Combined Elevation of Temperature and CO2 Impacts the Production and Sugar Composition of Aphid Honeydew.

Honeydew is the keystone of many interactions between aphids and their predators, parasitoids, and mutualistic partners. Despite the crucial importance of honeydew in aphid-ant mutualism, very few studies have investigated the potential impacts of climate change on its production and composition. Here, we quantified changes in sugar compounds and the amount of honeydew droplets released by Aphis fabae reared on Vicia faba plants under elevated temperature and/or CO2 conditions. Following the combined elevation of these two abiotic factors, we found a significant increase in the fructose content of A. fabae honeydew, accompanied by nonsignificant trends of increase in total honeydew production and melezitose content. The environmental conditions tested in this study did not significantly impact the other honeydew sugar contents. The observed changes may be related to changes in phloem composition under elevated CO2 conditions as well as to increases in aphid metabolism and sap ingestion under elevated temperatures. Although limited, such changes in aphid honeydew may concurrently reinforce ant attendance and mutualism under elevated temperature and CO2 conditions. Finally, we discuss the enhancing and counteracting effects of climate change on other biological agents (gut microorganisms, predators, and parasitoids) that interact with aphids in a complex multitrophic system.

Distributed leadership and adaptive decision-making in the ant Tetramorium caespitum.

In the ant species Tetramorium caespitum, communication and foraging patterns rely on group-mass recruitment. Scouts having discovered food recruit nestmates and behave as leaders by guiding groups of recruits to the food location. After a while, a mass recruitment takes place in which foragers follow a chemical trail. Since group recruitment is crucial to the whole foraging process, we investigated whether food characteristics induce a tuning of recruiting stimuli by leaders that act upon the dynamics and size of recruited groups. High sucrose concentration triggers the exit of a higher number of groups that contain twice as many ants and reach the food source twice as fast than towards a weakly concentrated one. Similar trends were found depending on food accessibility: for a cut mealworm, accessibility to haemolymph results in a faster formation of larger groups than for an entire mealworm. These data provide the background for developing a stochastic model accounting for exploitation patterns by group-mass recruiting species. This model demonstrates how the modulations performed by leaders drive the colony to select the most profitable food source among several ones. Our results highlight how a minority of individuals can influence collective decisions in societies based on a distributed leadership.

How to visualize the spider mite silk?

Tetranychus urticae (Acari: Tetranychidae) is a phytophagous mite that forms colonies of several thousand individuals. Like spiders, every individual produces abundant silk strands and is able to construct a common web for the entire colony. Despite the importance of this silk for the biology of this worldwide species, only one previous study suggested how to visualize it. To analyze the web structuration, we developed a simple technique to dye T. urticae'silk on both inert and living substrates. Fluorescent brightener 28 (FB) (Sigma F3543) diluted in different solvents at different concentrations regarding the substrate was used to observe single strands of silk. On glass lenses, a 0.5% dimethyl sulfoxide solution was used and on bean leaves, a 0.1% aqueous solution. A difference of silk deposit was observed depending the substrate: rectilinear threads on glass lenses and more sinuous ones on bean leaves. This visualizing technique will help to carry out future studies about the web architecture and silk used by T. urticae. It might also be useful for the study of other silk-spinning arthropods.

Social integration of robots into groups of cockroaches to control self-organized choices.

Collective behavior based on self-organization has been shown in group-living animals from insects to vertebrates. These findings have stimulated engineers to investigate approaches for the coordination of autonomous multirobot systems based on self-organization. In this experimental study, we show collective decision-making by mixed groups of cockroaches and socially integrated autonomous robots, leading to shared shelter selection. Individuals, natural or artificial, are perceived as equivalent, and the collective decision emerges from nonlinear feedbacks based on local interactions. Even when in the minority, robots can modulate the collective decision-making process and produce a global pattern not observed in their absence. These results demonstrate the possibility of using intelligent autonomous devices to study and control self-organized behavioral patterns in group-living animals.

Influence of seed size and seed nature on recruitment in the polymorphic harvester ant Messor barbarus.

In the polymorphic harvester ant Messor barbarus, the colony modulates response during foraging according to seed characteristics. The easiness of picking up small seeds (oat fragments or canary seeds) shortens the time lapse between food discovery and the return of scouts to the nest, favouring the onset of recruitment and higher mobilisation rates than bigger seeds (whole oat). However, size is not the only criterion, as canary seeds are preferred among small ones. A modulation of the trail laid by the first scouts also plays a role in the shaping of harvesting patterns and in the enhancement of recruitment towards small and/or preferred seed. This flexibility extends to the relative participation of the three worker sizes classes, which differs according to seed. Media are the most numerous at the foraging arena and largely responsible for trail-laying. Minor participate in seed harvesting and trail-laying, mostly when they can be efficient in carrying seeds, such as oat fragments. Major scarcely participate in harvesting and trail-laying, but are involved in the exploitation of bigger and/or preferred seed species. When faced with different seed species, M. barbarus seems thus to collectively adopt a "foraging time minimisation" rather than an "energetic gain per item harvested maximisation" strategy. Dynamics of foraging and division of labour between foragers of different sizes are related to both the workers' ability to carry seed items, and the transfer of information to nestmates by trail-laying, all parameters that vary according to seed characteristics.

Optimality of collective choices: a stochastic approach.

Amplifying communication is a characteristic of group-living animals. This study is concerned with food recruitment by chemical means, known to be associated with foraging in most ant colonies but also with defence or nest moving. A stochastic approach of collective choices made by ants faced with different sources is developed to account for the fluctuations inherent to the recruitment process. It has been established that ants are able to optimize their foraging by selecting the most rewarding source. Our results not only confirm that selection is the result of a trail modulation according to food quality but also show the existence of an optimal quantity of laid pheromone for which the selection of a source is at the maximum, whatever the difference between the two sources might be. In terms of colony size, large colonies more easily focus their activity on one source. Moreover, the selection of the rich source is more efficient if many individuals lay small quantities of pheromone, instead of a small group of individuals laying a higher trail amount. These properties due to the stochasticity of the recruitment process can be extended to other social phenomena in which competition between different sources of information occurs.

Dynamics of aggregation and emergence of cooperation.

Aggregation is one of the most basic social phenomena, and many activities of social insects are linked to it. For instance, the selection of a valuable site and the spatial organization of the population are very often by-products of amplifications based on the local density of nestmates. The patterns of aggregation are very diverse, ranging from the gathering of all animals in a unique site to their splitting between several ones. One might question how these multiple patterns emerge. Do ants actively initiate the formation of such patterns by modulating the emission of an attracting signal such as the trail pheromone? Alternatively, do patterns result from quantitative changes in the duration of interaction between animals once they have reached the gathering site, without any active modulation of the communications? To discuss these questions, we present two empirical studies: the gregarious behavior of cockroaches (Blatella) and self-assembly in the weaver ant (Oecophylla). Through experimental and theoretical studies, we show how a single behavior-the resting time-leads to a collective choice in both species. This behavior is a response to the density of conspecifics and can also be modulated by heterogeneities in the environment. In weaver ants, it allows the colony to focus the formation of chains in a given area among several potential sites. In cockroaches, it allows the gathering of individuals in particular shelters, depending on the proximity between strains. These results are discussed with emphasis on the role of aggregation processes in the emergence of cooperativity and task allocation.

The influence of the physical environment on the self-organised foraging patterns of ants.

Among social insects such as ants, scouts that modulate their recruiting behaviour, following simple rules based on local information, generate collective patterns of foraging. Here we demonstrate that features of the abiotic environment, specifically the foraging substrate, may also be influential in the emergence of group-level decisions such as the choice of one foraging path. Experimental data and theoretical analyses show that the collective patterns can arise independently of behavioural changes of individual scouts and can result, through self-organising processes, from the physico-chemical properties of the environment that alter the dynamics of information transfer by chemical trails.

Seed drops and caches by the harvester ant Messor barbarus: do they contribute to seed dispersal in Mediterranean grasslands?

To determine whether the harvester ant Messor barbarus acts as a seed disperser in Mediterranean grasslands, the accuracy level of seed processing was assessed in the field by quantifying seed drops by loaded foragers. In the vicinity of exploited seed patches 3 times as many diaspores were found as in controls due to seed losses by foragers. Over trails, up to 30% of harvested seeds were dropped, singly, by workers but all were recovered by nestmates within 24 h. Seeds were also dropped within temporary caches with very few viable diaspores being left per cache when ants no longer used the trail. Globally, ant-dispersed diaspores accounted for only 0.1% of seeds harvested by M. barbarus. We discuss the possible significance for grassland vegetation of harvester-ant-mediated seed dispersal.

How do ants assess food volume?

By comparing the behaviour of Lasius niger scouts at sucrose droplets of different volumes, we empirically identified the criterion used by each scout to assess the amount of food available as well as the rules governing its decision to lay a recruitment trail. When scouts discovered food volumes exceeding the capacity of their crop (3 or 6 µl), 90% immediately returned to the nest laying a recruitment trail. In contrast, when smaller food droplets (0.3, 0.7 or 1 µl) were offered, several scouts stayed on the foraging area, presumably exploring it for additional food. If unsuccessful, they returned to the nest without laying a trail. The droplet volume determined the percentage of trail-laying ants but had no influence on the intensity of marking when this was initiated. The key criterion that regulated the recruiting behaviour of scouts was their ability to ingest their own desired volume. This volume acted as a threshold triggering the trail-laying response of foragers. Collective regulation of foraging according to food size resulted from the interplay between the distribution of these desired volume thresholds among colony members and the food volume available. We relate some aspects of the foraging ecology of aphid-tending ants to this decision-making process. Copyright 2000 The Association for the Study of Animal Behaviour.

A new pheromone in the ant Pheidole pallidula (Myrmicinae).

In the dimorphic species Pheidole pallidula, an ethological factor, present on the VIIth abdominal sternite attracts ants from short distances and enhances the linear speed of those having approached it. Added to poison gland extract, it improves the workers' trail following behaviour. This factor is essentially presented by minors, majors' sternite having a much weaker ethological effect. Several functions and a glandular origin are suggested for this hitherto unknown pheromone in Pheidole pallidula.