All honey bee [Apis mellifera L. (Hymenoptera: Apidae)] subspecies (i.e., races) share the same defensive behaviors, which include threatening positions, flying, pursuing, and stinging. However, the onset and intensity of the defensive response may differ among them. For instance, African bees, Apis mellifera scutellata, are highly defensive (Collins et al., 1982), whereas defense is slower and less intense in Italian bees, Apis mellifera ligutica. The differences in defense are believed to have evolved through different selection regimes, such as those involving predators and climatic factors (DeGrandi-Hoffman et al., 1998). Although the honey bee is a domestic animal, these bees obtain their food from both natural and agricultural systems. To certain extent, they must protect themselves from climatic variation and natural enemies.
Honey bees provide us with a system where colony conditions, such as population demography (Eckert et al., 1994), diseases (Spivak & Reuter, 1998), genetic variation and composition (Clarke et al., 2001), resources (Danka et al., 1987) and allocation to defense (Collins et al., 1982; Giray et al., 2000; Rivera-Marchand, 2006), guarding (Guzmán-Novoa et al., 1994), foraging (Pankiw, 2003), and cleaning (undertaking; Robinson & Page, 1988), can be precisely manipulated and responses of colony members can be carefully monitored. As a result, honey bees are among the most studied social arthropods. Insights from honey bees have led to the understanding of different aspects of the evolution of social behavior including the relationship between behavior and genes (see Honey Bee Genome Sequencing Consortium, 2006). Within these studies, defensive behavior has received particular attention (see Breed et al. 2004 for a review). Defense is an important behavior required for the survival of the colony, since colonies contain attractive resources, such as brood and honey. The main threats to a honey bee colony include invertebrates, such as predatory wasps and other honey bees (which intend to rob honey; Fell, 1997), and vertebrates, such as mammals (Hood & Caron, 1997). Such threats usually cause a colony level response, which can include threatening postures, biting, and stinging.
Defensive behavior is a trait common to all social arthropods (Wilson, 1975) and can be costly. Organisms that invest in defense can be subjected to life history trade-offs, such as reduced growth and reduced reproduction. Studies of life history trade-offs due to defense include diverse taxonomic groups, such as algae (Chlorella vulgaris Beijerinck; Yoshida et al., 2004), plants (Triticum aestivum L.; Heil et al., 2000) and animals (Drosophila melanogaster Meigen; Kraaijeveld & Godfray, 1997). Nevertheless, studies of defense trade-offs are underrepresented in animals relative to other groups of organisms, such as plants. One reason for this lack of representation in animals may be the relative difficulty to perform defense trade-off studies, such as manipulations in a controlled environment. Lind & Cresswell (2005) have reviewed the literature on anti-predation (i.e., defense) behavior of animals. Among their findings was a relative absence of studies of life history trade-offs and defense. They cite four organisms that have been studied under this context, one spider and three stream-dwelling insects. Not even model organisms, such as honey bees, whose defensive behavior has been studied extensively, have been examined under a trade-off context.
In general, the current studies of social Hymenoptera have not focused on the possible costs of defense as related to life history traits. In this review, we highlight the circumstances where trade-offs due to defense may occur, focusing mainly on honey bees while including examples from other social insects. We begin by examining life history trade-offs, giving particular attention to defense. We also address the importance of resource availability in the cost of defense. Lastly, we offer suggestions for future research on trade-offs in honey bees and social insects in general. Although the current research contains gaps in this area, we hope to stimulate studies on this subject. Defense trade-offs may provide important insight into evolutionary mechanisms that affect social insects.
Life history traits and defense trade-offs
Life history traits are, by definition, linked with survival and reproduction. These traits include birth, growth, aging, mortality, and reproduction. All life history traits are associated with each other and are affected by internal (e.g., physiology) and external (e.g., resources) factors. Resources, such as nutrients, shelter, and space affect these traits. Many times the resources required for one activity may also be needed for another. For instance, resources will be needed for reproduction, maintenance, and growth. If resources are limited, trade-offs will occur. Trade-offs may take place between growth and reproduction, growth and defense, reproduction and defense, and maintenance and growth, among others (Stearns, 1992).
Defense is fundamental for survival, and for this reason it is linked to all life history traits. For example, parasite resistance is related to reproductive success in many birds, where bright plumage is an indicator of high immunity (Hamilton & Zuk, 1982; Lindström & Lundström, 2000). Defense evolves in response to natural enemies, such as pathogens, herbivores, and predators. However, many organisms are only able to invest in defense under certain circumstances or during specific stages in their life. The allocation of resources to defense may constrain growth (Vourc’h et al., 2001) and reproduction (Briggs & Schultz, 1990), ultimately affecting fitness (Komdeur & Kats, 1999). In plants, for instance, costly chemicals are only produced during stages of reduced growth (see Coley & Barone, 1996 for a review). Even though natural enemies are the main drivers behind the evolution of defense, resources and their allocation to growth and reproduction could influence its expression.
The life cycle of the social insect colony provides a template for the study of life history trade-offs. Because of the complexity of colony life cycles, some of the life history traits of insect societies have been redefined (Oster & Wilson, 1978). Birth is known as the founding stage, growth is the ergonomic stage, and reproduction is the reproductive stage. The honey bee colony is the prime example where all stages of colony life cycle have been studied in rigorous experiments (Otis, 1980). For practical reasons and because of the difference in life histories of different caste members (i.e., queens, workers, and males or drones; Winston, 1987), we will consider the honey bee colony as the reproductive unit and discuss life history traits of social insects using this example.
Founding stage and defense
A honey bee colony is founded by a reproductive swarm. A mated queen leaves her hive of origin with a group of worker bees and founds a new colony. Meanwhile, within her colony of origin a new queen emerges, and after mating she returns to the colony. Immediately after the reproductive swarm finds an appropriate site for the hive, workers begin to build the comb, the foundation of the hive where brood is reared and food is stored (Winston, 1980).
Defense is non-existent during the founding stage. Age demography may be the proximate reason for the absence of defensive behavior, while the lack of resources to defend may be the ultimate reason. The workers in the swarm are mostly very young (Winston, 1987), so they do not perform defense, a task mainly carried out by older workers. On the other hand, defense is only favorable when resources are present. Losing workers in a defensive response does not seem to outweigh the benefits of maintaining food and future worker generations. Without any food and brood, the swarm does not invest unnecessarily in this costly behavior.
Ergonomic stage and defense
The ergonomic stage of the colony begins with the queen laying eggs that will develop into workers and continues until colony reproduction. This stage may vary according to the initial size of the founding swarm, available resources, the condition of the queen, and the space for the hive. The duration of the ergonomic stage depends greatly on the performance of foragers and the resources they collect.
The main resources collected by bees are nectar and pollen (Danka et al., 1987; Eckert et al., 1994). Colony growth depends on the investment in one resource or the other. When investing in one, it may be difficult to invest in another. In temperate regions and islands where resources are not always available, it is better to invest more in foraging for nectar than for pollen (Pankiw, 2003). Colonies need to build honey stores to survive the seasons without nectar resources such as flowers. Should they invest mainly in pollen, it is very likely that, although they will grow fast, they will not survive the dearth season. In areas where resources are limited, the trade-off between foraging for nectar and pollen is more defined. By trading off nectar for pollen foraging, temperate bees may ultimately reduce the chance of survival, since they will not have a reliable source of energy when other resources become unavailable.
In addition to the influence of colony environment (Fewell & Page, 1993) and genetic mechanisms (Robinson & Page, 1989), defense also affects foraging investments. Workers that forage can also defend. When colonies invest in defense, they have reduced capacity for foraging (Giray et al., 2000; Rivera-Marchand, 2006) and, therefore, colony growth. The cost of defense is minor for Africanized bees in the mainland where resources are abundant. These bees can afford to defend instead of storing food, because flowers are available through the year. However, for temperate bees and Africanized bees on islands defense can affect growth. On the island of Puerto Rico, gentler colonies tend to forage more than aggressive colonies (Rivera-Marchand, 2006). Although the reduced defensiveness may be a result of genetic drift, selection may favor these gentle colonies on the island where resources are limited throughout most of the year.
Colony demography, worker aging, and defense
Although colony age may be determined by the age of the queen or the time since its founding, the demography of the workers may be its best indicator. The super organism analogy of the colony (Wheeler, 1928) describes the importance of worker demography as a determinant of age. Under this analogy workers are equivalent to cells, and the colony is the organism; as workers age so does the colony. Colony demography may be disturbed in several ways, including selective elimination of an age class by a catastrophe or during reproductive colony fission (see Giray & Robinson, 1996). If the workers are mainly young, the colony can be assumed to be young and undergoing an early ergonomic stage. If the workers are predominantly old, the colony is old and, under certain conditions, such as the death of the queen, may be approaching senescence.
Tasks performed by the workers are also related to their age (Seeley, 1982). Young bees perform tasks inside the hive, such as brood care, and older bees perform tasks outside, such as foraging and defense (Seeley, 1982). Even though tasks are age related, there is certain plasticity to their onset (Giray & Robinson, 1996). In a typical colony, young bees perform nursing tasks and at a later age will become foragers. If the forager population decreases, nurses can forage precociously. Similarly, if nurses diminish, foragers can revert to nursing (Robinson et al., 1989). If nurses become foragers ahead of time, nursing duties may be affected. In the same way, if foragers revert to nursing, foraging may be affected. The cost of reversion to nursing constitutes a higher trade-off than precocious foraging. When foragers go back to nursing, resources are depleted due to consumption of stored food by the brood that can not be replaced due to reduction in the number of foragers that would typically collect resources. Under these conditions, depending on the amount of stored resources, future colony growth may be compromised.
Colony demography also influences the ability of workers to defend the colony. Because the workers that defend the colony are the older bees (Giray et al., 2000; Breed et al., 2004; Guzmán-Novoa et al., 2004), colonies that contain primarily young bees have lower defensive capabilities than colonies with older bees. In addition, the relative defensive response also depends on foraging investments, because older bees are allocated to both foraging and defense jobs. In other words, there is potential for a trade-off between foraging and defense (Giray et al., 2000; Rivera-Marchand, 2006), because colonies that invest in one job have reduced numbers of individuals to allocate to the other job. Bees of areas where resources are abundant in time (e.g., Africanized bees) can afford high defensive levels, since less foraging is required for survival through times with low resource availability. However, bees that inhabit places where resources are limited in time (over the year) may not afford to give up foraging for high defense, because it is important to generate a surplus for lean times.
Reproductive stage and defense
The reproductive stage in honey bees consists of the production of a new queen and a reproductive swarm. The onset of colony reproduction depends on the size of the colony. The colony must attain a peak size to ensure the survival of both the reproductive swarm and the bees remaining in the hive. Peak size varies among races. For instance, European bees are known to swarm at a larger colony size than their tropical counterparts (Winston, 1979, 1987; Otis, 1980). The rate of colony reproduction varies between honey bee races. The differences are related to the availability of resources and the type of resources gathered. In the tropics, reproduction may be continuous (Otis, 1980). Temperate bees reproduce seasonally, taking advantage of the abundance of resources during a limited period in the year to grow, reproduce, and possibly store food for the period when it is not available (Winston, 1987).
Reproductive capacity is related to resource availability. For instance, drone production and maintenance diminishes when colony resources are depleted in a colony (Seeley & Mikheyev, 2003), that is, honey bees trade reproduction for future survival. Defense can also cause resource limitation within a colony. For example, defensive Africanized colonies of the island of Puerto Rico have a lower reproductive effort than gentler colonies (Rivera-Marchand, 2006). On islands where resources are limited, it seems that defensive colonies are not able to gather enough resources for reproduction. Colonies with low defensive levels may have a reproductive advantage over the more aggressive colonies on the island.
Colony mortality and defense
Colony mortality occurs if the queen dies and is not replaced, or if the worker population is reduced to a level where the necessary tasks can not be performed. The queen may die by way of illness, predation, or aging. Worker population reduction may be related to climate or natural enemies. If there is little stored food, death can come quickly during severe weather, such as cold, drought, or heavy rain. However, honey bees can survive severe local climate by absconding. Nevertheless, absconding may not be the answer when the climate is severe at a large scale. In this case, bees must rely on stored resources to survive. Only colonies that store large amounts of honey can survive such climatic threats.
An important cause of colony mortality occurs by failure to defend the colony against predators and pathogens. Predators can be warded off or killed by a defensive response, but if the predators cause large damage or can not be fended off, the colony may abscond. Other natural enemies such as parasites and pathogens may be attacked and removed (Mondragón et al., 2005; Rivera-Marchand, 2006) or the source of infection may be removed (Spivak & Reuter, 1998) or isolated. However, as with larger predators, if the threat can not be removed or isolated, the colony may abscond.
Defense in tropical honey bees (e.g., Africanized bees; Schneider et al., 2004), as well as most other tropical social insects, is an adaptation in response to the presence of predators and the abundance of resources. Differences in behavior occur in tropical and temperate species of other social insect genera as well. For example, the tropical paper wasp, Polistes stabilinus Richards, has a lower degree of parasitism than temperate species of the genus (Hughes et al., 2003). This is possibly due to its tendency of building multiple nests and moving from one to another when one of them is parasitized. In the presence of predators and a long growth season where resources are available, natural selection may have favored the evolution of this particular defense.
Environmental factors and defense trade-offs
The cost of defense may be negligible when resources are abundant (e.g., Heil et al., 2000), but, when resources, such as food, are limited, trade-offs may occur. The relationship between resource availability and defense can explain conflicting studies of ants.
Oster & Wilson (1978) proposed a model of caste allocation in ants with soldier castes (i.e., defense). According to this model, when colonies invest in reproduction, they also produce soldiers. This inducible defense production has been verified by a study of the ant Camponotus impressus Roger in Florida, USA (Walker & Stamps, 1986). However, studies of ants belonging to the genus Pheidole in Costa Rica contradict the model (Kaspari & Byrne, 1995). According to this study, soldiers are not inducible during reproduction; rather they are produced throughout the colony's life cycle. Resource availability and differences of life histories between these ants may be the underlying reason for the differences between the Pheidole study, and the model supported by the C. impressus study. The model assumes that food is limited. This may be the case for C. impressus where food may be seasonally limited in a temperate area such as Florida. These Florida ants only produce soldiers during reproduction, possibly coinciding with abundance of food. However, for Pheidole food is available throughout the year in the continental tropics. The food abundance of the tropics allows the Pheidole ants to grow quickly and reproduce often, producing soldiers throughout their life cycle. Moreover, with abundance or prolonged exposure to natural enemies in the tropics, natural selection will favor colonies that produce soldiers throughout the year. Exposure to natural enemies may be the immediate cause for the evolution of defense throughout the life cycle of these ants, and resource availability may permit investing a great amount of energy in defense.
Environmental variables play an important role in the evolution of defense, even though the presence of predators is considered the primary selective force (Kajobe & Roubik, 2006). Internal (i.e., hive) as well as external environments affect defense. In honey bee colonies, the individuals that defend the colony may also forage, therefore, there may be a trade-off between these two behaviors (Rivera-Marchand, 2006). The evolution and maintenance of defensive behavior ultimately depends on the immediate and long-term cost of the trade-off. Defensive behavior will be evolutionary stable, if, for the short-term, the response is effective enough to ward off a predator and, in the long term, does not compromise future survival and reproduction.
An example of how an environmental variable affects defense can be seen in the differences in defense investment between bee colonies with different nest structures, where open nests are found to be harder to defend than cavity nests. Colonies in open nests must invest more workers for defense than colonies built in cavities. In a comparative study of three species of Asian tropical bees in Thailand, Apis cerana Fabricius, a cavity nest builder, made a smaller investment in defense and produced more brood than the open nest builders, Apis dorsata Fabricius and Apis florea Fabricius (Seeley et al., 1982). The added protection allows A. cerana to invest more in foraging than defense, which translates into more brood. Even though the authors did not focus on the trade-offs between defense and foraging, it seems to be occurring in A. dorsata and A. florea, where the need to defend an open nest comes at the expense of foraging and brood production.
External environmental factors affect food availability, ultimately affecting the cost of defense. The abundance of nectar or flowers may influence the evolution of high defensiveness. Flowers may be patchy (i.e., irregularly distributed) or constant in space and time. Four possible scenarios of resource availability can be constructed from these conditions: (i) patchy in time (i.e., seasonal) and constant in space, (ii) constant in time and patchy in space, (iii) patchy in time and patchy in space, and (iv) constant in time and constant in space. Since we know of no place that complies with the fourth condition, we will not discuss it further. The first condition (Figure 1A), patchiness in time yet abundance in space, is typical of temperate regions. Plants tend to bloom abundantly over a large area, but only for a limited time, such as spring and summer. Because of the short time in which environmental factors (e.g., water, temperature, and sunlight) are appropriate for investing in flowering, the different species of plants tend to produce flowers either all at once or consecutively (Rathcke & Lacey, 1985). This pattern is seen in studies of plant communities in North America. Woodland herbs of the midwestern USA (Schemske et al., 1978) as well as plant communities along the east coast of North America and temperate Japan (Kochmer & Handel, 1986) tend to flower between April and August. This 5-month period represents a relatively short time in which honey bees must horde enough resources to reproduce and build stores for the winter.
Figure 1. Models of the relation between resource abundance of three environments and time and space. (A) continental temperate regions, (B) continental tropics, and (C) tropical oceanic islands.
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The second condition (Figure 1B) is seen in the continental tropics where flowers tend to be abundant in time and patchy in space. In the larger scale, environmental conditions are favorable throughout the year, which allows constant flower production (Morellato et al., 2000). Yet, due to local climatic conditions and increased competition for resources and pollinators, flowering in the tropics tends to be variable in space. But, at any point in time there are flowers available, even if they are not present everywhere at once. This hypothesis is supported by studies performed in Brazil (Henderson et al., 2000; Morellato et al., 2000), Venezuela (Ramírez, 2002), and Panamá (Ackerman, 1985). Plant communities in the Atlantic forest of Brazil tend to have synchronous flowering. At four study sites of different elevations, only two had significantly different flowering times (Morellato et al., 2000). Moreover, the Brazilian palm communities have staggered flowering periods with flowering of different species occurs consecutively, but with very little overlap of periods (Henderson et al., 2000). In the Venezuelan plains, flowering of different taxonomic groups occurs at different times, but at the community level there are always flowers available (Ramírez, 2002). In Barro Colorado Island in Panamá, the pattern is similar to Brazil and Venezuela, where floral resources are available all year although their composition may vary seasonally.
The third condition (Figure 1C) occurs on tropical oceanic islands (continental islands are not included, because they have similar ecological dynamics as their nearby continents). Islands have limited space and resources (MacArthur & Wilson, 1967). As a result, flowers are patchy both in space and time on tropical islands. Even though tropical islands and the mainland share similar climates, the climatic patterns of islands are different due to differences in scale. At a large scale, continental tropics do not vary much, but at a local scale, they may be highly variable. Because of their size, topical islands tend to have climates similar to the local scale on the continents. This island-wide variation in climate causes an effect similar to the patchiness in time of the temperate areas. Resources for flower production, such as rain, are irregular and often limited in time, therefore, flowering is also patchy in time (Rivera-Marchand & Ackerman, 2006). In general, the conditions of tropical oceanic island create an effect similar to the patchiness of floral resources in space of continental tropics and the patchiness in time of temperate regions (Perrott & Armstrong, 2000).
Studies on flowering phenology of trees in Hawai’i (Van Ripper, 1980; Tarayre et al., 2007) as well as in Puerto Rico support this hypothesis. After examining the flowering phenology (Little et al., 1981) of 27 trees reported to be visited by bees in Puerto Rico, we found that 62% flower between March and June and only 15% flower between November and February. Although there is no species abundance data for these plants, the flowering period strongly suggests a low floral availability during 4 months of the year on the island.
Defense in the tropics
Honey bees of the continental tropics can afford defense. At the continental scale, climate is stable throughout the year and flowers are abundant. Since there is less need to store resources, workers can be invested in defense without compromising future colony growth and reproduction. If colony resources are depleted, bees may abscond and avoid paying the costs of increased defense and reduced stores. This is because the risk related to absconding in the tropics is small, where the only possible limiting factor is nest space. Indeed, tropical and subtropical A. mellifera nests could be found in small spaces or in the open in tree branches or hanging from rock ledges. Temperate A. mellifera prefer large cavities in hard wood where environmental factors could be better controlled and colony can store a large surplus of honey for winter (Schmidt & Hurley, 1995; Seeley & Visscher, 2004).
The tropical colonies of the honey bee Apis cerana indica Fabricius were found to defend nests from wasps more efficiently than the temperate A. c. cerana and A. mellifera (Ken et al. 2005). These bees isolate and kill the wasps by isolating the intruders in a ball created by the bees within the colony. Inside the ball, temperatures reach over 46 °C, killing the wasp A. c. indica and A. c. cerana are efficient at eliminating the wasps, but show a decrease in foragers associated with their response. Temperate-adapted A. mellifera is less efficient at eliminating the wasps and show no significant reduction of foraging, thus demonstrating an important link between foraging and defense.
Defense in temperate regions
In temperate regions, where flowers are not available throughout the year, colonies must invest as many individuals as possible for foraging instead of defense in order to reproduce early in summer, and survive during the following colder seasons. If many workers remain in the colony to defend, it may compromise their early reproduction and, as a result, their survival through the winter. Moreover, temperate bees cannot afford to abscond, since the risk of not finding an appropriate nesting space in time to build up honey stores is too high. An example of a trade-off between foraging and defense in temperate bees was found in Italian honey bees in Illinois, USA (Figure 2; Giray et al., 2000). Defense was negatively correlated with foraging, where highly defensive colonies foraged less than gentler colonies. Aggressive colonies tend to invest in relatively fewer workers for foraging than gentle colonies, an expected pattern due to the temperate origins of these bees.
A similar situation to the bees in Illinois was found in the invasive ant Linepithema humile Mayr in the USA (Holway et al., 1998). In their native tropics, these ants tend to have high intraspecific aggression levels whereas in the USA the levels are low. This may be due to genetic drift or a release from natural enemies. The reduced defensive levels may have permitted their population to grow, increasing their impact as invasive species. The mechanisms underlying this change are unknown; however, selection may favor this behavior. By investing less in defense, these ants have been able to invest more in growth, perhaps by investing more to foraging.
Defense on islands
Almost all honey bees reported on islands have been introduced for apiculture. Most of the introduced honey bees are European in origin and initially arrived during the spread of the colonizers (Clarke et al., 2001). Nevertheless, there have been reports of Africanized honey bees in the Caribbean (Cox, 1994). The Africanized bees on the island of Puerto Rico have been found to be relatively gentle, behaving more like European bees than typical Africanized bees from the mainland (Rivera-Marchand, 2006). The Africanized honey bees on islands undergo a trade-off between foraging, including individual forager investment and honey stores, and defense (Figure 3; Rivera-Marchand, 2006). Aggressive colonies forage less and store less honey than gentle ones. Moreover, the aggressive colonies produce fewer queen cells (Figure 4), a measure of reproductive effort, than gentle colonies. This potential decrease in fitness may be due to the decrease in foraging.
Figure 3. Trade-off between defense (measured with a rank behavioral assay) and foraging (measured as ranked foraging effort) of Africanized bees on a tropical oceanic island (r = –0.69, P = 0.0186; from Rivera-Marchand, 2006).
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Figure 4. Trade-off between defense (measured with a behavioral rank assay) and queen cell production as a measure of reproductive effort of Africanized bees on a tropical oceanic island (r = –0.72, P = 0.0123; from Rivera-Marchand, 2006).
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The environmental dynamics of tropical islands make them more similar to temperate regions than to tropical continents. Because of the similarity, temperate bees should be more successful than tropical honey bees. If parasites and other tropical pests are absent from the island, then natural selection would favor temperate bees that are adapted to limited flower availability. Prior to the introduction of Varroa mites, a honey bee parasite, European honey bees were abundant on the island of Puerto Rico (Rivera-Marchand, 2006). After the arrival of the mite, the honey bee population decreased until the introduction of mite-resistant Africanized bees approximately 20 years later. It can be assumed that abundance of European honey bees was due to the absence of the parasite and their adaptations to surviving in an environment where resources are limited. Tropical bees, adapted to resource abundance over time, would have to adapt to the conditions of an island where resources are limited and where, due to limiting space, absconding is not a solution. Tropical honey bees on islands such as Puerto Rico (Rivera-Marchand, 2006) may invest more in foraging, instead of defense, avoiding compromising their reproduction.
Other than bees, loss of defense on islands has been reported for ants as well as plants. Solenopsis geminata Fabricius invaded most of the Caribbean during colonial times, over 200 years ago. Anecdotally, it has been said that these ants, known for their aggressive behavior, were a pest on the islands of Hispaniola, Jamaica (Wilson, 1971), and Puerto Rico (Torres, 1992) during the Spanish colonial times. In Puerto Rico, the capital city had to be moved from Guaynabo, 5 km from the coast, to San Juan (where it is currently), an islet off the north coast. Presently, these ants are not considered a problem on the island, to the extent that they are hardly recognized as a pest. The reduction in defensive behavior is evident although the cause is unknown. It may be possible that under the resource constraints of the island, natural selection has favored the less defensive nests.
Future research and applications
Social insects including honey bees are seen as important model organisms (West-Eberhard, 1996; Robinson, 2002) and have a significant economic value. Honey bees in particular are models for understanding social behavior, memory, communication, and flight as well as being pollinators of crops and producers of honey. Recent developments in socio-genomics have given a molecular perspective of these behaviors (Robinson et al., 2005; Honey Bee Genome Sequencing Consortium, 2006). For example, genes have been found that are important in defense (Guzmán-Novoa et al., 2002) and foraging (Fewell & Page, 1993), allowing research on the genetic basis of life history trade-offs.
Why an organism demonstrates defense under certain circumstances while not in others is important for different fields of research ranging from genetics to ecology. Recently, quantitative trait loci with genes coding for defense have been determined (Guzmán-Novoa et al., 2002). With the advent of the honeybee genome-wide studies of dense genetic markers (Whitfield et al., 2006), the genes that code for this behavior may soon be found. Yet, the only way to understand the mechanisms behind the evolution of this behavior is relating these genes to the ecological factors that affect them, that is, through studies on behavioral trade-offs in populations living under different conditions.
Trade-off studies may include intraspecific comparisons in different regions as well as interspecific comparisons within a region. These phylogeographic studies can provide tests for the hypotheses on association of foraging and defense effort with resource conditions. Controlled experiments can also be performed to corroborate trade-offs in these insects where food or other resources can be controlled and its effect on defense, survival, and reproduction can be determined.
Life history trade-offs are important in understanding ecological and evolutionary processes related to the spread and impact of invasive species including Africanized bees and fire ants. These invasive insects have a great impact, with defensive behavior causing the greatest concern. Conservation studies, including impact and spread of invasive organisms, have focused greatly on the defensive behavior of these bees, but almost never from the standpoint of life history trade-offs. Agriculture (e.g., apiculture) has not looked at defense from a trade-off perspective either. Management efforts in both conservation and agriculture could benefit from understanding the basis for colony variation in defensive behavior. For example, conservation management efforts may be able to predict possible invasion success of social insect pests based on resource availability. Apiculture may benefit by selecting colonies with important behaviors, such as foraging, while selecting against others such as defense.
Research on life history trade-offs in social insects is limited, but honey bees are an excellent model system for such studies. Both field and laboratory experimentation are possible and genomic resources are available. By incorporating additional studies of other social insects, generalizations on the trade-offs affecting the evolution of life histories are attainable.