Queen-worker conflict over sex ratio: a comparison of primary and secondary sex ratios in the Argentine ant,

We compare the primary sex ratio (proportion o haploid eggs laid by queens) and the secondary sex ratio (proportion of male pupae produced) in the Argentine ant Iridomyrmex humilis with the aim of investigating whether workers control the secondary sex ratio by selectively eliminating male brood. The proportion of haploid eggs produced by queens was close to 0.5 in late winter, decreased to less than 0.3 in spring and summer, and increased again to a value close to 0.5 in fall. Laboratory experiments indicate that temperture is a proximate factor influencing the primary sex ratio with a higher proportion of haploid eggs being laid at colder temperatures. Production of queen pupae ceased in mid-June, about three weeks before that of male pupae. After this time only worker pupae were produced. During the period of production of sexuals, the proportion of male pupae ranged from 0.30 to 0.38. Outside this period no males were reared although haploid eggs were produced all the year round by queens. Workers thus exert a control on the secondary sex ratio by eliminating a proportion of the male brood during the period of sexual production and eliminating all the males during the remainder of the cycle. These data are consistent with workers preferring a more female-biased sex ratio than queens. The evolutionary significance of the production of male eggs by queens all the year round is as yet unclear. It may be a mechanism allowing queen replacement in the case of the death of the queens in the colony.

Experimental manipulation of colony genetic diversity had no effect on short-term task efficiency in the Argentine ant Linepithema humile

Genetic diversity might increase the performance of social groups by improving task efficiency or disease resistance, but direct experimental tests of these hypotheses are rare. We manipulated the level of genetic diversity in colonies of the Argentine ant Linepithema humile, and then recorded the short-term task efficiency of these experimental colonies. The efficiency of low and high genetic diversity colonies did not differ significantly for any of the following tasks: exploring a new territory, foraging, moving to a new nest site, or removing corpses. The tests were powerful enough to detect large effects, but may have failed to detect small differences. Indeed, observed effect sizes were generally small, except for the time to create a trail during nest emigration. In addition, genetic diversity had no statistically significant impact on the number of workers, males and females produced by the colony, but these tests had low power. Higher genetic diversity also did not result in lower variance in task efficiency and productivity. In contrast to genetic diversity, colony size was positively correlated with the efficiency at performing most tasks and with colony productivity. Altogether, these results suggest that genetic diversity does not strongly improve short-term task efficiency in L. humile, but that worker number is a key factor determining the success of this invasive species.

Energy investment in gynes of the Argentine ant Iridomyrmex humilis (Mayr) in relation to the mode of colony founding in ants (Hymenoptera, Formicidae)

The increase in weight, fat and energy content of queens was studied in Iridomyrmex humilis (Mayr) in relation to the mode of colony founding in ants. The increase in energy content of gynes during the time between emergence and mating reaches only 80% in this species in which queens found colonies with the help of workers (dependent mode), whereas it can reach 470% in species in which queens found colonies without the help of workers (independent mode). These results are discussed with regard to the investment in energy required by each mode of colony founding.

Variation in the level of aggression, chemical and genetic distance among three supercolonies of the Argentine ant in Europe.

In their invasive ranges, Argentine ant populations often form one geographically vast supercolony, genetically and chemically uniform within which there is no intraspecific aggression. Here we present regional patterns of intraspecific aggression, cuticular hydrocarbons (CHCs) and population genetics of 18 nesting sites across Corsica and the French mainland. Aggression tests confirm the presence of a third European supercolony, the Corsican supercolony, which exhibits moderate to high levels of aggression, depending on nesting sites, with the Main supercolony, and invariably high levels of aggression with the Catalonian supercolony. The chemical analyses corroborated the behavioural data, with workers of the Corsican supercolony showing moderate differences in CHCs compared to workers of the European Main supercolony and strong differences compared to workers of the Catalonian supercolony. Interestingly, there were also clear genetic differences between workers of the Catalonian supercolony and the two other supercolonies at both nuclear and mitochondrial markers, but only very weak genetic differentiation between nesting sites of the Corsican and Main supercolonies (F(ST) = 0.06). A detailed comparison of the genetic composition of supercolonies also revealed that, if one of the last two supercolonies derived from the other, it is the Main supercolony that derived from the Corsican supercolony rather than the reverse. Overall, these findings highlight the importance of conducting more qualitative and quantitative analyses of the level of aggression between supercolonies, which has to be correlated with genetic and chemical data.

Evolutionary implications of polygyny in the Argentine ant, Iridomyrmex humilis (Mayr) (Hymenoptera: Formicidae): an experimental study

A comparison of several physiological parameters of queens of Iridomyrmex humilis in experimental monogynous and polygynous colonies showed that queens in monogynous colonies became heavier, had more developed ovaries and laid about twice as many eggs. Workers in monogynous colonies were more attracted to queens, which therefore probably received more food. This may partially explain the higher weight and fecundity of queens in monogynous colonies of Iridomyrmex humilis and possibly other ant species. In polygynous colonies, queens differed greatly in their fecundity. These differences did not appear to be the result of a dominance hierarchy. These results are discussed from an evolutionary point of view. Two hypotheses of mutualism and colony level selection are proposed as an alternative to kin selection which is unlikely to be the exclusive selective influence in the evolution of polygyny either in I. humilis or in most other ant species.

Population genetic structure in the native and introduced range of the Argentine ant/

SUMMARY : The evolution of animal societies, where some individuals forego their own reproductive opportunities to help others to reproduce, poses an evolutionary paradox that can be traced back to Darwin. Altruism may evolve through kin selection when the donor and recipient of altruistic acts are related to each other. In social insects, workers are generally highly related to the brood they rear when colonies are headed by a single queen. Yet some ants have an extraordinary social organization, called unicoloniality, whereby individuals from separate nests mix freely to form large supercolonies, which in some cases extend over hundreds of km. These supercolonies are characterised by a high number of queens (polygyny) and an absence of clear colony boundaries. This type of social organization represents an evolutionary paradox because relatedness between nestmates is effectively zero. In such conditions, kin selection cannot account for the evolution of reproductive altruism. Moreover, unicoloniality is thought to be unstable over time, because workers that can no longer aid close relatives may evolve more selfish strategies. The Argentine ant (Linepithema humile) is a highly invasive species listed among the hundred world's worst invaders by the UICN. Native from South America, L. humile has been accidentally introduced throughout the world. Native populations have been described as noninvasive with a family-based organization. In contrast, within its introduction range, they form unicolonial supercolonies that contain numerous nests without intraspecific aggression. The development of such unicolonial populations has been explained as a direct consequence of the ant's introduction into a new habitat, favouring a transition from family-based to open colonies. To determine if the social structure of the Argentine ant is fundamentally different between the native and the introduced range, we studied genetically and behaviourally native and introduced populations of L. humile over different geographic scales. Our results clearly indicated that there are no fundamental differences in the social organisation of the Argentine ant between the two ranges. Our investigations revealed that, contrary to previous claims, native populations have a unicolonial social organisation very similar to that observed in the introduced range. Consequently, the unicolonial social structure of the Argentine ant does not stem from a shift in social organization associated with introduction into new habitats but evolved in the native range and is likely a stable, evolutionarily ancient adaptation to the local environment. Our study on native populations of L. humile also gave important insight in the comprehension of the evolution of unicoloniality in the Argentine ant. Native supercolonies are relatively small compared to introduced ones and may co-habit in a same population. These supercolonies are genetically highly differentiated leading to a significant relatedness among nestmate workers when the different supercolonies of a population are taken as a reference population. This provides the necessary conditions for loin selection to operate. Furthermore, we examined a native population over time, which revealed a high supercolony extinction rate. If more competitive supercolonies are more likely to survive or replace other supercolonies, a subtle dynamical process between the spread of selfish traits within supercolony and the selective elimination of supercolonies with such traits may allow a stable equilibrium and the persistence of unicoloniality over time. Finally, a worldwide study of the Argentine ant showed that the introduced supercolonies originate from numerous independent introduction events. In conclusion, the success of the Argentine ant does not stem from a shift in social organization associated with its introduction into new habitats, but is most probably explained by the intrinsic characteristics developed in its native range. RESUME : L'altruisme de reproduction où certains individus renoncent à leur propre reproduction pour aider d'autres individus à se reproduire constitue l'un des plus grand paradoxe de l'évolution. En effet, comment expliquer l'évolution de comportements qui tendent à augmenter les chances de survie et le succès reproductif d'autres individus, alors que ces actes diminuent l'aptitude de leurs auteurs ? La théorie de la sélection de parentèle permet de résoudre ce problème. Cette théorie stipule qu'en aidant de proches parents à se reproduire, les individus peuvent promouvoir indirectement la transmission de copies de leurs propres gènes à la génération suivante. Chez les insectes sociaux, l'altruisme des ouvrières s'explique par la théorie de sélection de parentèle lorsque les colonies sont monogynes (constituées d'une seule reine) puisque les ouvrières sont fortement apparentées aux couvains dont elles s'occupent. Par contre, les espèces dites unicoloniales, dont les colonies forment des réseaux de nids appelés supercolonies, représentent toujours un paradoxe pour les théories de l'évolution puisque l'apparentement entre les différents individus d'un nid est nulle. De plus, l'unicolonialité ne devrait pas être stable sur le long terme parce que les ouvrières qui ne s'occupent plus de leur apparentés devraient développer des stratégies plus égoïstes au cours du temps. La fourmi d'Argentine (Linepithema humile) est une espèce invasive ayant un impact considérable sur son environnement. Originaire d'Amérique du Sud, elle a été introduite dans pratiquement toutes les régions du monde dont le climat est de type méditerranéen. Son incroyable succès invasif s'explique par sa structure sociale unicoloniale observée dans chacun des pays où elle a été introduite. Par contre, les rares études effectuées en Argentine ont suggéré que la fourmi d'Argentine n'était pas unicoloniale dans son aire native. L'unicolonialité chez la fourmi d'Argentine était donc considéré comme une conséquence de son introduction dans de nouveaux environnements. Durant cette thèse, nous avons vérifié si la structure sociale de cette espèce différait fondamentalement entre l'aire native et introduite. Pour cela, nous avons étudié, à différentes échelles géographiques, des populations introduites et argentines avec une approche génétique et comportementale. L'ensemble de nos résultats montrent que les différences entre les deux structure sociales ne sont pas aussi importantes que ce que l'on imaginait. Les populations natives sont aussi constituées de réseaux de nids coopérants. La taille de ses supercolonies est toutefois bien moins importante en Argentine et il n'est pas rare de trouver plusieurs supercolonies cohabitantes dans une même population. Nous avons démontré que ces réseaux de nids étaient constitués d'individus qui sont plus apparentés entre eux qu'ils ne le sont avec les individus d'autres supercolonies, ainsi l'unicolonialité dans son aire d'origine ne représente pas un réel paradoxe pour les théories de l'évolution. Finalement nous avons étudié la même population en Argentine à six ans d'intervalle et avons constaté que les supercolonies avaient un taux de survie très faible ce qui pourrait expliquer la stabilité de l'unicolonialité au cours du temps. Si les supercolonies les plus compétitives survivent mieux que les supercolonies dans lesquelles apparaissent des traits égoïstes, on devrait alors observer une dynamique entre l'apparition de traits égoïstes et l'élimination des supercolonies dans lesquelles ces traits égoïstes évolueraient. Finalement, une étude mondiale nous a montré que les supercolonies étaient originaires de nombreux événements d'introductions indépendants. En conclusion, le succès invasif de la fourmi d'Argentine n'est donc pas dû à un changement de comportement associé à son introduction mais est lié aux caractéristiques qu'elle a développées en Argentine.

Mate availability and male dispersal in the Argentine ant Linepithema humile (Mayr) (=Iridomyrmex humilis)

In the Argentine ant Linepithema humile (=Iridomyrmex humilis) only males disperse whereas female sexuals (unmated winged queens) stay in their mother nest where they mate. This study investigated (1) whether dispersing males are accepted into foreign colonies, (2) whether they can mate with resident female sexuals, and (3) whether the propensity of males to disperse is affected by the expectation of mating in their mother nest. Field experiments demonstrated that males were accepted into foreign colonies only when these colonies contained female sexuals or queen pupae. Before and after the time of (sic) female sexuals, workers attacked and killed most of the foreign males. Laboratory experiments snowed that males that successfully enter foreign colonies can mate with resident female sexuals. The propensity of males to disperse was significantly influenced by the presence of female sexuals in their nest. Males were more likely to fly out from colonies containing no female sexuals than from those with them. These results are consistent with males preferentially dispersing when there is little or no opportunity to mate in their mother nest. Thus there are two mating strategies available for males: staying in their mother nest when an opportunity to mate arises or dispersing and attempting to mate in a foreign nest when there are no female sexuals in their mother nest. This latter behaviour could mediate gene flow between colonies and account for the lack of significant inbreeding previously documented in this species.

Efecto de la temperatura en la tasa de puesta de reinas de la hormiga argentina, Linepithema humile (Mayr, 1868), (Hymenoptera, Dolichoderine) bajo condiciones experimentales de monoginia y poliginia = Effect of temperature on the oviposition rate of the argentine ant queens, Linepithema humile (Mayr 1868), (Hymenoptera, Dolichoderine) under experimental conditions

Data concerning the effect of temperature on different physiological parameters of an invasive species can be a useful tool to predict its potential distribution range through the use of modelling approaches. In the case of the Argentine ant these data are too scarce and incomplete. The aim of the present study is to compile new data regarding the effect of temperature on the oviposition rate of the Argentine ant queens. We analysed the oviposition rate of queens at twelve controlled temperatures, ranging from 10ºC to 34ºC under different monogynous and polygynous conditions. The oviposition rate of the Argentine ant queens is affected by temperature in the same manner, independently of the number of queens in the nest. The optimal temperature for egg laying was 28ºC, and its upper and lower limits depended on the degree of polygyny

Niche Differentiation and Fine-Scale Projections for Argentine Ants Based on Remotely Sensed Data

Modeling ecological niches of species is a promising approach for predicting the geographic potential of invasive species in new environments. Argentine ants (Linepithema humile) rank among the most successful invasive species: native to South America, they have invaded broad areas worldwide. Despite their widespread success, little is known about what makes an area susceptible - or not - to invasion. Here, we use a genetic algorithm approach to ecological niche modeling based on high-resolution remote-sensing data to examine the roles of niche similarity and difference in predicting invasions by this species. Our comparisons support a picture of general conservatism of the species' ecological characteristics, in spite of distinct geographic and community contexts

Effect of Seasonal Dynamics on Queen Densities of the Argentine Ant (Linepithema Humile) in an Invaded Natural Area of the NE Iberian Peninsula

The annual elimination of large numbers of Argentine ant queens near the advance front of an invasion could be a useful tool for weakening the species’ dispersion and, therefore, limiting its establishment in non-invaded areas. However, before carrying out trials to test the effectiveness of this method it would be essential to have sufficient knowledge of the effect of seasonal dynamics acting on the queens’ densities of the species in order to determine the most favourable period of the year to act. We analyzed the seasonal densities and nest dynamics of Argentine ant queens in an invaded Mediterranean natural ecosystem. We observed that the queens’ density varied depending on the season of the year and that this variation was mainly due to the seasonal dynamics of nest aggregations in winter and ant dispersions in summer. The greatest densities per litre of nest soil were observed in winter (December to March, approximately) and the lowest densities were observed in summer ( June to July). This information is essential for improving current knowledge of the Argentine ant’s biology and developing control methods based on the elimination of queens in invaded natural areas

Numerical Dominance of the Argentine Ant vs Native Ants and Consequences on Soil Resource Searching in Mediterranean Cork-Oak Forests (Hymenoptera: Formicidae)

This study is focused on the dominance exerted by the invasive Argentine ant over native ants in a coastal Mediterranean area. Theimpact of this invasive ant on native ant assemblages and its consequences on total ant biomass and on the intensity of habitat explorationwere evaluated. Foraging ants were observed and their trajectories recorded during 5-minute periods in two study zones, one invaded andthe other non-invaded. Ant species detected, ant worker abundance, ant biomass and the intensity of soil surface searching done by antswere compared between the two zones. The Argentine ant invasion provoked a drastic reduction of the ant species richness. Apparentlyonly one native ant species is able to coexist with the Argentine ant, the cryptic Plagiolepis pygmaea. Ant worker abundance was also modified after the invasion: the number of Argentine ant workers detected, which represented 92% of the invaded zone, was two times higher than the number of native ant workers detected in the non-invaded zone. The total ant biomass was inversely affected, becoming four times lower in the invaded zone highly dominated by Linepithema humile. The higher number of Argentine ant workers and their fast tempo of activity implied an alteration of the intensity of soil surface searching: scanning by the Argentine ants in the invaded zone was higher than that done by the native ants in the non-invaded zone, and the estimated time for a complete soil surface scan was 64 minutes in the invaded zone and 108 minutes in the non-invaded zone. Consequently, resources will be discovered faster by ants in the invaded zone than in the non-invaded zone. The increase of the mean temperature and the decrease of the relative humidity from May to August reduced the ant activity in the two study zones but this reduction was greater in the invaded zone

Effect of Temperature on the Development and Survival of the Argentine Ant, Linepithema humile

The influence of temperature on the developmental times and survival of insects can largely determine their distribution. For invasive species, like the Argentine ant, Linepithema humile Mayr (Hymenoptera: Formicidae), these data are essential for predicting their potential range based on mechanistic models. In the case of this species, such data are too scarce and incomplete to make accurate predictions based on its physiological needs. This research provides comprehensive new data about brood survival and developmental times at a wide range of temperatures under laboratory conditions. Temperature affected both the complete brood development from egg to adult worker and each of the immature stages separately. The higher the temperature, the shorter the development times. Brood survival from egg to adult was low, with the maximum survival rate being only 16% at 26º C. Temperature also affected survival of each of the immature stages differently: eggs were negatively affected by high temperatures, while larvae were negatively affected by low temperatures, and the survival of pupae was apparently independent of environmental temperature. At 32º C no eggs survived, while at 18º C less than 2% of the eggs hatched into larva. The data from the present study are essential for developing prediction models about the distribution range of this tramp species based on its physiological needs in relation to temperature

Predicting Argentine ant invasion across spatial scales via ecological niche models

La formiga argentina (Linepithema humile) es troba entre les espècies més invasores: originària d'Amèrica del Sud, actualment ha envaït nombroses àrees arreu del món. Aquesta tesi doctoral intenta fer una primera anàlisi integrada i multiescalar de la distribució de la formiga argentina mitjançant l'ús de models de nínxol ecològic. D'acord amb els resultats obtinguts, es preveu que la formiga argentina assoleixi una distribució més àmplia que l'actual. Les prediccions obtingudes a partir dels models concorden amb la distribució actualment coneguda i, a més, indiquen àrees a prop de la costa i dels rius principals com a altament favorables per a l'espècie. Aquests resultats corroboren la idea que la formiga argentina no es troba actualment en equilibri amb el medi. D'altra banda, amb el canvi climàtic, s'espera que la distribució de la formiga argentina s'estengui cap a latituds més elevades en ambdós hemisferis, i sofreixi una retracció en els tròpics a escales globals.