FORAGING AND RECHUITMENT ABILITIES OF SOLENOPSIS INVICTA BOREN, CO*!PARED WITH OTHER ANT SPECIES INDIGENOUS TO TEXAS
by STANLEY R. JONES, B.S. A THESIS IN ENTOMOLOGY Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved
May, 1985 yv
C-C^^'^ ACKN0^LEDGE?1ENTS
I would like to express my sincere appreciation to Dr. Sherman Phillips, committee chairman, for his guidance and support of this research. To Dr. Bernard Hartaan and Dr. James Wangberg I extend my gratitude for serving as commit tee members and for review of this thesis. I ar^ grateful to Dr. a. Willig for his helpful comments and assistance with data analysis, to Dr. Oscar Francke for verification of ant species identification, and to Is. Mary Peek for technical assistance. Finally, I thank James Cok- endolpher for review of preliminary drafts of this thesis. Special recognition is given to my parents for their encouragement and support of my education. To my wife Jo Ann, I express r^y deepest appreciation for her understand ing, support, and consideration during my graduate studies. This research was conducted through the facilities and financial resources provided by '^exas Tech University. To this institution I aii indebted.
11 ABSTRACT
Since its accidental introduction into "•obile, Alabama, the red imported fire ant, Solenopsis invlcta Buren, has be come an economic pest of much of the southern United States. Each year this ant continues to expand its range. Why S. invicta is able to successfully colonize areas previously inhabited by multifarious ant species is not understood. Possible explanations include its aggressive behavior, large colony size, and foraging efficiency. This study was con ducted to compare the foraging efficiency of S. invicta to three ant species native to south central Texas. Native ants studied include Pheidole dentata Mayr, wonomorium mini mum (Buckley) , and Forelius foetid us (Buckley) . Temporal recruitment and food retrieval patterns were recorded and con^pared for all four species held at equal colony strengths. Results indicate that interspecific differences in recruitment patterns do occur. However, these differenc- 9.5 are subtle and do not suggest that S. invicta is a more efficient forager under laboratory conditions, than the oth er three species.
Ill TABLE OF CONTENT'S
ACK>:OWLEDGEME!yTS 11
ABSTRACT 111 LIST OF TABLES iv
LIST OF FIGUPES V
CHAPTER
I. INTPODUC^ION AND LITERATURE REVIEW 1 Foraging and Pecruitment 5 Aggression 21 Competitive Avoidance 24 Objectives 31 General .Species biology 32 Pheidole dentata 32 Solenopsis invicta 32 Forelius foetidus 3 3 Monomorium minimum 3U II. r^STIIODS AND MATERIALS 36 Confrontation 37 Aggressive Ability 37 Defensive Ability 38 Recruitment and Foraging 39 Distance Travelled and Turns "Executed 44 Recruitment 47 Paits 48 Foraging 51 III. PiSUL'^:^ 53 Confrontation 53 Aggressive Ability 53 Defensive Ability 58 Recruitment and Foraging 61 Distance Travelled and Turns Executed 61 Pecruitment 63 Foraging 67
IV IV. DISCUSSION 71
LIT^RATUFE CITED 87
APPENDIX
A. ANALYSIS OF VARIANCE TABLES FOR FLFVEN PARA'IETERS 94 LIST OF TABLES
1. Analysis of variance table for number of contacts made by 17 species-caste combinations 94 2. Nested analysis of variance table for distance travelled 94 3. Nested analysis of variance table for number of turns executed 95 4. Nested analysis of variance table for initiation of recruitment 95 5. Nested analysis of variance table for time to reach peak numbers 96 6. 'tested analysis of variance table for peak numbers recruited 96 7. Nested analysis of variance table for bait retrieval time 97 ^. Nested analysis of variance table for numbers foraging prior to bait placement 97 9. Nested analysis of variance table for time required to discover a bait following placement 98 10. Nested analysis of variance table for numbers foraging after bait retrieval 98 11. Nested analysis of variance table for time to retrieve single baits 99
VI LIST OF FIGURES
1. Top ard side view (with detail) of a single ant foraging tray/myrmicary container 4 0 2. Side view of a free water dispenser, constructed from ^0X20 mm petri dishes 43 3. Top view of single ant foraging tray/myrmicary container 45 4. Results of confrontation studies between four ant species for 17 species-caste combinations 54 5. Results of confrontation studies between four ant species 57 6. Comparison of defensive abilities between 17 species-caste combinations 5 9 7. Comparison between species for distance travelled and number of turns executed 62 8. Comparative recruitment patterns of fotir ant species 64 9. Peak numbers recruited to food baits and bait retrieval times for four ant species 65 10. A comparison between species for the number of ants foraging prior to bait placement and after bai^ retrieval 68 11. A comparison between species for the time required to retrieve single baits 22 cm 69
vii CHAPTFIR I INTRODUCTION AND LITERATURE REVIEW
The manner in which various biotic and abiotic factors interplay in the environment determines not only the diver sity of an ecosystem, but also the particular habitats and functions of the taxa involved (Odum, 1975). Widely differ ing taxa, or similar taxa which utilize different resources, may have widely overlapping ranges with little or no effect on each other. Conversely, similar or dissimilar taxa with similar abiotic and/or biotic requirements may interact in various ways leading to resource partitioning (Wilson, 1971). Thus, organisms in an ecosystem occupy more or less well defined areas or niches. The niche concept has been variously defined by such early authors as Grinnell (1924), and Elton (1927) . In 1957, Hutchinson defined niche in a more complex but perhaps more accurate manner. His niche concept, known as a multi dimensional hypervolume, accounts for all environmental variables important to an organisms survival, and defines the organism's fundamental niche. Hutchinson further refined the concept to include an organism's realized niche which is the portion of the fundamental niche actually occupied by an organism after interactions with other organisms are accounted for. In this paper, Hutchinson's definition of niche will be assumed. Ants are conspicuous by their numbers and typically play a prominent role in most terrestrial habitats. Ants are generally among the principal predators of other in vertebrates. Their consumption of biomass and energy ex ceeds that of vertebrates in most terrestrial habitats. This large accumulation of biotic material is achieved via the continual searching of foragers night and day (Wilson, 1971) . because ants are so prominent in terrestrial habitats, inter- and intraspecific interactions are common and are im portant in shaping the distribution of ant species in a com munity, other biotic and abiotic factors are also important and include vegetation type and cover, food abundance, dis persion and dependability, temperature, rainfall and soil type (Sherba, 1<364) . With respect to food utilization, most ant species are omnivorous (Carroll and Janzen, 1973). Even predacious ants which utilize mobile insects as prey, sup plement their diet with scavenged resources. This behavior is due in part to the variable location in time and space of mobile prey, as well as the large energy expenditure required to locate and capture these items (Carroll and Janzen, 1973). Omnivorous food habits are usually correlated with broad niches and low food availability. Niche breadth for any one population should increase as a result o^ selection acting through the resource in short supply C^ernstein, 1979). Species diversity may also be re duced by lowered resource availability (Bernstein, 1979). For example, ant colonies are often more widely spaced in areas of low productivity than in areas of high productivi ty, with a corresponding reduction in diversity. This cir cumstance was described by Davidson (1977) for several spec ies of granivorous ants. This relationship indicates that food is probably a limiting resource (Davidson, 1977). Species diversity within a community may also depend on moisture gradients and general environmental heterogeniety (Davidson, 1977). Bernstein (1979) identified three compo nents of environmental heterogeniety which are important when considering niche breadth in ants. These factors in clude: the range of temperatures utilized for foraging, the size of the foraging area, and the diversity in size and distribution of food sources utilized. 7hen environmental conditions favor an increase in niche breadth, indicated by low species diversity and omnivorous food habits, populations may respond with an increase in either the within phenotype component or the between phenotype component of niche breadth. The between phenotype component is shaped by the above ecological factors and is expressed as differences among individuals within a population. In the extreme case, these ecological factors could lead to a population composed of individuals highly specialized for individual tasks. This condition is unlikely because the diversity of genes within a population limits the phenotypic variability, and genetic recombination tends to dilute high ly specialized genotypes (Bernstein, 1979). The within phe notype component of niche breadth involves the variety of resources collected and used by each individual within a population. In the extreme case, the population would be totally composed of generalists, each of whose niche breadth would equal that of the population as a whole. A population of this type would be highly inefficient in resource utili zation (Pernstein, 1979). However, in areas of low food availability, populations will tend to be generalists in or der to secure as much of the available resources as possi ble. Ants, due to their social behavior, have been able to increase the efficiency of resource utilization despite an increase in the within-phenotype component of niche breadth
(Bernstein, 1979). In ant communities, a gradation of generalists (within-phenotype component) to specialists (between-phenotype component) can be found. As stated previously, reduced resources favor niche broadening. As niches widen and populations utilize a broader food base, overlap in resource utilization results in increased intra- and interspecific competition. Inter specific competition between predominant ant species acts to oppose niche broadening. To avoid competition, yet remain relatively efficient in resource utilization, ecological variability within a population may increase. Ecological variability is achieved through change in phenotype, physi ology, or behavior ("Bernstein, 19''9) . The resulting commu nity is composed of populations differing in a vast array of scological components. Equilibrium in niche breadth within the community will depend on the^ efficiency of resource re covery, resource density, dependability, dispersion, and competitive abilities of the various populations (Bernstein, 1979) .
Foraging and Pecruitment The fitness of individual colonies depends to a large extent on an optimal diet. The ability of individuals with in a colony to gather food determines the extent to which an optimal ^iet is realized. Foraging strategies are shaped by natural selection (within the limits of genetic variation) such that nutrient gains are optimized and risks of foraging are minimized (Hassell and Southwood, 1978). A large amount of literature is available which deals with the subject of optimal foraging. Much of this work indicates that animals may indeed forage optimally (Pyke et al., 1977). Organisms such as ants, which return gathered food to a central location, employ specialized foraging strategies as compared to solitary foragers (Bernstein, 1975). Ant work ers are specialized for food gathering and display fairly fixed ecological roles based on their behavioral and morpho logical adaptations (Davidson, 1977). The teraporo-spatial structure of the environment can have a decisive effect on the strategies and rates of foraging by different ant spec ies (Hassell and Southwood, 1978) . Lowered environmental temperatures will reduce the rate of travel by all species foraging, but may differentially effect some more than oth ers. For example, decreased temperatures slowed the rate of foraging of both Formica subsericea Say, and Camponotus pennsvlvanicus (DeGeer) , but slowed F. subsericea more than C« pennsvlvanicus thus allowing the latter to collect more of the available resources (Klotz, 1984) . According to By ron et al. (1980), ants are potentially active at temperatures between 2*'c and 62 C. The foraging rate and strategy employed by a species while searching for food is most effected by food distribution, density, and renewal rate (Bernstein, 1^''*^) . Py employing a variety of foraging strategies, ants are able to obtain required food and reduce competition. Many species forage strictly by chemical cues and markers, others visually, and some employ a combination of visual and chemi cal stimuli (Carroll and Janzen, 1973). Foraging strategies in ants are highly variable, and are divided into separate categories strictly for conven ience. For example, Davidson (197"^) identified two general foraging strategies utilized by desert granivorous ants- Individual foraging occurs when each individual ant searches independently of all others, and the total area surrounding the colony is continuously searched. Group foraging in volves use of a permanent foraging trail or column, usually in restricted areas around the nest, "'his categorization is somewhat arbitrary in that group foragers generally forage individually at the distal ends of the foraging columns. Bernstein (1975) categorizes foragers into three general types. "'he first two categories are similar to those de scribed by Davidson (1977), and include individual foraging and group foraging. The third category is recruitment for aging which begins initially as individual foraging. However, upon discovery of a large food source, retrieval is accomplished and organized as a group effort. Many ant species utilize? only one of the above foraging categories. 8 However, some species are capable of switching from one strategy to another depending upon environmental factors. For example, Veromessor pergandei (Mayr) may switch from group to individual foraging strategies when food diversity decreases (Bernstein, 1975). Pogonomyrmex rugosus Emery ex hibits this same characteristic (Davidson, 1977). Ant spec ies, capable of changing foraging strategies as food densi ties change, are at an advantage in heterogeneous environments. Some ant species may employ several foraging strategies simultaneously. Lasius neoniger Emery workers search for and retrieve small baits individually- Larger, yet movable prey are cooperatively retrieved by several workers in an area. Lasius neoniger retrieves medium sized, immovable prey by recruiting a group of workers from the nest. Finally, large immovable food items are retrieved via mass recruitment. This ability ascribes a flexibility in food utilization to colonies of L. neoniger and increases efficiency (Traniello, 1983) - Traniello (1983) also demon strated that for L- neoniger, the majority of food in terms of biomass was retrieved individually, but 85T of the usable food was retrieved via mass recruitment. The above study demonstrates the advantage associated with mass foraging- Wilson (1962) determined that the efficiency of mass foraging depends upon the rate of buildup of individuals at a food source, swiftness of local exploration around the food site, rate of individual feeding, individual retrieval ability, and gut capacity. Due to the time required to re cruit individuals to a newly discovered food source, the food remains available to other ant species in the communi ty, ••ass recruitment may, therefore, be a less efficient means of food gathering than individual retrieval (Oster and Wilson, 1978) . However, somewhat earlier, Schoener (1971) ascribed several advantages to group foraging over individu al foraging. Group foraging may increase the colony's abil ity to defend its feeling area and should result in reduced feeding and defending time per individual. Schoener also pointed out that increased forager activity through group foraging may cause a reduction in overlap of foraging areas which should decrease aggressive encounters. Communal for aging may also result in increased foraging areas and in creased ability of colony members to subdue and utilize large prey. >^cre recently, Davidson (1977) demonstrated that group foragers spend less time searching for food and more time traveling to and from a food source than do indi vidual foragers. In general, group foragers are more successful in locating food than are individual foragers (Davidson, 1977) . However, some individual foraging is involved in all group foraging species. At the distal ends 10 of columns and even at all points along the columns, individual foragers are found. The number of individual foragers are greater distally from the nest than near the nest perimeter. Examples of group foraging species include: Pogonomyrmex barba tus (Smith), P. rugosus, Pheidole xerophi- la wheeler, and V. pergandei (Davidson, 1977). Types of foraging strategies are often correlated with food resource distribution. Group foragers are typically specialists on high density or clumped resources, whereas individual foragers such as Novomessor cockerelli (E. Andre) are specialists on highly dispersed food resources (David son, 1977). Mass foragers specialize on patchy resources which appear suddenly and randomly within the colony's for aging area. This strategy is most efficient for omnivorous and predacious genera such as Solenopsis Westwood, Pheidole Westwood, and Iridomyrmex Mayr (Carroll and Janzen, 1973) . Fluctuations in foraging occur both seasonally and dai ly. In temperate zones, all ant species rednce or arrest foraging during the winter months. In general, seasonal fluctuations in foraging activity are more pronounced in group foraging species than in individual foraging species (Davidson, 1977). Diel fluctuations may be governed by competition, temperature, or light. For example, Camponotus sem^itestaceus (Emery), a nocturnal forager, depends on a 11 combination of light and temperature cues to begin foraging activity. »*aximum activity begins in late evening when light intensity approximates one foot-candle and tempera tures decrease (Gano and Rogers, 1983).
Ants tend to increase efficiency and minimize risks of foraging. Wilson (1*^63) has postulated that foragers are older members o^ a colony. Since risks of foraging are high, older foragers may represent a low-cost caste which are expendable (Mackay, 1983). Older workers have already contributed labor to the colony, thereby contributing to colony fitness. Expendable workers can afford to be more aggressive, a factor which may enhance a colony's foraging success. Finally, older foragsns may be more familiar with the foraging area, again increasing foraging efficiency (Carroll and Janzen, 1973). Within limits, the larger the number of foragers, the greater the chance of discovering food. Ant colonies, therefore, should optimize energy intake and expenditure with number of foragers. However, not all ants outside a nest are foragers. Studies of foraging behavior have made use of numbers of individuals entering and leaving a nest as a measure of foraging activity. As Gordon (1983) points out, this measure assesses colony activity, not foraging activity. Gordon (1983) demonstrated that many of the ants 12 outside the nest may be involved in nest rebuilding, or midden work, and will not retrieve food sources. In any case, the number of ants foraging may be related to colony success. Under natural field conditions, Baroni-Urbani and Kannowski (1974) recorded the number of four ant species crossing an artificial line in 30 min. During the observa tional period, 19 Camponotus pylartes Wheeler, 18 Solenopsis invicta Buren, 1^ '^onomorium minimum (Buckley), and 10 Pseu- domyrmex flavidula F. Smith crossed the imaginary point. These numbers are surprisingly constant when considering differences in colony size, distribution, territoriality, and environmental heterogeneity within ant communities. When a forager returns to the colony after discovery of a food source, excitement is generated within the colony, resulting in increased foraging activity (Carroll and Jan zen, 1971). This increase in foraging activity is correlat ed with the increased probability of finding additional food in the vicinity of the first discovery. Such activity in creases are common among ants. Colonies of both P. bar^atus and N. cockerelli increased foraging activity after contact with millet seeds (Davidson, 197*7). in a laboratory experiment (Ayre, 1968), the number of ants passing over a predeter^iined area was recorded both before and after prey introduction. .Results demonstrated that for the three 13 species tested, foraging activity increased after food discovery. Myrmica aaericana Weber increased activity from 44 to 505 individuals. Creaatogaster lineolata (Say) in creased from 263 to 6 08 individuals, and Formica obscariven- tris 3ayr increased from 137 to 244 individuals (Ayre, 1968) . Regardless of the numbers of ants foraging, an individ ual food source is typically discovered by a single worker. The time required for an individual to locate a food source after leaving the nest entrance is defined as search time (Davidson, 1977). Factors which may influence the search time include speed of travel of the forager, radius of the perceptual field, radius of the prey, and olfactory cues. Laing (1938) developed a simple model which predicts rate of discovery over a two-dimensional surface. His model ac counts for speed of the forager, perceptual field, radius of the prey and density of both foragers and prey items. Ayre (1968) demonstrated that the distance of perception for M. americana, C. lineolata and F. obscuriventris was limited to one cm. Prey discovery by these species appeared to be a completely random process. Both visual and olfactory cues were involved in final prey location. In a meadow containing 25-50 S. invicta mounds/acre, one to two h were required for foragers to discover peanut butter baits placed within 30 cm of nest entrances (Horton et al., 1975). 14 Ants may search randomly for food items, but the movements of individuals may serve to increase the probabil ity of contact. Ants generally forage in an asymmetrical pattern of loops and turns. These turns may be analogous to the side-to-side casting movements seen in coccinellid lar vae and other organisms (Pyke et al., 1977). Once a food source has been discovered, it must be re turned to the colony. Return of a food item involves han dling time, defined as the time elapsed between food loca tion and return, usually by the most direct route (Davidson, 1977). "he rapidity with which small food items are re turned, depends directly upon the speed of the individual retriever. In some cases, more than one individual retriev ing a food source in unison may actually increase handling time. For example, times for three ant species to retrieve a housefly larva a known distance were recorded (Ayre, 1968). Individual workers of F. obscuriventris and H. americana transported larvae at rates of 0.1693 and 0.1820 cm/s respectively. When two workers simultaneously trans ported a single larva, transport rates were 0.5502 and 0-0465 cm/s, respectively. Three or more individuals involved in transport completely negated any organized retrieval (Ayre, 1969) . '-ass recruitment of workers to a food source allows a colony to retrieve a source much more 15 rapidly than could individual foragers discovering the food independently, lapid return of a large food source is espe cially important in habitats with limited food resources, and for organisms such as ants whose foraging territories often overlap with severe competitors (Carroll and Janzen, 1973). Group or mass foraging may impart an advantage in terms of food gathering efficiency. ^oaonornvrmex barbatus, a group forager, retrieved 200 seeds in 5-10 min whereas N. cockerelli. an individual forager required 60-90 min to re trieve 200 seeds (Davidson, 1977). Among omnivorous ant species, behavioral differences in mass recruitment and prey retrieval appear to be important in species interactions and success. i^onoroorium minimum is successful at retrieving both large and small baits, but must dissect the baits. La sius neoniger is successful at retrieving small to medium sized baits through rapid carriage and removal of the prey (Adams and Traniello, 1981) . In general, ant species with body lengths less than 3.4 mm are group foragers (Davidson, 1977). Small size may be, in part, related to strength in numbers. In a heterogeneous environment rich in competi tion, mass recruitment of many small individuals rray be a means by which workers are rapidly mobilized to surround and defend a food source. However, the strength in numbers concept assumes that, for example, a 1 gm force of small 16 ants is a superior competitive force to a 1 gm force of large ants (Carroll and Janzen, 1973). ""ajor workers of E!l£lil2l^ fallax '^ayr are rarely seen outside the nest until foragers discover a large food source. In this situation, majors are rapidly recruited and aid in defense of the food, or aid in displacement of a species already on the source (Ttzkowitz and Haley, 1983). Individual ants retrieve food
either in their mandibles or crop. The amount of food car ried by each individual influences the handling time for a food source (Carroll and Janzen, 1973). For example, an in dividual of S. invicta removes approximately 0.05 mg ••• or - 0.0017/visit to a peanut butter bait. Thus each individual retrieves about 4,5°!'i of its body weight (Horton et al. , 1975).
The size of a colony's foraging area governs the amount of food which it can utilize. ?1any factors are in volved in determining a colony's foraging arena. One of these factors involves the rate at which individual ants travel. In general, larger foragers are able to travel greater distances in search of food. This ability may be a result of greater navigational skills (Bernstein, 1979), or of leg length and physiological properties. Another generality is that ants with body lengths less than 3 mm typically travel no farther than 3 to 4 m from their nest. 17 Ants with body sizes exceeding 9 mm have been observed traveling over 40 m from their nest (Davidson, 1977). Iri- i2.El£E£5c spp. have been observed traveling 70 m before be ginning to search for insects (Carroll and Janzen, 1973). Formica fusca Linnaeus workers may travel 200 m from their nest in order to obtain energy rich honeydew from aphids (Brian, 195^) . Ant diets may be shaped to some degree by interference from other ants. Traniello (1983) demonstrated that L. neo- 5.ia^£ rarely lost small baits (8 mg) to competitors due to rapid retrieval rates. However, larger baits (20-130 mg) were often lost due to reduced retrieval rate, thus exposing the prey to competitors for longer periods. Lasius neoniger overlaps with several other ant species in nest site prefer ence, diurnal and seasonal foraging activity, prey type, and size. These sympatric species include ^;_. americana, M. min imum, Formica schaufussi Wheeler and Tetramori^um caespitum (Linnaeus). These species affect the foraging success of L. neoniger in two ways: by indirect methods such as resource preemption, where food is discovered.and removed before L. neoniger locates it; and by direct methods including direct aggression, displacement at food baits, and chemical interference. The ability of a colony to quickly recruit workers to a ^ooi source, and to secure a food source. 18 greatly affects its foraging success (Traniello, 1983). The above scenario is not uncommon among ants and illustrates the importance of rapid recruitment ability in successful competition.
Recruitment is initiated when colony workers respond to pheromone trails and motor stimuli provided by returning foragers (Crawford, 19<^3). Recruitment in some ants (for example in Camponotus socius Roger) is initiated soley by motor display. Up to 30 ants may be recruited by this meth od in 5 to 15 seconds- Trail pheroaones are established by the returning foragers, but these pheromones do not stimu late recruitment. The trail serves only as a guide back to the food item, Pecruitment is a short term process as compared to for aging. Pecruitment involves an active buildup of individu als at a food source followed by a reversal of this process after the food is retrieved (Gordon, 1983). Variability ex ists in the recruitment rate of particular colonies (Holldo- bler, 1976) . This variability may be a result of several biotic and abiotic factors including interspecific competi tion, characteristics of the food source, the hunger or satiation level of the colony, and distance from the colony (Gordon, 1983). In addition, the number of individuals comprising a colony may effect the distance over which 19 recruitment can occur as well as the speed with which individuals can build up at a food source (Wilson, 1962). Graded complexities in recruitment behaviors exist among ants. The most simple recruitment behavior is tandem running, in which follower ants keep close antennal contact with a leader, and follow the leader to the food. The most complex form of recruitment involves chemicals whereby ants are stimulated to follow a chemical trail even in the ab sence of a leader. Intermediate stages between these ex tremes do occur (Holldobler, 1971). Two types of rapid re cruitment to a food source are known. The first involves chemical trails and was described above. Use of chemical trails results in an exponential increase in ant numbers at a food source until a maximum is reached. The second type is group recruitment which results in a pulse of workers ar riving at a food source, with little subsequent recruitment (Carroll and Janzen, 1973). Lasius neoniger uses two commu nication systems in recruitment. The first system, termed short range, chemically attracts workers within a 5 cm radi us of the emitting ant toward a food source. If short range recruitment does not attract sufficient foragers to rapidly retrieve the food source, the second communication system is used. The second system is mass recruitment via a pheromone trail. 20 A considerable amount of inaccuracy in trail following is exhibited by recruited workers. However, this inaccuracy is not related to distance of the bait from the nest (Wil son, 1962) . Less accurate recruitment may benefit the colo ny in that ants which miss the target may discover new food sources and initiate new recruitment (Pasteels et al., 1982). Pecruitment is determined not only by the size of a bait, but also by its moveability. Ayre (1968) demonstrated that M. americana will form a recruitment trail to an immov able food source, no matter how small. Wilson (1962) demon strated that recruitment trails are not solid trails, but are made up of a series of short streaks. In Solenopsis saevissima Forel, recruitment pheromone is released from the Dufours' gland and applied via the extruded sting which is touched periodically to the substrate (Wilson, 1962) . Indi vidual workers are able to detect a newly laid recruitment trail from as far as 10 ram. Upon detection, workers respond by immediately following the trail away from the colony en trance. Artificial application of trail pheromone elicits the full response (Wilson, 1962). Taylor (1977) demonstrat ed that recruited Solenopsis geminata (Fabricius) workers decrease in number as distance to a food source increases. Conversely, numbers recruited increase with an increase in food quality. Crawford and Rissing (1983) suggest that 21 worker ants of Formica oreas 'rheeler receive information about the quality of a food source from stimuli provided by returning foragers. Recruiters are therefore able to commu nicate not only location of a food source, but also food quality. "^n this manner, colonies are able to maximize ef ficiency by recruiting a number of individuals to a food source proportional to the quality of the source.
Aggression Pecruitment and foraging activity often lead to direct intra- and interspecific aggressivity. Confrontations be tween ant species are common, but appear especially promi nent in spring when ants are re-establishing new foraging boundaries after winter contraction (Brian et al-, 1966). Levins et al., (1973) state that the adjustment of species niches to local physical and biotic conditions is primarily a. result of the way in which species interact. Aggression in ants is a consequence of cost benefit relations between the risk of loosing workers, energetic drains of aggressive behavior, and the reward of secured resources. Loss of for agers may be more costly to small colonies than to large colonies, a principle directly related to agonistic fervor (Carroll and Janzen, 1973). 22 Numerous authors have documented cases of aggression in Formicidae. For example, intensity of aggression between wyrmica rubra Buckley, Myrmica subuleti Forester, T. caespi tum, Lasius flavus Mayr, and Lasius niger Mayr were quanti fied by recording frequencies of mandible opening, seizing, gaster flexing, and carriage of the enemy (De Vroey, 1979). 8hatkar (1973) used 27 species of ants native to Florida to test their combative ability against the aggressive S- in victa. He discovered that several species were able to de feat S. invicta during aggressive encounters. Pheidole den tata Mayr has apparently adapted a species specific alarm-recruitment pheromone which alerts the colony of ap proaching s. invicta workers. Minor workers of P. dentata, upon contact with S. invicta, rapidly recruit soldier work ers from the nest to the area of contact. Pheidole dentata soldiers have large heads and mandi>^les, and are well adapt ed for combat (Wilson, 1976). In a heterogeneous environment, habitats may be divided into most preferred, moderately preferred, and least pre ferred- In areas with low population densities, such as areas of colonization, organisms will tend to fill the most preferred habitats first. In this situation, niche breadths may be stable, and unfilled niches may exist. However, as population densities and diversity accrue, the less 23 preferred niches will begin to fill, with competition for preferred habitats heightening (Vandermeer, 1972). Due to the frequent interactions of ants and their ag gressive behavior, competitive displacement is common (Mack- ay and Mackay, 1982). Competition generally occurs over nest sites and food- These two factors are interrelated and difficult to separate, because a decrease in available food resources may influence the number of suitable nest sites (Carroll and Janzen, 1973) . The process of competitive dis placement occurs when two or more sympatric species, utiliz ing a vital resource in short supply, compete in an asymme trical manner, such that the most efficient species eliminates the other (Mackay and Mackay, 1982) - The compet itive edge may go to the species most suited to a particular environment- Thus the importance of the environment to species interactions is clear. For example, Scherba (1964) demonstrated that aggression is the major cause of species replacement between the ants F. fusca and Formica opaciven- tris Emery. In forest habitats, F- fusca excluded F. opaci- ventris. In meadow habitats, the reverse was true. At the borders between ^orest and meadow, an area of coexistance occurred despite active reciprocal replacement by both species. Areas established by S. saevissima have resulted in total extinction of the native species Solenopsis xvloni 24 ^cCook. Solenopsis geminata has wider habitat preferences and has competed with S. saevissima more successfully (Wil son and Brown, 1957). Ants native to Burrauda have been re peatedly invaded and replaced by numerous introduced genera including i^dontomachus. Ponera, Pheidole, Iridomyrmex, Moiio- t2£i!iE# Brachvmyrmex. and Tetramorium (Crowell, 1968). Interspecific competition occurs when the densities of one or more colonies are lower than would be the case if each colony were present alone (Pontin, 1961). competition is often difficult to demonstrate, but is indicated by such factors as aggression, displacement, or overdispersed colony spacing. Pontin (1961) demonstrated competition between L. flavus (F) and X. niger (L) , by increased alate production when interacting colonies were separated. Despite the occu rence of competition, equilibrium densities can be achieved and coexistence maintained (Portin, 1961).
Competitive Avoidance The relationship between niche overlap and competition remains unclear. Niche overlap represents the degree to which two species overlap or co-occupy a habitat, but does not define or elucidate the interaction between these two species (Vandermeer, 1972). Competitive avoidance and lack of aggression is as common as overt aggression. Ants tend 25 to partition food and habitat in at least the three following dimensions: microhabitat, food type, and time, often, as a result of competition, ant colony distributions result which minimize, but may not exclude, overlap- For example, two species which are able to compete evenly with each other ( L. neoniger and S. invicta) may develop spatial patterns allowing coexistence (Apperson and Powell, 1984) . Species of similar size and food habit may coexist if dif ferent foraging strategies are used. However, species of similar body size using similar foraging strategies never coexist, and may act as ecological replacements (Davidson, 1977) . Ants often display niche shifts in order to reduce com petitive pressure. For example, the niche breadth of the polymorphic species V. pergandei is dependent upon competi tion with other ant species. Veromessor pergandei is able to shift its caste ratios in such a way that competitive pressures are reduced (Davidson, 1978). In addition, diel partitioning is used as a mechanism for resource partition ing. Klotz (1984) demonstrated that C. pennsvlvanicus and F. subsericea forage at different times of the day, thus avoiding direct interaction. Trunk trails used by P. rugosus an^ P. barbatus are initially formed to exploit a productive resource patch but may, once established, act to reduce interspecific confrontation. 26 Territoriality is common among ants and plays an important role in resource partitioning and competitive avoidance. A territory is an area occupied more or less ex clusively by a colony, and is maintained by direct aggres sion or posturing (Jaffe and Puche, 1984). Jaffe and Puche (1984) reported the occurrence of a territorial pheromone or marker produced by S. geminata used in territory mainte nance- Similar pheromone markers have been reported in oth er ant species (Holldobler, 1979; Jaffe et al., 1979; Holl- dobler and Wilson, 1977) , but do not at present appear to be of common occurence. Territory size is determined by the number of ants in a colony as well as the amount of food in an area (Wilson et al., 1971). The following three charac teristics typify ants possessing territories: colony forag ing areas are stable and contain a rich food supply, colo nies consist of numerous small workers, and colonies are located in habitats of low heterogeneity (Carroll and Jan zen, 1<^73). When two individuals of similar or disparate taxa con tact, one of the following results nay occur: both ants may exhibit overt aggression, a more aggressive ant may attack - a less aggressive ant with the escape of the latter, one ant may pose defensively and be ignored by the second, or both ants nay ignore each other. Despite the aggressive nature 27 of S. invicta, Bhatkar (1973) demonstrated that several ant species possess specialized defensive adaptations which al low them to exceed the defensive potential of S. invicta in laboratory confrontation studies (Bhatkar, 1973). In the majority of species tested by Bhatkar (1973), momentary de- fensiveness was displayed prior to escape. The important word here is escape, which reveals that aggressivity in one ant may be countered by escape in another. Thus not all ant encounters result in fighting. Paratrechina melanderi are- nivaqa (Wheeler) is a small swift ant which is typically ig nored by 5. invicta. Solenopsis molesta Say, is also small, and is also ignored by S- invicta (C'lTeal, 19^3). Baits oc cupied by M_. minimum were maintained by chemical interfer ence, repelling individual workers of S. invicta. However, as greater numbers of S» invicta arrived at the bait, M- minimum left- No conflict resulting in death of either species occurred (Baroni-Urbani and Kannowski, 1974)- Phei- dole floridana Emery avoided conflict with 5^ invicta by fleeing at the slightest contact (Howard and Oliver, 1979). Food appeasement between species of distant colonies has also ^een locumented. This may be a mechanism by which the 3onor art distracts an aggressive recipient long enough to escape without harm (Bhatkar, 1979). 28 SolenoT^sis invicta has rapidly increased its range, since its accidental introduction into the United States, while simultaneously displacing native ant species (Mackay and Mackay, 1982). Wilson and Brown (1957) report that the rate of displacement is occurring at approximately 8 km/y in the Gulf Coast States. After the establishment of S. invic ta in a new area, elimination of the weaker colonies rapidly occurs (Hays, 1959). Success of an invading species may de pend upon altitude, moisture, soil type, vegetation, colony size, aggresivity, and resistance from other organisms (Bernstein, 1975). Loss of foragers may be more costly to small colonies than to large colonies so that members of small colonies may be timid, whereas individuals of larger colonies may be more aggressive. Colonies of S. invicta are often very large and may exceed 44,000 individuals (Baroni- Urbani and Kannowski, 1974). Bhatkar (1973) found that of 27 ant species common to Florida, population densities of S. invicta exceeded these in every case (except in a few local ized areas). Whitcorab et al. (1972) discovered that S- in victa was the most aggressive ant species found in soybean fields of northern Florida. In addition to aggressive behavior, S. invicta has been touted as a highly efficient and effective forager (Horton et al., 1975). When these factors are considered, the ability of S. invicta to 29 increase its range, is apparently due to large colony size, aggressive behavior, and foraging efficiency. However, sev eral ant species common to the southern United States exceed the aggressive potential of S. invicta (Bhatkar, 1973; Bhat kar et al. , 197*:) on a one to one basis. Therefore, colony size and foraging ability may be more important factors in the success of S,. invicta, than aggressive behavior. Ants in the genera Pheidole^ Solenopsis, and Monomorium are known to be scavengers (Carroll and Janzen, 1973)- In areas where these ants exist, overlap in resource utiliza tion should lead to competition. In addition, Davidson (1977) has reported that species utilizing similar food re sources with similar foraging strategies cannot coexist. Scavengers typically have colonies composed of numerous small workers with highly developed chemical communication abilities, allowing rapid recruitment of individuals to food sources (Carroll and Janzen, 19^3) . This description exem plifies species of all three genera (Wilson, 1962; Wilson, 1976; Adams and '^raniello, 1981). When S. invicta invades an area occupied by ^heidole and '^onomoriui, some competi tive interactions should occur due to the similarities described. Wilson (1976) has described S. invicta as a major eneny of P. dentata. In addition, Baroni-Urbani and Kannowski (19'^4), as well as Adams and Traniello (1981), 30 have described competitive interactions between ^. minimum and S. invicta. The accounts of these interactions indicate that both 'A_* minimum and P. dentata are somewhat successful in competition with S. invicta. In a recent study conducted by Phillips (unpublished), S. invicta, P. dentata, M. mini mum, and a ^ourth species, Forelius foet idus (Buckley) , oc curred repeatedly at high frequencies during a year-long sampling study conducted in Kerr and Bandera counties, Tex as. This study indicated that the above ant species are sympatric in distribution. In addition, given the high fre quency of occurrance of P. dentata, M. minimum, and F. foe- tidus in areas established by S. invicta, these species are apparently coexisting with, and successfully competing against s. invicta. Environmental conditions required for the survival of S. invicta may become less favorable as this ant moves north (low winter temperatures), and west (low humidity). As a result, a decrease in population density should be expected. Under these conditions, the ability of S- invicta to compete with resident ants should also decrease. Abiotic and biotic resistance will thus effectively increase. The competitive abilities of resident ants may then exceed those of S. invicta colony for colony as well as on an individual to individual basis. The realization of this concept may 31 significantly affect the further expansion of S. invicta. In order to determine the effects of interactions between populations, analysis and demonstration of interactions at the individual level are needed (Pontin, 1961).
Object ives The following study was designed to compare the re cruitment and foraging abilities of P. dentata, S. invicta, M. minimum, and _^- foetidus on an individual basis when num bers of individuals within colonies were equal. Specifical ly, the study compared the following parameters: aggressive abilities, defensive abilities, recruitment abilities, and foraging abilities- These parameters involve phenotypic and behavioral aspects considered important to colony foraging success. Due to the difficulty of determining accurate col ony numbers in ^he field, and due to the wide assortment of variables which might confound separation of foraging effi ciency from other aspects of niche delineation, a laboratory design was used. These studies should aid in understanding the following: the factors enabling the coexistence of the above four ant species in central Texas, the ability of S. invicta to invade new areas and displace ants native to Texas, and the ability of resident ant species to compete with S. invicta as the latter invades new territories. 32 General ^ecies Biology Pheidole dentata Mayr "^hese ants are common fron North Carolina to Florida and westward to Texas (Creighton, 1950). They construct mounds in soil or old logs, and prefer lower elevations and shady areas (Dennis, 1938). Their colonies are populous, containing many thousand individuals ("^ilson, 1976). Only one queen/colony is common, but some are polygynous. Colony members are divided into two distinct castes, minor workers and majors (soldiers) . Approximately 8 to 20'?' of these col onies consist of majors (Wilson, 1976). The majority of P. dentata diet consists of insects and other arthropods (Smith and College, 1924). Seeds have also been found in some nests (Cole, 1940). Pecruitment is accomplished via a trail pheromone emitted from the poison gland of the sting- Only minor workers lay pheromone trails. Foraging behavior in cludes individual foraging and mass recruitment (Wilson, 1976)- -ody lengths of minor workers are approximately 3mm, and majors, 6ram.
Solenopsis invicta Buren The red imported fire ant is native to southern Brazil (Buren, 1972) . This species was accidentally imported into the United States, and was first reported from Mobile, Ala- 33 bama, between 1930 and 1945 (Buren et al., 1974). Colonies are large, with numbers of individuals ranging from 44,000 to 250,000 (Baroni-Urbani and Kannowski, 1974; Adkins, 1970). "^referred habitats include cultivated fields and pastures (Adkins, 1970). These ants are polygynous with as many as 63 queens reported fror a single colony (Fletcher et al., 1980). Glancey et al- (1975) reported an extraordinary case of polygyny in which a single colony contained 200 fer tile queens. Colony workers are typically divided into three castes; minors, mediums, and majors. '^hese divisions are based on size and behavior (Mirenda and Vinson, 1981). Red imported fire ants are omnivorous in food habit (Wilson and Oliver, 1969). Pecruitment is accomplished via a trail pheromone emitted from the poison gland of the sting- All three castes are able to recruit, but most foraging is done by minors- Foraging behavior includes individual foraging and mass recruitment (Wilson, 1962). ^ody sizes range from 2 mm for minors to 6 mm for majors (Mirenda and Vinson, 1981) .
.Forelius foetidus (Buckley) These are small, active ants ranging from Texas to southern California and south into Mexico (Creighton, 1950). ^^ests are found under stones or in open habitats, and have a 34 soil crater around the nest entrance. Workers are mono- morphic. These ants are aggressive but harmless, and appear very similar to Iridomyrmex pruinosus (Roger) (Cole, 1937). Colonies are populous and highly polygynous (Personal obser vation) . The diet of F. foetidus consists of arthropods and honeydew (Wheeler and Wheeler, 1973). These ants employ three methods of foraging: indivilual foraging, group for aging, and mass recruitment. During conflict with another ant, a sticky fluid, discharged from the repugnatorial gland, is smeared on the body of the "enemy." The fluid acts as a repellent, irritant, and in some cases may be fa tal to the attacking ant (Wheeler, 1910) . Body length is approximately 2.5 to 3mm-
Monomorium minim_um (Buckley) '^hese ants are small and range from southeastern Canada and the northeastern United States southwest to the Pacific coast (Creighton, 1950). Nests are constructed in most ha bitats, often under objects (Wheeler, 1910). These ants are monoffiorphic and highly polygynous (Dennis, 1938). Colonies are highly variable in size, but typically contain several thousand individuals (Gregg, 1944). The diet of M. minimum consists of arthropods and honeydew (Wheeler and Wheeler, 1973), These ants exhibit activity peaks during the 35 hottest parts of the day, and rarely forage nocturnally (Paroni-Urbani and Kannowski, 1974). Foraging is accom plished by three methods: individual foraging, group forag ing, and mass recruitment. Recruitment is initiated and maintained via pheromone trails laid down with the sting. Monomorium minimum employs gaster flagging as an effective defense. Worker body sizes range from 1 to 1-5 mm (Wheeler and Wheeler, 1973; Howard and Oliver, 1979). CHAP'^FR II METHODS AND MATFRIALS
All ants were collected between 30 June and 5 July, 1984. Colonies of S- invicta were collected from Kerr Coun ty, Texas, and placed into separate 21 liter containers for transport. Colonies of P. dentata, F. foetidus, and M. min imum were collected in Lubbock County, Texas. All colonies were collected during their periods of lowest foraging ac tivity, corresponding to early morning for F. foetidus and 2.- ntinimum, and early afternoon for 2- dentata and S. invic- ta (Phillips and Claborn, 1984). Colonies of P. dentata, S. invicta, and F. foetidus were separated from the soil using water saturation tech niques described by ^anks et al. (1981) and Jouvenaz et al- (1977). Successful separation of M. minimum colonies from the soil was accomplished using a technique similar to that described by Markin (1968). Soil and ants were spread onto a tray. A plastic container with moist castone plaster was placed in the center of the tray. As thp soil dried, M. minimum workers with brood, and queens, moved into the moist container. This technique was satisfactory, but required 2 to 3 days, as compared to 1 day for the soil saturation technique. All colonies were removed from the soil between 30 June and 6 July, 1984. 37 Colonies were stored in plastic shoe boxes measuring 16x30x9 cm, coated on the sides with Fluon to prevent ant escape. Bach plastic shoe box served as a foraging tray. Another container, measuring 11x11x4 cm with lid, was placed in the center of each shoebox. This container served as the nest box and was coated on the bottom with castone plaster kept moist via a water reservoir underneath.
Confrontation Aggressive Ability Overt aggression was determined by placing individuals of different species-castes into plastic containers measur ing 11x11x4 cm. Fach container was lined on the bottom with castone plaster which provided a rough substrate for the ants in each trial. In addition, the castone plaster served as a water reservoir to prevent ant desiccation. One cubic cm of distilled water was added via a hypodermic syringe to each container prior to each test. The sides of each con tainer were coated with Fluon to prevent ant escape. Each test was conducted at 30*C, and under artificially lighted conditions. Humility remained relatively constant at 70 to 75"^. Ten individuals of one species-caste were simultaneously placed with ten individuals of another species-caste. Individuals were left undisturbed for 3 h. 38 after which numbers alive and dead were recorded. Individuals that were still moving but incapacitated were recorded as dead. Ten individuals of each species-caste combination were maintained in separate containers and served as controls. Individuals alive and dead in the con trol containers were recorded at the end of the 3 h test periods. These procedures were replicated eight times for each species-caste combination, resulting in 17 different confrontation pairs. Data were analyzed by one-tailed t- tests, yielding a comparative indication of species and caste combative ability.
Defensive Ability Defensive ability and avoidance of conflict was demon strated by placing one individual of one species-caste with an individual of another species-caste. Again, 17 combina tions were tested. Temperature and humidity conditions were as above. Containers for these trials consisted of petri dishes (60X20mra) coated on the sides with Fluon. Following placement of two individuals into a container, the number of contacts made between each was recorded every 60 sec for a maximum of 10 min. If a contact resulted in overt aggression leading to death, the time of this occurrance was recorded. Also, the aggressor, if obvious, was noted- Each 39 trial was replicated 15 times for each of the 17 possible pair combinations- Data were analyzed by a nested ANOVA, followed by a Student-Newman-Kuel's test.
Recruitment and Foraging Ants used for the remaining tests were housed in spe cially designed foraging tray/ayrmicaries (Fig. 1) . Fach myrmicary was constructed of 3.2 mm acrylic plastic. *^he foraging tray measured 26X51-3 cm surrounded by a 7-6 cm siding coated with Fluon to prevent ant escape, '^he nest container consisted of five circular galleries. The outer four galleries were ^.5 cm in diameter and 3.2 mm in height- The center chamber measured 4.5 cm in diameter and 6.4 mm in height. Each chamber was connected by a passageway leading to a single exit. The nest container was constructed of 3.2 mm red acrylic plastic to reduce disturbance of the ants by light. The nest consisted of a bi-layer of plastic, at tached to the foraging tray. ^he top of the nest effec*-ive- ly served as part of the foraging area. Nest chambers were cut into the bot^-om layer of plastic with holes slightly larger than the nest chambers cut into the top layer. This arrangement provided circular, removable tops for each chamber, allowing cleaning and brood removal. 40 41
reservoir foraging tray myrmicary entrance 26 cm passageway chamber
cover ^'£y
51.3cm
myrmicary TOP VIEW
side foraging tray SIDE VIEW chamber cover central chamber
7^^^..^.^wter-T--t ^ yy)y:^/7f^///////y)yy%//////////M^ reservoir
['•.•:.'/ /.'•• •<*:'••• •• •-' DETAIL "A" Z castone plaster 42 Two of the outer chambers were humidified via a water reservoir underneath. Reservoirs consisted of 60x20 mm plastic petri dishes. Moisture was wicked into the chambers via castone plaster, forming a connection between water and chamber. The water reservoirs required filling only once/ month. A plastic solvent, methylene chloride, was used to ce ment acrylic parts together- Because methylene chloride is toxic to most insects, the myrmicaries were air and sun dried for a minimum of 3 wks prior to use. After this peri od, introduced ants did not appear to exhibit higher mortal ity than those in more conventional myrmicaries. A total of twelve myrmicaries were constructed. Free water was provided via 6 0x30 mm plastic petri dishes and lids (Fig. 2). Vertical slots, 3.2 mm in length, were cut Into the rim of petri dishes. Cotton plugs were inserted into these slots. Pitri dishes were filled with water and capped, inverted, and placed onto the foraging tray in designated areas. These free water reservoirs pro vided water for about 2 wks between fillings. Three queen-right colonies each of P. den tata, S. i-OLZl£l^» I- foetidus, and ]1. minimum were selected from those previously collected in the field. A simple introduction device was employed, allowing introduction 43
water petri dish
cotton
Figure 2. Side view of a free water dispenser, constructed from 60X20 mm petri dishes. 44 of a known number of ants, while minimizing physical damage to the ants. This device consisted of a 16x30x9 cm plastic shoe box coated inside with Fluon. At one corner, an acryl ic cylinder measuring 1.3 cm in diameter by 5.5 cm in length was attached. This apparatus was placed above a myrmicary with the cylinder nearly touching the foraging tray. A sin gle colony was transferred from its previous nest container into this device. Ants were agitated by tapping the side of the container. Ants were unable to grip the Fluon and slid toward the cylinder opening. Individual ants were counted as they descended to the foraging tray. Twelve colonies, three of each species, were introduced into individual myr micary containers. Fach colony ultimately contained 1,000 individuals, a queen, and approximately 20 developing brood- All colonies were held at 30 C, 15% humidity, and exposed to a 12:12 light-dark photo period. Individuals within each colony were counted weekly to ensure constancy of number. A 1.27 cm grid was placed beneath each semi-transparent myrmi cary nest to facilitate counting (Fig. 3) .
Distance Travelled and Turns Executed
Assessment of distance travelled and turns made by in dividuals of each species while foraging, were conducted by ^he following method: A 3.2 mm sheet of glass was placed 45 46
myrmicary
Myrmicary Entrance
25 cm
Foraging Tray
Arc 47 over a myrmicary. The path of an individual forager was traced with a marker for a period of 1 min. The tracing was then transferred to paper. This procedure was repeated 12 times/colony yielding 36 replications/species. Distances travelled/min were assessed by placing each tracing over graph paper with a 3.94 square/cm grid spacing, and counting the number of squares through which the ant path traversed. These numbers were converted to cm/s giving an estimate of rate of travel for each species. Numbers of turns executed were assessed by counting the number of turns greater than or equal to 90 made during one min. These data were ob tained from the tracings described above. Data were com pared by a nested ANOVA, followed by a Student-Newman-Kuel»s test.
Pecruitment recruitment studies involved a comparison between the four species for the following parameters: rapidity of re cruitment, the time at which the largest numbers of individ uals were recruited to a food source, peak numbers recruit ed, and the time at which a foo'^ source was retrieved. Each parameter was replicated 10 tin-^s/colony for a total of 30 replications/species. Data were analyzed by a nested ANOVA, followed by a Student-Mewman-Kuel*s test. Because these 48 parameters each involved the placement of a known quantity of bait, only baits eliciting maximum recruitment responses were used.
Bajjts In an attempt to discover the bait most attractive to sach species, numerous baits were positioned individually on the foraging trays of each colony. lumbers recruited to these food sources in 5 min were recorded. ^aits tested in cluded freshly killed insects, peanut butter, jelly, dog food, fish food, cheese, cracker crumbs, bread, fish, honey, fruits, and vegetables. of these, the five baits yielding maximum recruitment were placed simultaneously, in equal quantities, on the foraging tray of each colony- The four most attractive baits were: beef dog food (Fine Fare), chicken dog food (Fine Fare) , grape jelly (Welches) , ground oatmeal (generic), and fish food (Tetralin), The numbers of ants recruited to each of these five baits within 5 min were then recorded for each colony. The proportion of individu als recruited to each bait was used as an index by which to mix the baits. All four species were exposed to the same food types, but in proportions eliciting maximum response.
The bait most attractive to P. dentata was- a mixture of oatmeal and chicken dog food. The bait most attractive to 49 ^* iInvicta and n, minimum was beef dog food, whereas grape jelly was r'ost attractive to F. foetidus. Final bait mix tures used for recruitment studies (by percent of total) were as follows: P. dentata chicken dog food 30%, powdered oatmeal 30^, fish food 25%, and grape jelly 15%; S. invicta^ beef dog food 48*^, powdered oatmeal 29%, grape jelly 14%, and fish food 9"; F. foetidus, grape jelly 77""., powdered oatmeal 9"?, fish food Q%, and chicken dog food 7*^,; ?f. mini- mura, beef dog food 487", powdered oatmeal 29f, grape jelly 14''^, and fish food 9*^. All baits were finely ground in a mortar, then mixed with water until a paste was formed. Eoual bait auantities were maintained between succes- sive trials using the following apparatus: A 0.32X6.5 cm plastic rod was fitted into a 0.32 (inside diameter) X4.5 cm piece of flexible tubing, thus forming a plunger. A refer ence mark was placed on the flexible tubing 3 mm from one end. The plunger (plastic rod) was inserted into the tubing from the opposite end until it reached the reference nark, forming a constant volume. Bait mixtures were packed into this apparatus. ^aits were then expelled via the plunger onto a 1x1 cm card prior to placement on a foraging tray. Thus, approximately constant bait volumes and surface areas were maintained from one trial to the next. 50 Four colonies were tested per day, one of each species. Colonies were tested at randomized times within each day, ranging from 0*^30 h to 1300 h. Food was withheld for 2 days between successive trials for each colony. Each trial com menced with the placement of a bait on the foraging tray of the colony being tested. Baits were positioned at random points along an arc drawn 25 cm from nest entrances (Fig. 3) . This procedure prevented ants from learning and subseq uently concentrating foraging at particular areas of forag ing trays. Following bait placement, the time required for an individual forager to discover the bait was recorded. Ants on the 1x1 CTH card, or on the bait, were counted and recorded every 15 s for 40 min. These data yielded a gener al recruitment rate curve for each species. Recruitment to a food source was maintained by the ants until the food was totally retrieved, ""he time of complete retrieval was re corded. The above sequence typified a single recruitment trial. This procedur?^ was followed for each colony, 7 d/wk until 10 replications/colony, or 30 replications/species had result ed. 51
Foraging Foraging parameters studied included the following: the numbers of individuals foraging prior to food bait placement and after food retrieval, the time required to discover a food bait following placement, and the time required for cin individual to retrieve a single dry food bait. The above parameters were replicated 30 times/species, with the excep tion of single bait retrieval times, which were replicated 75 times/species. All parameters were analyzed using a
nested ANOVA, followed by a Student-Newman-Kuel's test. Prior to bait placement and recruitment trials, the number of ants foraging on a myrmicary foraging tray were counted. Following this count, baits were positioned, and the time from placement to discovery by a forager was re corded- Following the 40 min recruitment test period, the number of ants foraging were again recorded. This number, when compared to the number of ants foraging prior to bait placement, indicated the level of increase in foraging ac-
tivity- The ability of individual ants to retrieve a single food bait was recorded. Ability in this case was measured as the time required for an individual to retrieve a single bait over a distance of 22.5 cm. ^aits were composed of equal parts of powdered oatmeal, fish food, grape jelly, and 52 water. These ingredients were finely mixed and ground in a mortar. A sheet of typing paper was cut into 2 cm wide strips, the edge of which was dipped into the bait mixture. This paper, with a thin coating of food mixture, was allowed to dry. Once dry (48 h), 1 mm square pieces were cut such that each square was coated on both sides with the dry bait mixture. Squares were weighed, and averaged 0.446 mg (N=20, standard deviation .07) . A single trial involved placement of one square bait onto the foraging tray distally from the nest entrance. Timing was initiated after a forager carry ing a bait crossed a point 25 cm from the nest entrance. Timing was terminated when the forager crossed a point 3 cm from the nest entrance (Fig. 3). This procedure was repli cated 25 times/colony, 75 times/species. CHAPTER III PFSUITS
Confrontation Aggressive Ability Pesults of the aggressive abilities for the 17 species- caste combinations are presented in figure 4. The combative ability of S. invicta minors and mediums were significantly greater than that of ?. dentata minors (t=2.05, .01
•^3 54 55
CONFRONTATION PAIRS (SPECIES - CASTE)
R dentata minor S. invicta minor R dentata minor S. invicta medium P. dentate minor S. invicta major B R dentata minor M. minimum R dentata minor V/////////////A A F. foetidus < B P. dentata major y////////////////////////////77?.- A S. invicta minor B R dentata major yj///////////////////////////}r- A S. invicta medium R dentata major S. invicta major P. dentata major V//////////////////////////A^ A M. minimum R dentata major V//////////////////////////7777A A F. foetidus S. invicta minor M. minimum S. invicta minor F. foetidus S. invicta medium M. minimum S. invicta medium F. foetidus S. invicta major M, nniinimum S. invicta major F foetidus M. minimum R. foetidus 1 \ 1 r 0 12 3 4 5 6 7 8 9 10 MEAN NUMBER ALIVE AFTER 3 HOURS 56 significantly greater than that of F. foetidus (t=5.31, P<.001; t=3.87, .001
CONFRONTATION RMRS BY SPECIES
R dentata S invicta R dentata V/////////////////////////77A A R foetidus
R dentata M. minimum
S invicta F. foetidus
S Invicta M. minimum
R foetidus M. minimum
2 4 6 8 MEAN NUMBER ALIVE AFTER 3 HOURS
Figure 5. Results of confrontation studies between four ant species. Data an alyzed by t-tests. Means with the same letter are not significantly different within species pair. 58
Defensive Ability Trials involving defensive abilities demonstrated that there was a highly significant difference between species- caste pairs (?<.001, F=24) (Fig. 6). However, no significant difference was found between the mean number of contacts for species-caste combinations 1-8. The above group (1-8) was significantly different from species-caste combinations 9-15, but no difference was found between species-caste pairs within this second group. Species-caste combinations 16 and 17 were significantly different from both groups above, but not between themselves. The number of contacts can be used as an indication of aggresivity. In general, these results indicated that P. dentata and S. invicta were aggressive towards each other, and toward F. foetidus and M. minimum, while the latter two were primarily defense oriented. Very large ants such as P. dentata majors and S. invicta majors were less aggressive toward ant castes of similar size or toward very small ants. Large ants exhibited most aggression toward ant castes slightly smaller than themselves. 59 60
SPECIES- CASTE COMBINATIONS
1 S. invicta mojor F. foetidus 2 M. minimum F. foetidus 3 S. invicta minor F. foetidus 4 S. invicta medium F. foetidus 5 S. invlcta mojor M. minimum 6 S. invicta medium M. minimum ^ P. dentata minor F. foetidus 8 P. dentata minor M. minimum 9 S. invicta minor M. minimum 10 P. dentata major M. minimum 11 P. dentata major F. foetidus 12 P. dentota minor S. invicta major 13 p. dentata minor S. invicta minor 14p. dentota major S. invicta major 15 P. dentata minor S. invicta medium 16 P. dentata major S. invlcta minor 17 P. dentata major S. invicta medium 0 I 234 56789 10 MEAN NO. CONTACTS MADE BY SPECIES- CASTE COMBINATIONS PRIOR TO OVERT AGGRESSION 61 Recruitment and Foraging Distance Travelled and Turns Executed Comparing travel rates for the four species, signifi cant differences were found (P<.001, F=60.98) (Fig. 7)- Travel rate was significantly greater (P<-05) for the three larger species, 2.- dentata, S. invicta, and F. foetidus. Despite the smaller size of F. foetidus compared to S. in victa, F- foetidus travelled significantly farther in 1 min than did S. invicta- Pheidole dentata also travelled sig nificantly farther in 1 min than did S. invicta. No differ ence was detected between P- dentata and F. foetidus. Mono morium minimum was significantly slower than the other three species.
Significant differences (?<.05) were also found between species for the number of turns executed (Fig. 7) . Although no difference was found between ^. invicta and F. foetidus, both species executed a significantly greater number of turns than did ^. minimum. Pheidole dentata was not differ ent from any of the other three species. 62
401 9 !< I X 30- o o CO UJ z _l Q: 20- Ul ID > 1- NO , 10- O )AN D H CO CO (T < O CD R dentata S. invicta F foetidus M.minimum
Figure 7 Comparison between species for distance travelled and number of turns executed. Data analyzed by Student-Newraan-Kuel»s test. Means with the same letter are not significantly different within parameter. 63
Pecruitment Pesults of the recruitment trials are presented in Fig ure 8. Note the similarity between F. foeitidus and M. nin- imum, and the similarity between P. dentata and S. invicta recruitment curves. However, no significant difference was detected between species for initiation of recruitment (P>. 10) nor for time to peak numbers (P>. 10). Significant differences were detected between species for peak numbers recruited (?<.001) (Fig. 9) . Forelius foetidus and M.minimum recruited the largest number of workers to the food source, but were not different from each other (P>.05) . P.k£il2l® dentata and 5. invicta recruited significantly fewer indi viduals to the food source. No significant difference was found between ?. dentata and S. invicta (P>.05). Significant differences between species also occurred for bait retrieval time (P<.001) (Fig. 9) . Forelius foetidus and f1. minimum were not different from each other (?>.05), but were considerably slower in bait retrieval than either ^- dentata or S. invicta. These latter two species were not different from each other in retrieval times (^>.05)- Note that despite greater numbers recruited, f. foetidus and M. minimum were still much slower in bait retrieval than either p. dentata or 5. invicta. 64
50 n
M. minimum
20 MINUTES
Figure 8 Comparative recruitment patterns of four sympatric ant species. Data points represent mean numbers of ants recorded at baits. 65 66
B B Q 50- UJ I 40- o UJ a: 30- 20- < UJ 10-
< UJ Q. P dentata S. invicta F foetidus M. minimum
B B z 30- Ul
-I 20-
Ul a:
< CD P dentata S. invicta F foetidus M. minimum 67 Foraging Results of foraging activity are presented in figure 10. When the nu-nber of individual foragers on a foraging tray were counted prior to bait placement, no difference was detected between species (?>.10). Also, no difference be tween species for the time required to initially discover the bait was detected (P>. 10) . The numher of ants foraging following bait retrieval was significantly higher for each species than the number foraging prior to bait placement (?<.001 for all species). A difference was detected between species for numbers foraging after food retrieval (.01
R dentata S. invicta F. foetidus M minimum
Figure 10 A comparison between species for the number of ants foraging prioi to bait placement and after bait retrieval. Data analyzed by Stu- dent-Newman-Kuel*s test. Means with the same letter are not significantly different within parameter. 69 70
R dentata S.invictd R foetidus M.minimum CHAPTEP IV DISCUSSION
Parameters with little or no significant differences between two species indicates a parameter enabling coexis tence. Parameters with highly significant differences indi cate a factor enabling competitive superiority of one spec ies over another. In the case of S. invicta, these are parameters which might partially explain its ability to in vade niches and displace ants native to the southern United States. Keep in mind that for all tested parameters, colony sizes were held equal. Any superiority demonstrated by one species over another for a particular parameter should therefore be a function of aggressivity or of foraging effi ciency, and not of colony size. Aggressive ability may be important to species success through territory maintenance, food source maintenance, and displacement of a competitor from a food source. Aggression Ttay also be important in displacement of an entire colony from a preferred nesting site. Typically, overt aggression between ant species is observed in areas where species have been recently introduced, where species are highly territorial, and where habitats have been artificially simplified (!7ilson, 1971). 71 72 In situations involving overt aggression, the advantage often is realized by the species having the greatest num bers. For example, Phatkar et al. (1972) demonstrated dis placement of L. neoniger by S. invicta. Despite the superi or individual confrontation ability of L. neoniger, S. invicta was successful in displacement due to overwhelming numbers. l^aclcay and .lackay (1982) demonstrated that large numbers of Formica haemorrhoidalis Emery enable this species to quickly attack and overwhelm the less numerous Camponotus laevigatus (Smith). Monomorium minimum often detected and occupied food baits more quickly than did S- invicta. ?1ono- morium' minimum was capable of averting displacement by other ant species via chemical interference. However, as the num ber of S. invicta increased at the bait, ^. minimum was dis placed (Howard and Oliver, 1979) . In order to nullify the effects of disproportionate numbers, aggressive abilities in this study were conducted using equal numbers. Species which might ordinarily utilize a defensive secretion or behavior to escape the attack of a more aggressive species, were forced to remain in the vicin ity of the aggressor for a considerable period of time- Personal observations indicated that the defensive secretions of the less aggressive and smaller species of ^- miniaum and F. foetidus were somewhat noxious to the more 73 aggressive species. However, these chemicals were not usually effective over these extended test periods. To attempt a numerical interpretation of the overall confrontation and foraging abilities of the four species studied, a rating system was developed. The confrontation data will be used to demonstrate th€ use of this rating sys tem, "'he rating system will be used to compare the abili ties of all four species for all test categories, and will appear at the end of discussions of each parameter. After analyzing the confrontation data, a 0 was as signed to each species-caste combination for each confronta tion pair for which no significant difference was found. If a significant difference was found between two members of a confrontation pair, a •••I was assigned to the "winner" and a -1 was assigned to the "loser." When these numbers were summed for each species, results indicated that S. invicta had the greatest combative ability with a +^, P. dentata was second with a +1, ?1- minimum was third with a -1, and final ly, ^. foetidus was last with a -4. The actual advantage gained by a species having the greatest confrontation abili ty is difficult to determine. Tor example, the ^6 assigned to S,. invicta may not mean that this species is 6 times superior to the other three species. Therefore, species acquiring positive numbers will simply be assigned a •I, and 7U species with negative numbers will he assigned a -1. Consequently, S. invicta and P. dentata both were assigned a +1, indicating superior combative ability over F. foetidus and ^. minimum. This rating therefore, does not indicate that S. invicta is superior to P. dentata. Similarily, F. foetidus and f!. minimum were both assigned a -1 indicating that they have equal aggressive abilities, but that both are perhaps inferior to S. invicta and P. dentata-
The above rating system will be used for all tested pa rameters, indicating species performance as compared to the other species for each parameter. Numbers will be summed for each species for all test parameters yielding an indica tion of a species overall performance. The confrontation studies forced all four species into a combative situation without allowing for the escape abili ties of smaller species such as F. foetidus and ^. minimum. The second parameter tested, defensive ability, was designed to reveal escape or defensive abilities. In natural environments, ants have assumed numerous be haviors geared toward the avoidance of conflict. For exam ple, Bhatkar (1979) has demonstrated that several ant species regurgitate liquid food to appease an aggressive individual of another species. The regurgitate odor nay cover the colony odor of the donor individual thus averting 75 an aggressive encounter. Pheidole floridana and Leptothorax schaumi ''oger often occupy different portions of a food source, but are never observed in conflict (Howard and Oli ver, 1979). Cyphomyrmex rimosa minutus Mayr often foraged near S. invicta nests and were ignored by the latter- This behavior is most likely a result of the small size and slow movements of C. rimosus minitus (Howard and Oliver, ^919). Howard and Oliver (19*79) also demonstrated that despite dis placement of ^. minimum from food baits by S. invicta, ag gressive interactions rarely occurred.
Territoriality may be another sechanism by which ant species reduce overt aggression (Jaffe and Puche, 1984). Territory maintenance may be a function of the number of ants foraging at any particular time, the aggressivity or timidity of the species involved, and the number of contacts made between colony members. Davidson (1977) reported that ant worker density is typically greatest near the colony .en trance and declines with distance from the colony- Intui tively then, territories should be most successfully defend ed near the nest. A theory on territoriality can then be postulated. Ant worker aggression may be positively reinforced by the number of contacts made with siblings/unit time. Ants near the nest entrance will contact siblings numerous times, thus enhancing their tenacity. Workers 76 farther from the colony will contact siblings less often and may, therefore, become somewhat less aggressive. »hen ter ritories of two ant species overlap, contacts of one species with another will increase as one species leaves its terri tory and crosses into the rivals' territory. When this situation occurs, the intruding individual should be neg atively reinforced, and should become less aggressive- The individual forager should then reorient toward its own ter ritory and colony. Its aggressive tendency should again in crease as the number of contacts with siblings increase- Consequently, a flexible, yet definite territory may be maintained. Areas of territorial overlap therefore, should be areas of low foraging activity and lowered aggression, thus allowing two species to co-occupy a bait, or allowing a colony which first discovers a bait to maintain and com pletely retrieve the bait. Avoidance of aggression among ants is well documented and may impart some advantage to the species most adept at this avoidance. Fhen two individuals of different species meet while foraging, one generally flees the other (personal observation). These types of interactions appear to be common, and rarely result in combat. Defensive ability or avoidance of aggression, in this study, was determined using single individuals of each species-caste combination- 77 Seventeen species-caste combinations were again tested. Contacts made between individuals were recorded for 10 min. ^nly the first ten contacts were used in the data analysis due to the highly variable n umber cf contacts made within the 10 min period. This variation resulted from the differ ential activity levels of various individuals when placed into an arena. Higher activity levels resulted in a greater number of contacts between non aggressive species-caste com binations. Any contact which resulted in aggression and combat to the death of one or both individuals was recorded- Fesults of defensive abilities were ranked. Species making the greatest number of contacts were considered to have the advantage in this case- Solenopsis invicta, F- foetidus, and f^. minimum all received -••1 ratings, while P- ^entata received a 0. When the actual number of contacts made between species-caste combinations were considered, contact pairs involving P. dentata made the least contacts before combat resulted. Solenopsis invicta was next, fol lowed by 1. minimum and finally F. foetidus. Pheidole den tata and S. invicta were generally aggressive toward each other and toward F. foetidus and !1. minimum. i^.onomorium minimum was effective in escaping its rivals via gaster flagging. Often, the smaller species were ignored by the larger castes of S. invicta, presumably due to the former's 78 small size and slow movements. Forelius foetidus was the most adept at gaster flagging and rapid escape. However, this species performed poorly in forced confrontation tri als, but its success at escape tactics demonstrates that an aggressive ants' tendencies nay be counteracted by another's defense. When overall species performances were summed for the first two parameters, confrontation and defense, the follow ing ratings were obtained: S. invicta was first with a *2, followed by P. dentata with a ••I. F. foetidus and M. mini- miim were last, each with a 0 rating. Bernstein (1979) demonstrated that larger ants typical ly travel farther than smaller a.nts in search of food. Sim ilarily, Davidson (1977) demonstrated a correlation between large body length and greater distance travelled. Results of the present study tend to substantiate the above correla tions. An indication of the importance of forager rate of travel can be visualized by the following hypothetical situ ation. Assume that 100 individuals of each tested species were foraging simultaneously, and that each forager 'discovered a food source during each meter of travel, during a 1 h foraging period. Using the average speeds obtained during the present study for each species, P. dentata 79 travelled 62 m/h, S. invicta 55 m/h, f. foetidus 65 m/h, and 1' ™i.S.i§.!ia ^2 m/h. In terms of food gathered, F. foetidus and P. dentata would discover approximately 1000 more food iteras/h than ^. invicta^ and approximately .1000 more food items than ?^. minimum. These large differences should have a significant effect- on colony fitness. Pating the four species performances on distance tra velled yields the following results: P. dentata +1, S. in- victa -1, Z- foetidus -••1, and ^, minimum -1- In overall rating, 2- dentata rates a +2, S. invicta a -H, F. foetidus a ••1, and T. minimum a -1. The number of turns executed by an individual forager may also be important to forager success- 5Tide to side casting in insects is considered to increase the width of a foragers perceptual field, thus making food discovery more likely (Pyke et al., 1977), when individual species were rated for number of turns executed/min, the following re sults were obtained: P- dentata 0, S. invicta +1, F. foeti dus +1, and K. minimum -1- Overall, for all parameters, P. dentata received a •2, S- invicta received a ^2, 7. foetidus a -»-2, and ?1. minimum a -2. ?^ass recruitment ^y ants to a food source, generally increases the efficiency of resource recovery (Davidson, 1977; Schoener, 1971; Traniello, 1983). The success of a 80 colony at food bait retrieval aay depend in large part on the rapidity of recruitment. First arrivals at a food source are likely to maintain the food source against rival colonies. If displacement of one colony by another at a food source does occur, the ability of the displaced colony to arrive rapidly at the source will dictate the portion of the source utilized by that colony. Surprisingly, no dif ferences between species were found for time to initiate re cruitment, nor for time to reach peak numbers at the food source. The lack of a difference between species for the above parameters is particularly surprising when the small size and slow rate of speed of ^. ninimu m is considered.
The numbers of individuals recruited to a food source may be important in successful food recovery. The two less aggressive species, F. foetidus and ^. minimum, recruited significantly greater numbers to a food source than did ei ther P- dentata or S. invicta. This behavior could have two adaptive advantages: first, larger numbers should be capa ble of rore rapid removal of a food source. At least this principle should be true to the point at which intraspecific competition interferes with individual efficiency. Second, larger numbers at a food source should be an effective force in warding o^f intruders- Since ?. foetidus and M. minimum were considerably less efficient in bait retrieval than P- 81 dentata or S- invicta, the larger numbers recruited by the former two species is apparently an adaptation for maintain ing a food source against intruders. The rapid bait re trieval observed for ^» dentata and S. invicta appears to be a function of worker body size. Both of these species re cruit a number of major workers to food sources. ?!aiors were observed to sever the baits into pieces which were then either transported to the nest by the major, or by a minor worker. The presence of major workers at a food source should also increase the ability of the colony to maintain a food source against intruders.
All species received 0 ratings for time to initiate re cruitment, and for time to reach peak numbers. For peak numbers recruited to a bait, ?. dentata received a -1, S- invicta a -1, F. foetidus a +1, and ^. minimum a +1. Rat ings for food retrieval times were just the opposite of above. Therefore, overall ratings remained: P. dentata +2, S- invicta +2, F. foetidus *-2, and ^. minimum -2- According to Gordon (1^33), the numbers of ants forag ing at any one time is not correlated with the rate of re cruitment. Pecruitment and nuiribers foraging are separate behaviors. The number of ants foraging is important to colony success for two significant reasons. Colonies with large numbers of foragers should discover food baits more 82 rapidly than colonies with fewer foragers, and should defend territories more readily. The present study demonstrated that no differences between species were found for numbers foraging prior to bait placement. As expected therefore, no differences between species were ^ound for time required to discover food baits. Many insects, upon discovery of a food source, increase foraging speed and casting motions (Pyke et al., 1977). Ants behave similarly, but may extend their ad vantage further by simultaneously increasing the numbers of foragers following food discovery. This principle was found to hold true for the four tested species. Forelius foetidus and M. minimum demonstrated the greatest increase in forag ing activity, while S. invicta demonstrated the least. Pheidole dentata exhibited an intermediate increase in ac tivity. Because no difference was found between species for the number of foragers prior to bait placement, nor for time required to discover a bait, O's were assigned to all spec ies. For numbers foraging following bait retrieval, P. den tata received a 0, S. invicta a -1, F. foetidus a *^, and B. minimum a +1. Overall, foraging abilities of the four spec ies for all parameters thus far considered are as follows: o. dentata +2, S. invicta •1, F. foetidus +.3, and M. minimum -1. 83 The final parameter studied involved the ability of an individual forager to rapidly retrieve a single bait. In a natural community, inter- and intraspecific interference is common (Fluker and Beardsley, 1970; Holldobler, 1982). The longer the time required for a worker to retrieve a single bait, the higher the probability of interference. Ratings for this parameter were as follows: P. dentata and S^ in victa each received a -•• 1, F, foetidus a 0, and ^. minimum a -1. Finally, for all tested parameters, under laboratory conditions, P. dentata and F. foetidus both received a >3 rating, S. invicta received a +2 rating, and H. minimum was last with a -2 rating. Even though the rating system em ployed is artificial, it is conservative, and based on sta tistical analysis of actual data. Therefore, this system is useful in making some predictions as well as some assump tions about the comparative foraging efficiencies of these four ant species. In Kerr and Bandera counties, Texas, P. dentata, S. in victa F. foetidus and ^. minimum are dominant species (Phillips, Unpublished). All four species are omnivorous in food habits (Carroll and Janzen, 1973; i^heeler and Wheeler, 1973), a trait indicative of low resource availability (Schoener, 1971). In such areas, competition is usually keen due to overlapping resource utilization.
,^JUJBiJU«i>t' 84 Since the accidental introduction of S. invicta into .'Mobile Alabama, this ant has demonstrated a remarkable abil ity to displace ants native to the southern United States. r^any species, such as S. xyloni, have become locally extinct in areas established by S. invicta (Howard and Oliver, 1979). Other species however, are considered to be strong competitors of S_. invicta. Fxaiiples of these species in clude; PheidgJLe spp., ]!. minimum, and L. neoniger. Apperson and Powell (1984) demonstrated the difficulty with which the red imported fire ant invaded areas occupied by L. neoniger. Baroni-Urbani and Kannowski (1974) documented the success of 5.- minimum in competition with S. invicta, and Buren et al. (1974) have postulated that Pheidole spp. are largely re sponsible for maintenance of the narrow north south distri bution of S,. invicta in South America. In addition, a num ber of ant species have been known to prey upon the red imported fire ant (Hung, 1974; Nickerson et al. , 1975; O' Neal, 1^73; Whitcomb et al., 1973). The ability of S. invicta to displace ants native to the southern United States and to establish new territories, is generally believed to be a result of the following three factors: high fecundity (Baroni-Urbani and Kannowski, 1974; Bhatkar, 1*^73), aggressivity (Bhatkar, 1973; Whitcorab et al., 1972), and foraging efficiency (Horton et al., 1975). 85 This study, as well as those by Bhatkar (1973), demonstrate that S.. invicta is not dramatically superior in combative ability to several ant species native to the southern United States during one on one confrontations. This study demon strates that S. invicta is not superior in foraging ability to p. dentata, ^. foetidus, or ^. minimum in laboratory studies. In fact, F. foetidus and ?. dentata are apparently the most efficient. The success of ^. invicta may therefore be more a function of high fecundity and overwhelming num bers, than of aggressivity or foraging efficiency. As the red imported fire ant increases its range north and west, lowered winter temperatures and reduced humidity should low er population densities of this ant. As the densities of S. LmlStS. approach those of resident species, effective com- netitivG resistance bv native ants should increase. Wilson and Oliver (1969) demonstrated that S. invicta foraged both night and day in southeastern Louisiana. Also, daytime nutritive yield was higher than nocturnal yield. In Kerr and Bandera counties, Texas, S. invicta is predominant ly a crepuscular and nocturnal forager (Phillips and Cla born, 1984). Lower humidity in central Texas has perhaps forced this shift in foraging patterns. However, lower humidity in combination with the effective diurnal foraging and defensive abilities of F. foetidus and ^. minimum are 86 more probable reasons for this diel shift in foraging. By shifting to a nocturnal foraging pattern, S. invicta not only avoids high diurnal temperatures, but trades direct competition with two species ( F. foetidus and ^. minimum), for direct competition with only one (P. dentata). All four species studied forage to greater and lesser degrees on a 24 h basis, with greatest overlap occurring during crepuscular hours. The effect of this overlap and of resource preemp tion is not known and awaits further study. LITERATUR2 CITED
Adams, A.S, and J.F.A. Traniello, 1981. Chemical interfer ence competition by ?!onomorium minimum (Hymenoptera: For micidae) . Oecologia. 51: 265-270.
Adkins, R.G. 1970- The imported fire ant in the southern United States. Ann. Ass. Amer. Geograph. 60: 578-592.
Apperson, C.S. and E.F. Powell. Foraging activity of ants (Hymenoptera: Formicidae) in a pasture inhabited by the red imported fire ant. Fla. Entomol. 67: 383-393.
Ayre, G.L- 1968. Comparative studies on the behavior of three species "of ants (Hymenoptera: Formicidae) . I. Prey finding, capture and transport. Can. Entomol. 100: 165-172.
Banks, W.A,, C.S. Lofgren, D.'*. Jouvenaz, C.S. Stringer, P."^. Bishop, D.F. Williams, D.P. Wojcik, and B. W. Glan cey. 198 1. Techniques for collecting, rearing, and han dling imported fire ants. U.S. Dep. Agric. Sci. Educ. Adm. Publ. AAT-S-21: 1-9.
Baroni-Urbani, C, P-B. Kannowski- 1974. Patterns in the red imported fire ant settlement of a Louisiana pasture: some demographic parameters, interspecific competition, and food sharing. Environ. Entomol. 3: "^55-76 0.
Bernstein, R.A. 1^75. Foraging strategies of ants in re sponse to variable food density. "cology. 56: 213-219.
Bernstein, T'* 1979. Evolution of niche breadth in popula tions of ants. Am. Nat. 114: 5 3 3-544.
Bhatkar, A.P. 1973. Confrontation behavior between Solenop- sis invicta Buren and certain ant species native to Flo rida. Univ. ^la., Gainesville; 1R1 pp. dissertation,
Bhatkar, A.P. 1979. Evidence of intercolonial food exchange in fire ants and other ^yrmicinae, using radioactive phosphorus. Txperientia. ?5: 1172-1173.
Phatkar, A., W.H. Whitcomb, W.F. Buren, p. Callahan, and T. Carlyslp. 19"^?. Confrontation behavior between Lasius neoniger (Hymenoptera: Formicidae) and the imported fire ant. Environ. Entomol. 1: 274-279. 88
Brian, f!-V. 1955. ^ood collection by a Scottish ant community. J. Anim- Ecol. 24:336-351. Brian, M.V., J. nibble, and A.F. Kelley. 1966. The disper sion of ant species in a southern English heath- J. Anim. Ecol. 35:281-290. Buren, W.F. 1972. Pevisionary studies on the taxonomy of the imported fire ants. J. Ga. Entomol- Soc. 7:1-27. ''uren, W.F., G.T^. Allen, W-H-Whitcomb, F.E. Lennartz, and R.N. Williams. 1974. Zoogeography of the imported fire ants. J. NY. Entomol. Soc. 82: 113-123- Byron, P.A., E.P. Byron, R.A- Bernstein. 19R0. Evidence of competition between two species of desert ants. Insectes Soc- 27:351-360. Carroll, C.P.. and D.F. Janzen. 1973. Ecology of foraging by ants. Annu. Rev. Ecol. Syst. 4:231-2*^7. Cole, A.C., Jr. 1937. An annotated list of the ants of Ari zona (Hym.: Formicidae). Entomol. News. 48:137. Cole, A.C. , Jr. 1^40- Ants of the Great Smokey Mountains, Am. Midi. ^Tat. 24:29-44- Crawford, D.L- and S.W. Pissing. 1983. Regulation of re cruitment by individual scouts in formica greas Wheeler (Hymenoptera, Formicidae)- Insectes Soc, 30:177-183. Creighton, w.s. 1950. The ants of North America. Bull. J^us- Comp. Zool- Harv. Univ. 104:1-585. ^rowell, K.L. ^968. Rates of competitive exclusion by the argentine ant in Burmuda- Ecology. 49:551-555. Davidson, D.W- 1977- Foraging ecology and community organi zation in desert seed-eating ants. Ecology- 58:725-737. navidson, D.W. 1^78- Size variability in the worker caste of a social insect (Veromessor oernandei Mayr) as a function of the competitive environment. Am. Nat. 112:523-532. Dennis, C.'^. 1939. "he distribution of ant species in Tennessee with reference to ecological factors. Ann. Entomol. Soc. Am. 31:281-304. 89 De Vroey, C. 1979. Aggression and Cause's law in ants. Physiol. Entomol. 4:217-222. Elton, c. 1927. Animal Ecology. London: sidgewick and Jackson. 204 p. Fletcher, D.J.C., M.S. Blum, T.V.Whitt, and N. Temple. 1980. Monogyny and polygyny in the fire ant, Solenopsis invic ta. Ann- Entomol. Soc. Am. 73:658-661. Fluker, S.S. and J.W. '^'eardsley. 1970. Sympatric associa tions of three ants: Iridomyrmex humilis, Pheidole rae- gacephala, and Anoplolepis longines in Hawaii- Ann. En tomol. Soc. Am. 63:1290-T296. Gano, K.A. and L-E. Rogers. 1983. Colony density and activ ity times of the ant Camponotus semitestaceus in a steppe community. Ann. Entomol. Soc. Am- 76:958-963. Glancey, B.H., C-E- Stringer, C.H. Craig, and P.K. Bishop. 1975. An extraordinary case of polygyny in the red im ported fire ant. Ann. Entomol. Soc. Am. 68: 922. Gordon, D.^. 1983. The relation of recruitment rate to ac tivity rhythms in the harvester ant, Pogonomyrmex barba tus (F. Smith)(Hymenoptera: Formicidae). J- Kans- Ento mol- Soc- 56:277-2 85. Gregg, R.E. 1944. The ants of the Chicago region. Ann. En tomol- Soc. Am. 37:454-466. Grinnell, J- 19.'^4. Geography and evolution. Ecology- 5:225-2-^9. Hassell, ?1.P. and T.r.E. Southwood. 1978. Foraging strat egies of insects. Annu. Rev. Ecol. Syst. 9:75-98. Hays, K.L. 1950. Ecological observations on the imported fire ant, Solenopsis saevissima richteri Forel, in Ala- bar.a. J. Ala. Acad. SciT" 30: 14-18. f^olldobler, P. 1971. Recruitment behavior in Camponotus socius (Hym. ^ormicidae) . Z- ^^ergl- Physiologic. 75:123-142. Holldobler, ^. 197'^. Recruitment behavior, home range orientation and territoriality in harvester ants, Pogonomyrmex. Behav. Ecol. Sociobiol. 1:3-44, 90 Holldobler, B- 1979. Territoriality in ants- Proc. Am- Philos. Soc. 123:211-218. Holldobler, B. 1982. Interference strategy of Iridomxrmex pruinosum (Hymenoptera: Formicidae) during foraging. Oe cologia. 52:208-213. Holldobler, B., and E.O. Wilson- 1977. Colony specific ter ritorial pheromone in the African weaver ant. Proc. Nat. Acad. Sci- U.S.A. 74:2072-2 075. Horton, ?.^., S.B. Hays and J.R. Holman. 1975. Food carry ing ability and recruitment time of the red imported fire ant. J. Ga. Entomol. Soc. 10:207-213. Howard, F.W. and A.D. Oliver. 1979, Field ovservations of ants (Hymenoptera: Formicidae) associated with red im ported fire ants, Solenopsis invicta Buren, in Louisiana pastures. J. Ga. ^ntomol- Soc- 14:259-263- Hung, A.C.F. 1974- Ants recovered from refuse pile of the pyramid ant, Conomyrma insana (Buckley)(Hymenoptera: For micidae) - Ann. Entomol. Soc. Am. 67:522-523. Hutchinson, G.F. 1957, Concluding remarks. Cold Spring Harbor Symp. Ouant. Biol. 22:4 15-427. Itzkowitz, ?!. and !»!. Haley. 1^8 3. The food retrieval tac tics of the ant Pheidole fallax Mayr. Insectes Soc, 30:317-322. Jaffe K., and H. Puche. 1994. Colony-specific territorial marking with the metapleural gland secretion in the ant Solenopsis geminata (Fabr.). J. Insect Physiol. 30:265-270. Jaffe K., M. Bazire-Benazet, and P.E. Rowse. 1979. An integumentary pheromone secreting gland in A tta spp.; territorial marking with a colony specific pheromone in Atta cephalotes. J. Insect Physiol. 25:833-839. Jouvenaz, D.P., G-E. Allen, W.A. Banks, and D.P- Wojcik, 1977. A survey for pathogens of fire ants, Sol^enogsis spp., in the southeastern United States- Fla. Entomol. 60:275-279. Klotz, J.H. 1984. Diel differences in foraging in two ant species (Hymenoptera: Formicidae). J. Kans. Entomol. Soc, 57: 1 n-118.
y^ 91 Laing, J- 1938. ITost finding by insect parasites. II, the chance of Trichogramma eyanescens finding its hosts. J. Exp. Biol. 1*^:281-302. Levins, E., M.L. Pressick, H. Heatwole. 1973. Coexistence patterns in insular ants. Am. Sci. 61:463-472. ?!ackay, W-P. 1933. Stratification of workers in harvester ant nests (Hymenoptera: Formicidae) . J. Kans. Entomol. Soc. 56:538-542- flackay, W. and E. >!ackay. 1982. Coexistence and competitive displacement involving two native ant species (Hymenopt era: Formicidae). Southwest Nat. 27:135-142. ffarkin, G.r, 1968. Handling techniques for large quantities of ants. J. Econ. Entomol. 61:1744-1745. mirenda, J.T., and B.S. Vinson- 1981. Division of labour and specification of cafrtes in the red imported fire ant Solenopsis invicta Buren. Anim, Behav. 29:410-420. Nickerson, J.C.E., W.H. Whitcomb, A.P. Bhatkar, and M.A. Naves. 1975. Predation on founding queens of Solenopsis invicta by workers of Conomyrma insana. Fla. Entomol. 58:75-82. Odum, F.P. 1971, Fundamentals of Ecology. 3rd el- Phila delphia: W.B. Saunders Co7; 574 p. O'Neal, J. 197,''- Predatory behavior exhibited by three species of ants on the imported fire ants, Solenopsis in victa Buren and S. richteri "^orel. Ann. Entomol. Soc. Am7 67:140. nster, G.F. and E.O. Wilson. 1978, Caste and Ecoloai in the Social Insects. N-J.: Princeton Univ. Press. Fasteels, J.??., J.C. Vergaeghe, and J. L. Deneubourg. 1982, '^he adaptive value of probabalistic behavior during food recruitment in ants: experimental and theoretical approaches. Biology of the social insects. Boulder, CO.: Westview Press; p. 279-301. Phillips, S.A. and D.^. Claborn. 1984. Foraging strategies of native ants in fire ant infested areas of central Texas. "^roc. of 1984 Fire Ant Conference; Gainseville, Florida. 92 ^ontin, A.J. 1961. Population stabilization and competition between the ants Lasius flavus (F.) and L, niger (L.). J. Anim. Ecol. 38:47-54. Pyke, G. H-, H.R. Pulliam, and F.L. Charnov. 1977. Optimal foraging: a selective review of theory and tests. o. Rev- Biol. 52:137-154-
Scherba, G. 1964. Species replacement as a factor affecting distribution of Formica opaciventris Emery (Hymenoptera: Formicidae). J. NYT Entomol. Soc. 72:231-237. Schoener, T.W. 1P71. Theory of feeding strategies. Annu. ^ev. Ecol. Syst. 11:369-404, Smith, ff.R. and .^. College. 1^24. An annotated list of the ants of !!ississippi (Hym.). Entomol. News 35:77. Taylor, F. 1977. "''oraging behavior of ants: experiments with two species of Hyrmecine ants. Behav. Ecol. Socio biol. 2:147-167. Traniello, J.F.A. 19R3. Social organization and foraging success in Lasius: behavioral and ecological aspects of recruitment communication. Oecologia. 59:94-100. Vandermeer, J.H- 1972- Niche theory. Annu. Rev. Ecol. Syst. 3:107-132- Wheeler, W.M. 1910. Ants: Their Structure, Development and Behavior. New York: Columbia University Press; 66 3 p. Wheeler, G,C. and J.W. Wheeler- 1973. Ants of Deep Canyon. Univ. Calif., Riverside: Philip L. Boyd Deep Canyon Re search Center; 162 p. whitcomb, W.H., H.A. Denmark, A.P. Bhatkar, and G.L. Green. 1972. Preliminary studies of the ants of Florida soybean field- Fla. Entomol. 5*^:129-142. Whitcomb, 'r.H., A. Bhatkar, and J.C. Nickerson. 19''3. Predators of Solenopsis invicta gueens prior to colony establishment. Environ."sntoraol. 2:1101-1103- wilson, E.O. 1962. Chemical communication among workers of the fire ant Solenopsis saevissiva (Fr. Smith). 1. the organization of mass-foraging. 2. an information analysis of the odor trail. 3. the experimental induction of social responses. Anim. Behav. 1">: 1 34-1 6'1. 93 Wilson, E.O. 19^3. The social biology of ants. Annu. Rev. Fntomol. 8:345-363. Wilson, W-0- 1971- The Insect Societies. Cambridge, MA: Harvard Uni'^- Press; 548 pp- wilson, E.O. 1976. The organization of colony defense in the ant Pheidole dentata ?1ayr (Hymenoptera: Formicidae). Behav. ^col. Sociobiol. 1:6 3-9 1. Wilson, E.O- and W.L. Brown, Jr. 1957. Recent changes in the introduced population of the fire ant Solenopsis sae vissima. ^volution- 12:211-218. Wilson, N. L. and A.D- Oliver. 1969. Food habits of the im ported fire ant in pasture and pine forest areas in southeastern Louisiana. J. Econ. Fntomol. 62:1268-1271. Wilson, N.L., J.H- Dillier, and G-P. Warkin. 1971- Foraging territories of imported fir9 ants- Ann. Fntomol. Soc. Am. 64:660-665. APPENDIX A
ANALYSIS OF VARIANCE TABLES FOR ELEVEN PARAMETERS
ABLE 1. ANALYSIS OF VARIANCE TABLE FOR "'"^HBER OF CONTACTS MADE BY 17 SPECIES-CASTE COMBINATIONS
VARIANCE D.F. SUM OF SQ MEAN SQ VARIANCE SOURCE COMPONETfT
TOTAL 25 4 3733.67 14.70 16.68 100
CONTAC 1862.55 372.51 9. 16 55
ERROR 249 1871.12 7.51 7.51 45
MEAN 6.72
STANDARD DEVIATION 2.74
COEFFICIENT OF VARIATION 40.81
TABLE 2. NESTED ANALYSIS OF VARIANCE TABLE FOP DISTANCE TRAVELLED
VARIANCE D; F. SU" OF SQ MEAN SQ VARIANCE «r SOURCE COM^ONANT
TOTAL 143 18701.77 130.76 154.80 100
SPECIES 3 10637.04 3545.68 96.99 62
COLONY 8 433. 22 54. 15 -0.31 0
EPPOP 132 7631.51 57.81 57.81 37
MEAN 35. 13
STANDARD DEVIAT^O^^ 7.?0
COEFFICIENT OF VA-IATION 21. 64
94 95
ABLE 3. NESTED ANALYSIS OF VARIANCE TABLE FOR NUM3EH OF TURNS EXECUTED
VARIANCE D-F, SUM OF SQ MEAN SQ VARIANCE
SOURCE COMPONENT 7113.83 49.75 51.80 100 TOTAL 14 3 420.24 140.08 3.01 6 SPECIES 3 253.67 31-71 -1-42 0 COLONY 8 6439.92 48.79 48.79 94 ERROR 132
MEA^•' 23.53
STANDARD DEVIATION 7.00
COEFFICIEN*^ OF VARIATION 29.68
TABLE 4. NESTED ANALYSIS OF VARIANCE TABLE FOR INITIATION OF RECRUITMENT
VARIANCE D.F. SUM OF SQ "^EAN SQ VARIANCE
SOURCE COMPONENT 1.59 100 TOTAL 119 185.70 ''.56 0. 10 3 SPECIES 3 16.57 5.52 0.26 16 COLONY 8 30.93 3.87 1.28 30 -^RROP 108 138.20 1.28
MEA*^ 2.05
STANDARD DEVIATION 1-13
COEFFICIENT OF VAPIATIO" 5^.18 96
T'ABLE 5. NESTED ANALYSIS OF VARIANCE '"ABLE FOR TIME TO PEACH PEAK NUMBERS
VARIANCE D-F. SU" OF SQ MEAN SQ VARIANCE SOURCE COMPONENTm
TOTAL 119 2537-97 21.32 22-30 100
SPECIES 3 414.30 138.10 1.94 9
COLONY 8 638.47 79.8 1 6.61 30
ERROR 108 1485.20 13.75 13.75 62
^EAN 9.68
STANDARD DEVIATION 3.71
COEFFICIENT OF VARIA'^ION 38.30
TABLE 6. NESTED ANALYSIS OF VARIANCE TABLE FOR PEAK NUMBERS RECRUITED
VARIANCE D.F. SU'^ OF SQ MEAN SQ VARIANCE %
SOURCE COMPONENT
TOTAL 119 32143.83 270.12 343.18 100
SPECIES 3 27114.67 9038-22 299.26 87
COLONY 8 483-00 60.38 1.83 0.5
ERROR 108 4546.16 42.0^ 42.OQ 12
MEAN 34.91
STANDARD DEVIATION ^^^9
COEFFICIENT OF VARIA'^ION 18.58 97
TABLE 7, NESTED ANALYSIS OF VARIANCE TABLE FOR BAIT RETPIEVAL -"IME
VARIANCE D.F. SU* OF SQ MEAN SQ VARIANCE •:
SOURCE COMPONENT.
TOTAL 119 13965.17 117-35 148.33 100
SPECIES 3 11503-30 3834.43 125.40 85
COLONY 8 578.87 72.36 5.49 4
ERROR 108 1883.00 17.44 17.44 12
MEAN 24.92
STANDARD DEVIATION 4.18
COEFFICIENT 0^ VARIATION 16.76
TABLE 8. NESTED ANALYSIS OF VARIANCE TABLE FOR NUMBERS FORAGING PRIOR TO BAI*^ PLACEMENT
VARIANCE D.F. SU"* OF SQ MEAN SQ VARIANCE % SOURCE COMPONENT
TOTAL 11^ 73885.70 620.89 679.5'=^ 100
SPECIES 3 22980.17 7660.06 1*^2.82 22
COLONY 8 24602.00 3075.32 283. 18 42
ERROR 108 26303.00 243.55 243.55 36
MEAN 29.95
STANDARD DEVIATION 15.61
COEFFICIENT OF VARIATION 52.11 98
TABLE 9. NESTED ANALYSIS OF VARIANCE FOR TIME REQUIRED TO DISCOVER A BAIT FOLLOWING PLACEMENT
VARIANCE D.F. SU'^f OF SQ M^^AN SQ VARIANCE % SOURCE COMPONENT
TOTAL 119 3-77 0.03 0.03 100
SPECIES 3 0.29 0.10 0.00 5
COLONY 8 0.42 0.05 0.00 5
ERROR 108 3.05 0.03 0.03 83
.»SAN 0.17
STANDARD DEVIATION 0. 17
COEFFICIENT OF VARIATION 98,27
TABLE 10, NESTED ANALYSIS OF VARIANCE TABLE FOR NUMBERS FORAGING AFTER BAIT RETRIEVAL
VARIANCE D.F. SUM OF SQ MEAN SQ VARIANCE %
SOURCE COMPONENT
TOTAL 119 178212.59 38170.'^7 1754.46 100
SPECIES 3 96967.36 32322.45 51S.89 SI
COLONY 8 44028.13 5503.52 515.89 29
ERROR 108 37217-10 344.60 344-60 20
MEAN 67.61
STANDARD DEVIATION 13.56
COEFFICIENT OF VARTATION 27.46 99
"^ABLE 11. NESTED ANALYSIS OF VAPIANCS TABLE FOR TIME TO RETRIEVE SINGLE BAITS
VARIANCE D.F. SUM OF SQ MEAN SQ VARIANCE % SOURCE COMPONENT
TOTAL 299 58339.00 195.11 232.42 100
SPECIES 3 43159.43 11386.48 149.76 64
COLONY P 1234,77 154,35 2.99 1
ERROR 288 22944.80 79.67 79-67 34
MEAN 35.30 STANDARD DEVIATION 8.93 COEFFICIENT OF VARIATION 25.29