Keywords

3.1 Introduction

Among the diversity of behaviours shown by the social wasps, foraging is probably the most popular topic in specialized literature. Through their foraging activity, wasps form a network of interactions that is ultimately responsible for their value in providing ecological services (such as pest predation, pollination or seed dispersal). There is a great deal of importance in investigating the foraging behaviour, for it leads to understanding the social wasps’ evolutionary history, basic biology and the ecological context in which they are inserted.

“Foraging activity” can be defined for social wasps as the functional group of behaviours of collecting environmental resources essential for survival at the colony level. These resources can be used either in the nourishing of adult wasps and their offspring or in the construction and maintenance of nests. Since the Polistinae social wasps are opportunistic and generalist predators, collected resources vary in nature and origin. This allows wasps to explore a wide array of ecological interactions. The foraging behaviour of social vespids has been detailed in literature review papers (Raveret-Richter 2000) and chapters in books dedicated to the group (Spradbery 1973; Carpenter and Marques 2001; Jeanne and Taylor 2009). In this chapter, we attempt to summarize and describe the general aspects of social wasp foraging, building on those previous studies by updating the references to reflect the state of the art. Furthermore, while previous literature reviews have been mostly skewed towards temperate-zone organisms, we here focus our efforts on the Neotropical Polistinae, aiming to bring a fresh perspective to the field of social foraging research.

3.2 Activity Schedule and Rhythm

The Polistinae clade is almost exclusively made of diurnal species (Spradbery 1973). A flow of individuals committed to the foraging activity coming in and out of the nest are often visible during daytime. At night, nests are usually more cryptic, since most of their inhabitants inside the nest, showing low levels of activity. Even in colonies of the Epiponini tribe, which sometimes keep guards around nest openings during the night (e.g. Chavarría-Pizarro and West-Eberhard 2010), such individuals will remain mostly motionless unless disturbed. The remarkable exception to this rule within the Polistinae is the wasps from the genus Apoica Lepeletier, which is exclusively comprised of nocturnal species and therefore presents foraging activity that differs from the rest of the subfamily (Hunt et al. 1995; Warrant 2008).

Diurnality is the main trend among insects, especially in tropical zones. This is partially due to a stimulation of their activity by environmental factors that are typically higher during the day, such as luminosity and temperature (Huffaker and Gutierrez 1999). Indeed, foraging activity in Neotropical social wasps is normally positively correlated to environmental temperature and negatively correlated to the relative air humidity across different genera (e.g. Ribeiro-Júnior et al. 2006; Santos et al. 2009; Cavenazzi and Noll 2011; Castro et al. 2011; Detoni et al. 2015; Jacques et al. 2018). The influence of wind speed in the process is not as clear, although there is consensus that strong winds discourage foraging by hindering the wasps’ ability to fly (Elisei et al. 2005, 2013). Luminosity regulates the foraging activity through physioethological thresholds (Spradbery 1973; Ribeiro-Júnior et al. 2006), which are required since wasps rely heavily on visual orientation during their foraging bouts. Furthermore, there is evidence that artificial light sources may attract large amounts of wasps around sunset, when foragers are usually returning to their nests (Barbosa et al. 2016). This argument reinforces the group’s positive phototropism and its relation with foraging light thresholds.

The diurnal habit of the social wasps may, however, be explained by more than only their stimulation by abiotic factors. Firstly, the localization of their prey and other alimentary resources is oriented to a good extent by visual clues, thus requiring a source of light (as seen in Jeanne 1972). Secondly, most of the prey captured by Polistinae wasps (especially caterpillars) are also active during the day (Heinrich 1979), which makes prey-predator encounters more likely and is therefore essential for the wasps’ success in protein foraging.

One of the main research interests in the social wasp foraging schedule is how does the foraging fluctuate in intensity through the day, a process that can be called “foraging activity rhythm”. A colony’s rhythm can be measured by counting how many foragers leave and return to the nest per time unit, generating a flow that varies according to the time of the day (Giannotti et al. 1995; Jacques et al. 2018). Spradbery (1973) has described a general pattern of foraging rhythm for diurnal social wasps: the activity begins around sunrise, being limited by the availability of solar light, and reaches its daily peak around noon and decreases through the afternoon until it ceases around sunset.

In theory, social wasp larvae will require a greater amount of energetic resources during the morning, after a period of deprivation during the night. That would explain why foraging tends to be more intense during the first half of the day, even if the amount of light available is similar to that in the afternoon (Gaul 1952). Alternatively, studies show that in many different species, the foraging activity peaks at the warmest hours of the day, which would also explain that positive correlation (Resende et al. 2001; Lima and Prezoto 2003; Elisei et al. 2005; Ribeiro-Júnior et al. 2006; Rocha and Giannotti 2007; Castro et al. 2011; Detoni et al. 2015).

There is a great difference between the foraging rhythm of the independent-founding social wasps (Mischocyttarini and Polistini) and the swarm-founding social wasps (Epiponini), the latter usually being more active. Colony size, as observed by Edwards (1980), is likely a variable that plays a role in that difference: while a Mischocyttarus or Polistes colony has a few dozen workers on average, nests of the Epiponini tribe commonly host hundreds, thousands or even millions of foragers (that last case being true for some nests of the extremely populous Agelaia colonies). Still, the difference in adult populations alone does not seem to explain rhythm differences across species. For example, Polistes simillimus Zikán (1951) (Polistini) and Protopolybia sedula (de Saussurre, 1854) have similar colony sizes, yet the rhythm of the swarm-founding Pr. sedula (average 105.4 forager departures and 108 returns per hour; Detoni et al. 2015) is clearly much more intense than that of Po. simillimus (average 18.4 forager departures and 20.9 returns per hour; Prezoto et al. 1994). A possible explanation for such phenomenon is that, due to the lower worker:brood ratio in Epiponini populations, the colony’s metabolic needs are higher and, therefore, an intense foraging rhythm is required. In fact, the worker:brood ratio seems to be a central element in the social biology of the Polistinae (London and Jeanne 2003).

Unlike diurnal Neotropical social wasps , the Apoica genus is distinguishable from other swarm-founding wasps (and social wasps, in general) for their nocturnal habits. Hunt et al. (1995), when studying Apoica pallens (Fabricius, 1804) in Venezuela, noted the complete absence of foraging activity during daytime, a period which workers spent by ventilating the nest and guarding the colony against threats. After sunset, the workers start foraging; the activity begins at a slow pace, but then an “explosive departure” occurs, with hundreds of foragers leaving the nest in few minutes, in which “the air was filled with wasps” (Hunt et al. 1995). The intense flow is normalized in a few instants and remains stable through most of the night time, decreasing in the hours prior to dawn. Similar events were observed in the species Apoica flavissima (Vecht, 1972) by Nascimento and Tannure-Nascimento (2005), which also verified the typical influence of temperature and humidity on the species’ foraging, as well as possible relationship to the phase of the moon. This exceptional pattern of activity has likely arisen as a strategy to avoid losses caused by direct competition (Smith et al. 2017).

3.3 Flight Range

Foraging behaviour shares dynamics across different groups of social insects. The need to not only locate and obtain food but also to bring it back to the nest is a challenge faced by all social foragers. The exploration of natural resources must, therefore, have the nest as its geographical reference. Studying the flight range of social wasps allows us to better understand the geographical reach of colonies when interacting with their ecosystems, especially regarding their impact as predators.

By looking at predator ants as an example, it is clear how social foraging is related to the nest site. Camponotini ants create trails that run for several dozen meters, on which they mark their progress through pheromones so that nestmates may easily find the way between nest and food. The species Camponotus sericeiventris Guérin, for example, usually builds one or more foraging trails radiating from the nest and reaching up to 86 meters in length, either vertically or horizontally (Elisei et al. 2012).

A major biological difference between ant foragers and wasps resides in the latter’s ability to fly. Flight makes dispersion easier, if not more effective, especially when regarding the vertical exploration of ecosystems. On the other hand, the lack of attachment to the substrate while foraging means that a reliable pheromone-marked trail, as seen in ants, is not possible for wasps. That is undoubtedly one of the main drives for the wasps’ strong dependence on visual orientation (Raveret-Richter and Jeanne 1985). As a result, there is a cost-benefit relationship regarding exploration: the further away from the nest a forager flies, the smaller the chance that it will be able to find its way back, which can lead to worker loss in the colony. A colony set in a poor environment, where its workers must increase their foraging flight range in order to obtain food, is therefore at severe disadvantage (Prezoto et al. 2016b).

“Flight range ” is defined as the maximum distance a forager is able to fly with significant chances of returning to the nest and, therefore, being successful at foraging. By obtaining the flight range value, it is possible to deduct an effective radius in which foragers will be active around the colony (equal to ½ flight range, since that includes the flights from the nest and back) (Gobbi 1978; Santos et al. 1994).

Studies on the flight range of Brazilian social wasps are few and restricted to few taxa (Table 3.1). The method used to assess a species’ flight range is consistent among publications and is made by following these steps: (1) intercepting social wasp workers from colonies in the pre-emergence stage; (2) marking the workers to allow individual distinction; (3) releasing the workers at multiple distance intervals from the nest; and (4) assessing the proportion of workers that returned to the nest from each distance (Santos et al. 1994). Although this method allows the assessment of the maximum distance from which a worker can return to its nest, a return rate of at least 70% is used as a valid estimator of a species’ effective flight range (Santos et al. 2000). Following that assumption, most species would have an effective flight range within 300 meters (Gobbi 1978).

Table 3.1 Social wasp foraging flight range across studies
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3.4 Trophallaxis

Thus far we focused on how external factors influence on a colony’s foraging activity, especially those linked to external environmental variables. In addition, social wasps are also capable of an internal and social behavioural control of foraging. Of these behaviours, the most fascinating – and possibly, the most relevant for the group’s natural history – is trophallaxis.

The term trophallaxis was coined by author William Wheeler in 1918 to describe the reciprocal exchange of liquids between two insects through their respective mouths. The term’s definition has been updated and broadened to include unidirectional exchanges as well (Hunt and Nalepa 1994). This behaviour has implications for various aspects of social life other than just foraging; trophallaxis has been studied in the context of establishing social cohesion and the hierarchy of social wasp colonies (Roubaud 1916; Jeanne 1991).

According to Spradbery (1973), social wasp trophallaxis is divided into two distinct categories according to the life stage of its participants: adult-larva trophallaxis and adult-adult trophallaxis . We will explain each of these individually, as they present distinct effects on a colony’s foraging behaviour.

To initiate adult-larva trophallaxis, a reproductive wasp or worker will approach a larva, usually observable when the adult inserts its head inside a nest cell containing brood (see Fig. 3.1a). The adult wasp stimulates the larva by antennating its mouth parts (and also possibly by beating its gaster against the nest in Polistes). The wasp larva will then secrete saliva, which is transferred into the adult’s gastric tract. Pioneer studies on adult-larva trophallaxis showed that larval saliva is highly attractive to adults (Du Buysson 1903; Janet 1903); later, chemical analysis allowed the characterization of saliva as mainly composed of sugars and amino acids. Indeed, Hunt and colleagues (1982) describe larval saliva as being nutritionally much superior to wasps than the richest floral nectar.

Fig. 3.1
figure 1

(a) An adult Mischocyttarus sp. wasp inserts its heads inside a cell containing a larva; (b) a group of adult Polistes wasps engaged in trophallaxis; (c) an adult paper wasp (to the left) holds a mass of maxalated prey in her mandibles

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At first impression , this scenario seems to escape the logic of social foraging. Since wasp larvae are in constant need of nutritional intake in order to grow and develop, what would be the biological rationale behind giving away such a highly caloric resource to adult wasps, which are able to forage for self-sustenance? Researchers tried to solve this enigma by addressing the question from different perspectives. Ishay and Ikan (1968a, b) showed through radioactive marking that the sugary substances in larval saliva do not derive from the nectar collected from foragers but instead from proteolytic enzymes. This finding led to the conclusion that larvae provide rewards for adults feeding them, generating a positive feedback. However, other studies suggest that the process is not that simple. First, trophallaxis in Polistes seems to be not always reciprocal but rather a one-way exchange; this prevents larvae from rewarding foragers immediately after being fed (Suryanarayanan & Jeanne 2008). In addition, larva saliva provision seems to increase in frequency when colonies show minimal foraging activity (Ikan and Ishay 1966). In other words , larvae will dispense more resources despite being fed less often. Indeed, Mischocyttarus wasps seem to have higher sugar concentration in their saliva during the pre-emergence stage of the colony cycle, when worker populations are smaller and foraging is costlier when compared to post-emergence (Hunt 1988). In conjunction, these findings come together in the hypothesis that trophallaxis plays a major role in the social cohesion of a colony: in times when food is scarce or foraging is otherwise energetically costly, larvae will provide attractive resources to avoid being cannibalized by adults, a common strategy during periods of nutritional stress (Gadagkar 1991). Nevertheless, it is still possible that adult-larva trophallaxis has a role in foraging during periods of regular activity, and these nutritional interindividual dynamics constitute a cornerstone in the evolution of eusociality in Hymenopterans.

The second category , adult-adult trophallaxis (see Fig. 3.1b), is somewhat less studied than when immature wasps are involved in the process and its biological significance has been less explored by researchers. There is evidence for some groups that adult-adult trophallaxis is a mediator in determining the social hierarchy in social insects. In Polistinae, foraging load partitioning between adult nestmates seems to also play a role, or at least reflect, the hierarchical organization of a colony (O’Donnell 1995a; Tindo and Dejean 2000; Sinzato et al. 2003).

It has been suggested for some swarm-founding species that trophallaxis between adults could result in recruitment for foraging within the colony (Taylor et al. 2012). Conversely, a study with Agelaia multipicta (Haliday, 1836) does not support this hypothesis (Jeanne et al. 1995). Food recruitment in social wasps is a somehow elusive topic, and we recommend the reading of Jeanne and Taylor’s (2009) discussion about this phenomenon. Since trophallaxis is often observed immediately after a forager returns to its nest with sugary substances, it has also been used as a non-invasive behavioural indicator to distinguish the forager’s load (Giannotti et al. 1995).

3.5 Searching for Resources

A detailed narrative on the challenges faced by a social wasp worker when leaving the nest to forage is provided by Raveret-Richter and Jeanne (1991). The first step is locating the resource in the environment, whether it is sugary food, water, nest material or a prey. For the latter, the forager must still subjugate its prey, and, if it is too big to be carried back in one piece, it must be fragmented before the wasp can return to its nest. In this case, the wasp still needs to memorize where she left the remaining fragments, so it can return to claim them later – and potentially fight off opportunists to do so. The whole process involves a diverse series of behaviours, of which the most instigating include orientation during flights and the ability to locate resources.

Most behaviours linked to the foraging activity in social wasps are intrinsically connected to a learning process. Prior to being able to fly out in the open, young foragers-to-be make short trips around the nest in order to memorize the surrounding landscape. These visual cues will be used to find the colony upon returning from a foraging trip. Gaul (1951) suggests that temperate-zone vespids make a series of three flights, in which the forager executes progressively wider loops around the nest opening, culminating in a sudden departure for the actual foraging during the third flight. This behavioural display is named orientation flight (Spradbery 1973).

After their departure, foragers must be able to detect resources in the environment using sensorial stimuli. Authors attribute different weights to the importance of different cues for the localization and identification of food sources, but there is a consensus on the conjoined relevance of visual and olfactory cues (Raveret-Richter 2000). Pioneer studies on resource identification in social wasps stressed the role of olfactory cues (Iwata 1976), and evidence has been added to the case of chemical perception in wasps over the years. A good example is how wasps are able to detect leaves that were damaged by chewing herbivores (especially by caterpillars, which they usually prey upon) and choose to land on those instead of healthy leaves as a substrate in order to maximize prey encounters (Saraiva et al. 2017). Furthermore, olfactory cues seem to be especially important for the detection and obtainment of sugary substances. Some wasps may consider a site’s scent more than its colour when choosing where to land (Mcpheron and Mills 2007) and are able to detect honeydew from long distances (Gaul 1952). Finally, carrion-feeding species (such as those in the tropical genus Agelaia) are likely to use mainly chemotactic cues to locate their food (F.P., pers. obs.).

Resources may also be located visually, which is especially true for prey. Wasps may even misdirect predation behaviours towards shapes that resemble prey (Jeanne 1972). Some researchers defend that visual orientation may be more important than olfactory, especially for the capture of highly active prey, since the wasp’s sight may rely heavily on the perception of movement (Spradbery 1973; Jeanne and Taylor 2009).

Overall, a forager may use visual cues to search for prey in a larger scale, chemical cues as it approaches the prey in a smaller scale to choose a landing site. This integration of different stimuli was suggested and described in detail for Polybia wasps by Raveret-Richter and Jeanne in 1985.

After identifying and capturing their prey, foragers must bring the food back to their nests. Prey will often have a much larger body mass than foragers, and so wasps need to cut their prey in fragments and carry them in multiple trips. Sometimes, wasps will execute orientation flights around the fragmented prey to memorize its location (Takagi et al. 1980).

Inter- and intraspecific interactions between social wasps will often take place at a food source. Forager aggregates in Agelaia sp. may be evidence that local enhancement (use of social cues to facilitate the finding of good foraging sites) is being used to a colony’s benefit (Forsyth 1978). Other species may use local enhancement to exploit a competitor’s efforts. Mischocyttarus immarginatus (Richards, 1940) wasps have been observed to attract foragers of three other different species (Polistes instabilis, Polybia diliguetana and Polybia occidentalis) that quickly robbed prey fragments after the original predator left the site (Raveret-Richter 1990). Conversely, niche overlap between species may not affect their direct survival in some environments (Jacques et al. 2018). When wasps compete with ants for food, they might show exceptional behaviours to overcome direct competition; a notable example is the reported removing of ants from sites by dropping them from heights (Ricioli et al. 2017). These are only some of the examples on the ways the social wasp’s search for resources might generate multi-level ecological interactions, constituting one of the group’s most relevant traits.

3.6 Items Foraged

Social wasps are notably generalist and opportunistic in their foraging behaviour. This implies that wasp foragers are able to explore a wide range of resources while showing varied behavioural strategies to do so. These habits were probably developed in the group in the answer to the evolution of sociality, which requires a nutritional and energetic income much higher to that of individual life. Greater foraging behavioural plasticity, in contrast with specialization, could allow colonies to survive periods of food shortage and extend their lifespans (Jeanne and Taylor 2009). These traits add to the complexity of social foraging and must be taken into account when studying social wasp diet and prey preferences.

The items foraged by wasps are usually divided into four groups (West-Eberhard 1969; Raveret-Richter 2000): (1) water, (2) nest-building materials (often called “vegetal fibre”), (3) sugary substances (also called “carbohydrates” or even “nectar”) and (4) protein (also called “prey”). These groups have different functions for the maintenance of the colony, as discussed below. Raveret-Richter (2000) has reviewed the foraging of each of these items in detail for social wasps, and a good part of the literature on them has remained untouched for Neotropical paper wasps. We here provide a summarized description of these behaviours and discuss more recent findings for the Polistinae and advise the reader to refer to Raveret-Richter’s work for a more extensive review.

3.6.1 Water

Social wasps imbibe water droplets during foraging activity, which is done from water bodies such as lakes and puddles or from accidental/temporary containers such as morning dew (Jeanne 1986). Wasp foragers may display unusual behavioural alternatives to avoid competition while foraging for water. For instance, wasps may choose to perform a short nimble landing on the water’s surface to drink from it instead of joining other wasps on the water’s edge (Prezoto et al. 2016a).

After its collection, water may be used in more than one way in the colony. In addition to being ingested by adults or larvae for metabolic purposes (Kasuya 1982), water is also added to vegetal fibre during nest construction and repair (Akre 1982). Workers will also place droplets on nest walls or inside cells. In conjunction with fanning (wing beating) behaviour, it makes up a behavioural strategy to thermoregulate the colony on hot days (Wilson 1971).

3.6.2 Nest-Building Materials

Vegetal fibre is the main construction material used by Polistinae, which has rendered them the popular name “paper wasps”. Fibre is collected from natural vegetal structures such as branches or trunks, but may also be obtained from untreated, unpainted wood worked by man such as old furniture (Wenzel 1991). Foragers land on the substrate and cut a shred of the fibre using their mandibles, which are then maxalated and mixed with saliva into a rounded shape. At least for one species, foragers will regurgitate water on the original source before cutting of the fibre – probably making its removal easier and helping in keeping the material’s cohesion (Jeanne 1986).

3.6.3 Sugary Substances

Sugary substances , or simply carbohydrates, are collected by foragers and used as a main source of energy by adult wasps, which feed almost exclusively on it (Jeanne and Taylor 2009). Carbohydrates are usually collected in the form of plant nectar or the honeydew exuded by homopterans (Evans and West-Ebehrard 1970; Moller et al. 1991). This resource is shared at the nest to both adult and immature wasps via trophallaxis.

Nectar can be collected from both extrafloral nectaries and flowers, although many of the latter are not accessible to wasps due to their morphology (Spradbery 1973; Köhler 2008). The flowers visited by wasps reflect the group’s generalist/opportunistic habits: favoured species vary as different flowering seasons coincide with a need for high carbohydrate intake at the colonies (Köhler 2008). Additionally, it is during flower visitation that wasps may act as pollinators.

One of the most studied alternatives for plant nectar as a carbohydrate source is the collection of honeydew. This substance is a metabolic by product secreted by aphids and scale insects after the ingestion of sap. Homopteran families such as Aphidae, Psyllidae and Coccidae are often visited by wasp foragers for the collection of honeydew and may even benefit from wasp protection against predators (Letourneau and Chloe 1987; Moller et al. 1991).

Other alternatives for carbohydrates exploited by wasp foragers include tree sap (Jeanne and Taylor 2009), bee honey robbing (Edwards 1980) and ripe fruit. The latter seems to be especially attractive to foragers when at the over-ripe or rotting stage: wasps have been observed foraging at Brazilian cherries (Souza et al. 2013), Brazilian grapes (De Souza et al. 2010), cactus fruits (Santos et al. 2007), cashews (Santos and Presley 2010), grapes (Hickel and Schuck 1995), guavas (Brugger et al. 2011), mangoes (Barbosa et al. 2014) and prunes (Prezoto and Braga 2013).

3.6.4 Protein

Protein foraging in social wasps is the main interest in a myriad of studies on the group’s ecology. In addition to its role in the social cohesion of a colony (see “Trophallaxis), protein foraging in Neotropical social wasps is the key behaviour behind the growing interest in exploiting the group as agents of pest control (Prezoto et al. 2019).

Even though adult wasps may eventually consume portions of prey, these are largely destined to larval nourishment. In addition to ingesting sugary substances, larvae require a constant protein intake in order to fully develop into adults. For that reason, it is generally expected that protein foraging intensifies during stages when the proportion of brood in a colony is higher (London and Jeanne 2003).

The main source of protein foraged by wasps comes from prey, namely, terrestrial arthropods (Prezoto et al. 2019). The standard method for assessing social wasp prey preference is the interception of foragers as they return to the nest and physically removing the load from their mandibles for analysis (Giannotti et al. 1995). This method is limited, however, because most wasps chew prey fragments prior to their transport (Fig. 3.1c). Visual taxon identification performed on prey matter is therefore restricted to higher taxonomical levels, such as families (Jeanne and Taylor 2009). Polybia occidentalis (Olivier, 1791) is a notable exception: since their prey is carried back almost intact, the species stands as a good model for cost-efficient diet assessment (Hunt et al. 1987).

Despite the challenges involved, various researchers have put effort into assessing social wasp diets. Jeanne and Taylor (2009) state that wasps, although generalists, tend to choose soft-bodied terrestrial arthropods as their prey. The biggest portion of these is represented by caterpillars (Lepidoptera) and adult flies (Diptera), with a smaller amount of Dermaptera, Orthoptera, Odonata, Hemiptera, Coleoptera, Hymenoptera and Arachnida being captured. The trend for targeting caterpillars during protein foraging has been verified in many Neotropical social wasps; Table 3.2 shows species and environments for which Lepidoptera larvae constituted at least 50% of their diets.

Table 3.2 Research showing species of social wasps that capture a majority (50% or more) of Lepidoptera as their prey in different Neotropical environments
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This preference for caterpillar as prey seems to be established across different genera of both individual- and swarm-founding social wasps, especially in environments altered by man. In environments dominated by fewer species, such as in monoculture plantations, the proportion of Lepidoptera in social wasp diets can reach values as high as 95–100% (Prezoto et al. 2006; Elisei et al. 2010). This supports the claim that social wasps, although generalist, may act as facultative specialists (Raveret-Richter 1990).

Nevertheless, a generalist diet still plays an important role in social wasp ecology. Diet composition may shift significantly away from lepidopterans to fit prey seasonality. As an example, research shows colonies preying on Diptera at high proportions for the species Polybia platycephala (Richards 1951) (Prezoto et al. 2005) and Synoeca cyanea (Fabricius, 1775) (Prezoto and Braga 2013).

The opportunistic aspect of social wasp foraging allows the exploitation of a range of alternatives for protein foraging. Agelaia wasps are notable for their carrion-feeding behaviour, a trait that sometimes is also seen in Angiopolybia, Brachygastra, Parachartergus, Polybia, Protonectarina and Synoeca (O’Donnell 1995b; Silveira et al. 2005). Wasp foragers may further act on corpse decomposition Gomes et al. (2007) observed Polybia ignobilis individuals capturing flies that were laying eggs on rotting carcasses. Another usual strategy is preying on alate ants and termites during nuptial flights, when they are most vulnerable (Holfling and Machado 1985; Raveret-Richter 2000). Finally, unusual behaviours have been recorded for social wasps such as robbing prey from spider webs and other arthropod predators (Chevalier 1924; Jeanne 1972), preying on vertebrate eggs (Warkentin 2000) and capturing prey from bait traps used in insect diversity studies (Maciel et al. 2014).

3.7 Resource Storage

To overcome the challenge of feeding a colony for extended periods of time, some vespids have developed strategies to store resources in their nests. This behaviour is comparable to the production of honey in social bees (Seeley 1989) or the storage of vegetal matter in subterranean ant nests (Taber 1999). In Polistinae these behaviours are somewhat simpler but still reflect the importance of resource storage in the success and evolution of eusociality (Hunt 1982, 1991).

In their “Observations on the social wasps of South America (Hymenoptera, Vespidae)”, Richards and Richards (1951) performed an extensive analysis on the nests of different social wasp species and found two types of resources: liquid storages (honey-like sugary substances) and solid storages (maxalated prey fragments). Both types were found inside nest cells, although each one showed different distribution patterns through the nest. Other studies analysed liquid storages and found them to be of vegetal origin, usually nectar from flowers surrounding the colonies. These substances are sometimes called “wasp honey” or simply “honey” (Strassman 1979; Guimarães et al. 2008).

Resource storages are found in nests of both independent- and swarm-founding species, but seem to be more common in the latter (Richards and Richards 1951; Prezoto and Gobbi 2003). The nests of swarm-founding paper wasps have multiple layers of combs surrounded by an envelope, which probably provides food stocks with better protection against robbers when compared to the exposed, single-combed nests of the independent-founding Polistes and Mischocyttarus. Honey storage has been verified for the swarm-founding species Metapolybia cingulata (Fabricius, 1804), Polybia bicyttarella (Richards, 1951), Polybia bistriata (Fabricius, 1804), Polybia ignobilis (Haliday, 1836), Polybia occidentalis (Olivier, 1791), Polybia platycephala (Richards, 1978), Polybia scutellaris (White, 1841), Protonectarina spp. and Protopolybia emortualis (de Saussurre, 1855) (Richards and Richards 1951; Hunt et al. 1987; Corujo et al. 2010; Rocha 2011) and for the independent-founding Mischocyttarus cassununga (von Ihering, 1903), Mischocyttarus drewseni (de Saussurre, 1854), Mischocyttarus socialis (de Saussurre, 1854) Polistes simillimus (Zikán, 1951) and Polistes versicolor (Olivier, 1791) (Richards and Richards 1951; Jeanne 1972; Prezoto and Gobbi 2003; Guimarães et al. 2008; Barbosa et al. 2017). The species Brachygastra lecheguana (Latreille, 1824) stands out for storing honey in remarkable amounts (Richards and Richards 1951).

Rossi and Hunt (1988) described two patterns for honey storage inside nests. Honey droplets may be placed inside empty cells (Strassman 1979; Machado 1984; Hunt et al. 1987) or cells shared by eggs or first instar larvae (and rarely by second instar larvae) (Rau 1928, 1939; Heldmann 1936; Jeanne 1972). Honey placed in cells also containing immature wasps was related to providing young larvae with additional nourishment by Jeanne (1972). This claim is supported by behavioural evidence, since adults may engage in trophallaxis with larvae immediately after visiting honey storages (Jeanne 1972; Guimarães et al. 2008). When stored in empty cells, honey is usually placed in older, unproductive combs (in swarm-founding species) or in the comb’s peripheral cells (in independent-founding species). These storages are possibly long-term food reserves to grant the colony its sustenance at the final phases of the cycle, which often coincide with food shortage periods (Strassman 1979). This claim is supported by the fact that honey storages seem to be more conspicuous during dry seasons (Hunt et al. 1987). Finally, from behavioural observations, when individual wasps do not interact with nestmates after visiting honey storages, it can be inferred that the reserves are purposed for individual adult nourishment (Prezoto and Gobbi 2003).

Solid resource storages , on the other hand, seem to be a more widespread behaviour in Neotropical social wasps – especially in the genus Polybia. Studies with Polybia platycephala and Polybia occidentalis (Richards and Richards 1951; Rocha 2011) report a high number of nest cells containing fragments of flies (Chironomidae) and alate ants (Hymenoptera) and termites (Blattodea). The high proportion of alates found in storages is further evidence of the wasp’s opportunistic foraging, since reproductive ants and termites are usually available as prey in short periods but in very high densities. Furthermore, the unusual abundance of prey could be the necessary stimulus for building solid resource storages.

Investigating resource storages in social wasp nests can provide insights outside of the wasp’s life history. As an example, pollen analysis performed on honey storages allows researchers to identify flowers visited by wasp foragers without needing to deal with the challenges of tracking wasps during their bouts (Corujo et al. 2010). Overall, storage analysis can be a complementary key to understand the ecological interaction webs that wasps take part in.

3.8 Future Directions

Due to many factors, the research on the foraging behaviour of social wasps in the Neotropical environments – especially in Brazil, where most of its research groups are based at – unfortunately lags behind studies carried out in other climatic zones, not unlike social wasp research that sometimes lags behind ant and social bee studies. Underfunding, allied to the relatively small number of researchers devoted to study the group, is probably the main factor responsible for that issue. Curiously, the general public’s negative perception of wasps as organisms (Sumner et al. 2018) could be one element involved in preventing wasp research to be funded while also pushing prospective researchers away from the field during their formation. Another considerable challenge faced by researchers is the impressive diversity of social wasps in the Neotropics. Many of these species have nothing but their taxonomical description available in literature, and even abundant, widely distributed ones lack published data on their life history. This often forces investigators to focus on general biology and behaviour prior to addressing the field’s trending theories, which is usually looked down by editors and funding organizations.

Yet, as research on basic biology and behavioural ecology progresses, scientists must look ahead to explore the frontiers of sociobiology. Here, we discuss what we consider promising methods and theories for the future of social wasp foraging behaviour research in Neotropical areas.

3.8.1 Biological Control

The use of paper wasps as agents of agricultural pest control has been suggested as early as in the 1950s (Rabb and Lawson 1957). In the Neotropics, this possibility has been repeatedly brought up as the inspiration behind investigations on wasp foraging behaviour and even materialized itself in the form of experimental studies (Prezoto and Machado 1999). Hard experimental evidence obtained in controlled settings (such as pest predation rates and plant productivity in greenhouses) has only very recently been published (Southon et al. 2019), finally opening the way for exploiting social wasps in carefully designed pest control plans. Current research on the matter has been mostly limited to anecdote reports on wasps nesting or foraging within crops. The allocation of scientific resources to explore social wasp foraging in controlled experiments could be hugely beneficial to the field. Additional possibilities were recently opened as wasps have been suggested as controllers of dipterans of medical interest (Prezoto et al. 2016b). The current epidemiological relevance of Aedes mosquitoes, vectors of the dengue, Zika and Chikungunya fevers, could be used to motivate researchers and funding agents to invest in social wasps foraging as a control alternative .

3.8.2 Molecular Analysis

The overwhelming majority of studies in the foraging behaviour of Neotropical social wasps rely on observation data. Conversely, the so-called Omics Age brings a wide range of new ways to answer questions on sociobiology (Ghoul et al. 2017). In addition to applying well-established genetics methods to social wasps – such as searching for genes linked to foraging success (Ingram et al. 2011) or identified genetic-physiological thresholds for the perception of specific foraged items (Değirmenci et al. 2018) – we also suggest that researchers explore the innovative fields on ecogenetics in their investigations. For instance, metabarcoding the gut content of foragers and larvae (Krehenwinkel et al. 2016) is a way to overcome the challenges of tracking foragers outside their nests while obtaining reliable data on their interactions with both vegetal species and prey. If successfully done, this could reinforce arguments on the ecological services provided by social wasps .

3.8.3 Collective Personalities

Since the early 2000s, the study of animal personalities has been experiencing a sharp increase in interest among behavioural ecology researchers. Previously interpreted as statistical noise, consistent behavioural variations are now investigated for their theorized role in success and evolution of organisms (Sih et al. 2004). Moreover, the study of social behaviour and the personality of superorganisms – or collective personalities – has presented itself as an exciting new subarea in which sociobiology may be explored through research (Jandt et al. 2014; Wright et al. 2019). For foraging behaviour, relationships between personality traits and general ecology – such as aggressiveness and foraging success or parental care – could be verified for Neotropical social wasps, helping not only in the understanding of their life history but also of the evolution of social behaviour as a whole.

3.9 Concluding Remarks

The foraging behaviour of Neotropical social wasps is one of the most fascinating topics to be explored on the group’s behavioural ecology. Here, we briefly presented some of the most unique aspects involved in the activity of wasp foragers and tried to show how its different particularities make up complex relationships with other traits such as nest building, pollination or social hierarchy. Studying the foraging behaviour of social wasps is one of the keys to understand the evolution of eusociality in Hymenoptera.

In our perspective, in the specific case of the Neotropical wasps, the group’s diversity should transition from being a challenge to being an advantage to researchers. Exploring the foraging activity of different clades with both classic behavioural approaches and innovative methods will surely lead to the discovery of unheard behavioural strategies and shed a new light on the existing knowledge not only for the taxon but also for Neotropical systems as a whole.