Learning in an exotic social wasp while relocating a food source

Context-dependent use of olfactory cues by foragers of Vespula germanica social wasps

Food search is guided by cues from different sensory modalities, such as olfactory and visual. In social wasps, olfaction plays a key role in locating new resources. However, while several studies have focused on the importance of odours in predation, less is known about their role during scavenging, when spatial memories become a relevant guidance mechanism. Here, we investigated whether the use of odours during carrion exploitation by Vespula germanica wasps depends on whether they are locating or relocating the resource. By means of field choice experiments, we evaluated wasp response to odours: an odour eliciting a spontaneous aversive response, a learnt odour eliciting an appetitive response, and the conspecifics' odour eliciting an attractive response. Experiments were conducted in different contexts, i.e., during food localisation by naïve foragers, re-localisation of a resource at the learnt site and re-localisation of a resource that had been displaced from the learnt site. All olfactory stimuli evaluated markedly influenced foraging decisions in naïve wasps and in experienced wasps when the food was moved from the learnt location. However, odours were ignored during the wasp's return to the foraging site. These results suggest a cue hierarchy, in which local landmarks are more reliable to relocate carrion, while olfaction would be useful to locate novel resources or relocate a known source when spatial memories fail. Our findings demonstrate a context-dependent use of odours during carrion exploitation by V. germanica wasps and highlight the importance of spatial memories as an important factor modulating odour response.

Spatio-Environmental Analysis of Vespula germanica Nest Records Explains Slow Invasion in South Africa

Simple Summary

Social wasp invasions can spread quickly and have serious impacts if they reach new regions with favourable climatic conditions. However, in areas less suitable to them, invasion patterns can show factors that may prevent their spread. We use nest records of the German wasp from the southern tip of South Africa, to map and analyse what habitats they seem to prefer. Factors investigated included temperature, rainfall, and moisture availability. We find that this invasive wasp prefers moister and cooler conditions, and because these habitats are patchy in the region they have invaded in South Africa, they can only spread slowly unassisted, and utmost between 50 and 200 km with the assistance of humans. This is the likely reason for the very slow invasion seen in South Africa. The spatial patterns we quantify here make it possible to use a remote-sensing approach to determine the suitability of an area for future invasions. Predicting the likelihood of invasions will greatly aid management actions. Public awareness around the potential accidental transport of the German wasp and similar species should also be undertaken.

Abstract

Investigating the distributions of invasive species in marginal habitats can give clues to the factors constraining invasive spread. Vespula germanica is the most widely distributed of all the invasive Vespids, which in the Southern Hemisphere typically have large extensive invasive populations. In contrast, the invasion into South Africa has been slow and is still confined to a small geographic area. Here we analyse the distribution of all recent nest records in South Africa (n = 405). The distance to main rivers, mean annual rainfall, summer normalised difference moisture index (NDMI) values, and mean annual temperatures (average, minimum, maximum, and summer maximum temperature) was measured for every nest. We find that value ranges of these variables are different between the value ranges recorded for nests, the general distribution area of the wasp, and the area of absence. Optimised Hot Spot Analysis was used to quantify spatial structure in the measured climatic variables. Generally, factors related to moisture stress set the environmental limits of V. germanica’s landscape distribution. Due to the strong preference of nesting sites close to river courses, for higher rainfall conditions, medium to medium-high NDMI values, and lower mean annual temperatures, it is unlikely that V. germanica will be able to spread uniformly where it is currently found in South Africa.

Evidence of associative blocking during foraging in the social wasp Vespula germanica

While foraging, Vespula germanica usually return to abundant food sites. During this relocation behavior, these wasps learn to identify contextual cues associated with food position. We analyzed associative blocking in this species, that is, how an association with a conditioned stimulus (CS1) blocks subsequent learning when a novel stimulus (CS2) is added on a second foraging visit. Three groups of wasps (A, B, and C; total 74 individual wasps) were observed while collecting meat during one or two consecutive visits. In group A, an environmental cue (CS1) was paired with food placed at a specific site, and on the second visit, a second cue (CS2) was added while food remained in the same position. In a subsequent testing phase, CS1 was removed and the food source displaced nearby. We then recorded the number of hovers performed over the empty dish (previously baited). Group A wasps appeared to ignore the addition of CS2 on their second visit because they performed fewer hovers over the learned site. For group A, the duration of the decision-making process to finally fly toward the baited dish was shorter than when CS1 and CS2 were presented together on their first visit (group B). This is the first study to demonstrate the occurrence of associative blocking in vespids, confirming that a prior foraging experience influences subsequent food relocation in V. germanica. Our findings reveal that first learning episodes block further associations with novel contextual cues, contributing to understanding of complex cognitive processes involved in V. germanica´s foraging behavior.

Spatial memory in Vespula germanica wasps: A pilot study using a Y-maze assay

In the present study we analysed spatial learning in Vespula germanica wasps when dealing with a walking Y-maze. We recorded the time taken to leave the maze during two consecutive visits and which of the two short arms was chosen to exit. Two treatments were conducted to evaluate whether wasps learned to leave the Y-maze guided either by spatial or visual cues. In Treatment 1, the colour of both arms remained unchanged between two consecutive visits; and in Treatment 2, the position of the coloured arm was switched after the first trial. Our results demonstrated that the time taken to exit the maze on the second trial was less than half in both treatments and wasps left the maze from the previously chosen arm, irrespective of its colour. This is the first study to demonstrate spatial learning in V. germanica wasps by using a walking Y-maze. Free flying wasps learned to enter the Y-maze on their own volition, walk through it, collect food and find their way out more rapidly after a single foraging experience. The current experimental device is suitable for the evaluation of spatial memory processes and exploratory behaviour in this species.

How behavioral plasticity enables foraging under changing environmental conditions in the social wasp Vespula germanica (Hymenoptera: Vespidae)

The foraging strategy at abundant resources of the social wasp Vespula germanica includes scanning in the direction of the nest while memorizing resource-specific landmarks and contextual cues. In the present study, we sought to explore wasps' behavioral plasticity on foraging trips to resources whose location and composition changed after a single visit. We evaluated how contextual modifications of food displacement and replacements 60 cm apart from the original site, affect re-orientation for re-finding previously memorized food resources. The results showed that wasps detected and collected the resource faster when more changes were introduced on the following visit. If returning foragers discovered several modifications on both the location and the kind of resource, they collected food more rapidly from the displaced dish, than if only a single parameter in the environment had been changed. These findings illustrate the grade of behavioral plasticity in V. germanica while foraging on abundant resources, which may contribute to the understanding of the prodigious invasive success of this species in anthropized environments.

Foraging behaviour of the exotic wasp Vespula germanica (Hymenoptera: Vespidae) on a native caterpillar defoliator

Vespula germanica is a social wasp and an opportunistic predator. While foraging, these wasps learn and integrate different kinds of cues. They have successfully invaded many parts of the world, including native Nothofagus and Lophozonia forests located in the Andean-Patagonian region, where they forage on native arthropods. Perzelia arda, a lepidopteron defoliator of Lophozonia obliqua, uses the foliage to hide in and feed on. The purpose of this work is to study whether V. germanica use olfactory cues when foraging on P. arda. To do this, we used a Y-tube olfactometer and established three treatments to compare pairs of all combinations of stimuli (larvae, leaves with larval traces, and leaves without larval traces) and controls. Data were analysed via two developed models that showed decisions made by V. germanica and allowed to establish a scale of preferences between the stimuli. The analysis demonstrates that V. germanica wasps choose P. arda as larval prey and are capable of discriminating between the offered stimuli (deviance information criterion (DIC) null model = 873.97; DIC simple model = 84.5, n = 152). According to the preference scale, V. germanica preferred leaves with traces of larvae, suggesting its ability to associate these traces with the presence of the prey. This may be because, under natural conditions, larvae are never exposed outside their shelters of leaves and therefore V. germanica uses indirect signals. The presence of V. germanica foraging on P. arda highlights the flexible foraging behaviour of this wasp which may also act as a positive biological control, reducing lepidopteran populations.

Long-term spatial memory in Vespula germanica social wasps: the influence of past experience on foraging behavior

Social insects exhibit complex learning and memory mechanisms while foraging. Vespula germanica (Fab.) (Hymenoptera: Vespidae) is an invasive social wasp that frequently forages on undepleted food sources, making several flights between the resource and the nest. Previous studies have shown that during this relocating behavior, wasps learn to associate food with a certain site, and can recall this association 1 h later. In this work, we evaluated whether this wasp species is capable of retrieving an established association after 24 h. For this purpose, we trained free flying individuals to collect proteinaceous food from an experimental plate (feeder) located in an experimental array. A total of 150 individuals were allowed 2, 4, or 8 visits. After the training phase, the array was removed and set up again 24 h later, but this time a second baited plate was placed opposite to the first. After 24 h we recorded the rate of wasps that returned to the experimental area and those which collected food from the previously learned feeding station or the nonlearned one. During the testing phase, we observed that a low rate of wasps trained with 2 collecting visits returned to the experimental area (22%), whereas the rate of returning wasps trained with 4 or 8 collecting visits was higher (51% and 41%, respectively). Moreover, wasps trained with 8 feeding visits collected food from the previously learned feeding station at a higher rate than those that did from the nonlearned one. In contrast, wasps trained 2 or 4 times chose both feeding stations at a similar rate. Thus, significantly more wasps returned to the previously learned feeding station after 8 repeated foraging flights but not after only 2 or 4 visits. This is the first report that demonstrates the existence of long-term spatial memory in V. germanica wasps.

Social Learning in Vespula Germanica Wasps: Do They Use Collective Foraging Strategies?

Vespula germanica is a social wasp that has become established outside its native range in many regions of the world, becoming a major pest in the invaded areas. In the present work we analyze social communication processes used by V. germanica when exploiting un-depleted food sources. For this purpose, we investigated the arrival pattern of wasps at a protein bait and evaluated whether a forager recruited conspecifics in three different situations: foragers were able to return to the nest (full communication), foragers were removed on arrival (communication impeded), or only one forager was allowed to return to the nest (local enhancement restricted). Results demonstrated the existence of recruitment in V. germanica, given that very different patterns of wasp arrivals and a higher frequency of wasp visits to the resource were observed when communication flow between experienced and naive foragers was allowed. Our findings showed that recruitment takes place at a distance from the food source, in addition to local enhancement. When both local enhancement and distant recruitment were occurring simultaneously, the pattern of wasp arrival was exponential. When recruitment occurred only distant from the feeder, the arrival pattern was linear, but the number of wasps arriving was twice as many as when neither communication nor local enhancement was allowed. Moreover, when return to the nest was impeded, wasp arrival at the bait was regular and constant, indicating that naive wasps forage individually and are not spatially aggregated. In conclusion, this is the first study to demonstrate recruitment in V. germanica at a distance from the food source by modelling wasps’ arrival to a protein-based resource. In addition, the existence of correlations when communication was allowed and reflected in tandem arrivals indicates that we were not in the presence of random processes.