S. invicta being invaded by this species was high. The S. invicta colony size had a significant effect on colony survival in the presence of resident species (Tables 3 and 4). Colony size is known to influence competitive ability (Holldobler 1981, Holway and Case 2001); foraging behavior (Gordon 1995, Herbers and Choiniere 1996); and the size of recruitment response of ants, which often decides the outcome of competitive interactions (Fellers 1987). These data support theoretical work on the importance of colony size in the functioning of social insect colonies (Pacala et al. 1996, Anderson and Ratneiks 1999). In this study, as Pacala et al. 1996 colony size increased, the time to destruction by resident ant species increased. These results suggest that once Pacala et al. 1996 colonies reach 240 workers or more, the risks to their survival posed by these ant species rapidly decreases. This may be the point after which S. invicta may not be directly vulnerable to them. In control experiments, where small S. invicta colonies interacted with a larger S. invicta colony, invasion by the larger S. invicta into small colonies rarely resulted in the death of the small colonies. The two colonies were observed to merge in most replications. Furthermore, the risk to small S. invicta colonies of being invaded and taken over by larger polygyne S. invicta colonies is not related to the size of smaller S. invicta colonies used here.

Both P. dentata and M. minimum were good recruiters during their interaction with the S. invicta colonies, with P. dentata mainly recruiting their major workers after the first encounter with the S. invicta colony (A.R., unpublished data). After the invasion and attack on S. invicta colonies, all resident species except Forelius sp. were observed to carry the S. invicta brood back to their colony (Rao 2002). In contrast to brood carrying, Forelius sp. was observed to carry dead S. invicta workers to their nest. These observations were not quantified in the current study.

Although several reports (Porter and Savignano 1990, Gotelli and Arnett 2000) have shown that native ant diversity is adversely affected by invading S. invicta, at least four native species discussed above seem to persist (Fowler et al. 1990, Helms and Vinson 2001). This coexistence, along with the demonstration that these native species are capable of successfully invading and killing small S. invicta colonies, suggests that, if encouraged, these native species could help sustain pressure on and possibly reduce the S. invicta population, particularly in concert with the introduction of biological control agents from South America (Porter 1998).

Predators may not always have a lethal effect on their prey. Nonlethal effects may be equally important (Lima 1998), for example, reducing or preventing workers from small S. invicta colonies from foraging. This effect was portrayed by Forelius sp. in this study. Further quantification and analysis of this effect is required to draw any conclusions. However, this qualitative observation concurs with work on F. pruinosis, which blocks the nest entrance of their competitors preventing them from leaving the nest altogether and also succeeds in displacing larger ants of Myrmecocystus workers from food bait (Ho¨lldobler 1982). This behavioral effect on foraging of S. invicta by Forelius sp., if proven, might be exploited in ways that would exert pressure on S. invicta colonies.

Considering that several native ant species can reduce the number of newly mated S. invicta queens that can initiate a colony and that S. invicta queens that escape remain vulnerable even after they have reared workers, suggests that better use of our native ant species may be possible. Nevertheless, more detailed work on these species in the field is necessary to establish their importance in suppressing S. invicta densities. An ideal density of native ants that can limit the invasion of the S. invicta into an area, and whether densities of native ant species can be increased through augmentation and conservation, needs to be determined. Furthermore, methods need to be developed to rear these native species in the laboratory and successfully introduce them in areas after removal of the S. invicta. After introduction, their ability to prevent S. invicta reinvasion need to be investigated.

Acknowledgments

We thank Sherry Ellison for collecting and maintaining native ant colonies and Sean OʼKeefe for identifying the voucher specimens of ants to species level. Voucher specimens are located at the Entomological Research Laboratory, Texas A&M University. We also thank P. V. Pietrantonio and K. R. Helms for valuable comments on the manuscript, and M. Sherman for reviewing the statistical analysis. This research was funded by the Texas Imported Fire Ant Research and Management Project.

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