The effect of ant attendance on the success of rosy apple aphid populations, natural enemy abundance and apple damage in orchards

1 The rosy apple aphid, Dysaphis plantaginea, is the most serious pest of apple in Europe and, although conventionally controlled by insecticides, alternative management measures are being sought. Colonies of D. plantaginea are commonly attended by ants, yet the effects of this relationship have received little attention. 2 An ant exclusion study was conducted in two distant orchards within the U.K. At both sites, ants were excluded from a subset of D. plantaginea infested trees at the beginning of the season and populations were monitored. The number of natural enemies observed on trees was also recorded and, before harvest, the percentage of apples damaged by D. plantaginea calculated. 3 Overall, the exclusion of ants reduced the growth and eventual size of D. plantaginea populations. On trees accessed by ants, greater numbers of natural enemies were recorded, presumably because aphid populations were often greater on such trees. However, this increased natural enemy presence was diluted by the larger aphid populations such that individual aphids on ant‐attended trees were subjected to a lower natural enemy pressure compared with those on ant‐excluded trees. 4 At harvest, apple trees that had been accessed by ants bore a greater proportion of apples damaged by D. plantaginea. There were also differences in cultivar susceptibility to D. plantaginea damage. 5 The present study highlights the importance of the ant–D. plantaginea relationship and it ia suggested that ant manipulation, whether physically or by semiochemicals that disrupt the relationship, should be considered as a more prominent component in the development of future integrated pest management strategies.     

Asexual reproduction in introduced and native populations of the ant Cerapachys biroi

Asexual reproduction is particularly common among introduced species, probably because it helps to overcome the negative effects associated with low population densities during colonization. The ant Cerapachys biroi has been introduced to tropical and subtropical islands around the world since the beginning of the last century. In this species, workers can reproduce via thelytokous parthenogenesis. Here, we use genetic markers to reconstruct the history of anthropogenic introductions of C. biroi, and to address the prevalence of female parthenogenesis in introduced and native populations. We show that at least four genetically distinct lineages have been introduced from continental Asia and have led to the species' circumtropical establishment. Our analyses demonstrate that asexual reproduction dominates in the introduced range and is also common in the native range. Given that C. biroi is the only dorylomorph ant that has successfully become established outside of its native range, this unusual mode of reproduction probably facilitated the species' worldwide spread. On the other hand, the rare occurrence of haploid males and at least one clear case of sexual recombination in the introduced range show that C. biroi has not lost the potential for sex. Finally, we show that thelytoky in C. biroi probably has a genetic rather than an infectious origin, and that automixis with central fusion is the most likely underlying cytological mechanism. This is in accordance with what is known for other thelytokous eusocial Hymenoptera.     

Exotic species richness and native species endemism increase the impact of exotic species on islands

Aim Exotic species pose one of the most significant threats to biodiversity, especially on islands. The impacts of exotic species vary in severity among islands, yet little is known about what makes some islands more susceptible than others. Here we determine which characteristics of an island influence how severely exotic species affect its native biota. Location We studied 65 islands and archipelagos from around the world, ranging from latitude 65° N to 54° S. Methods We compiled a global database of 10 island characteristics for 65 islands and determined the relative importance of each characteristic in predicting the impact of exotic species using multivariate modelling and hierarchical partitioning. We defined the impact of exotic species as the number of bird, amphibian and mammal (BAM) species listed by the International Union for Conservation of Nature (IUCN) as threatened by exotics, relative to the total number of BAM species on that island. Results We found that the impact of exotic species is more severe on islands with more exotic species and a greater proportion of native species that are endemic. Unexpectedly, the level of anthropogenic disturbance did not influence an island's susceptibility to the impacts of exotic species. Main conclusions By coupling our results with studies on the introduction and establishment of exotic species, we conclude that colonization pressure, or invasion opportunities, influences all stages of the invasion process. However, species endemism, the other important factor determining the impact of exotic species, is not known to contribute to introduction and establishment success on islands. This demonstrates that different factors correlate with the initial stages of the invasion process and the subsequent impacts of those invaders, highlighting the importance of studying the impacts of exotic species directly. Our study helps identify islands that are at risk of impact by exotics and where investment should focus on preventing further invasions.     

Genetic relationships between native and introduced populations of the little fire ant Wasmannia auropunctata

Native to much of Central and South America, the little fire ant Wasmannia auropunctata has been rapidly spreading throughout the world. In its introduced range, W. auropunctata is frequently linked with drastic reductions of ant diversity; anecdotal reports of damaging attacks on vertebrates are also common. As it poses an ever-increasing threat to biodiversity, W. auropunctata has emerged as a model system for the study of ecological differences between native and invasive ant populations. These studies have been hampered by a lack of information on the genetic relatedness between native and introduced populations. By investigating the genetic structure of W. auropunctata populations, we provide a framework for conducting phylogenetically independent tests of differences between these ants in their native and invasive ranges. Phylogenetic analyses, based on the cox1 / cox2 region of mtDNA, revealed at least three separate source populations of W. auropunctata distributed across two large clades. Much of the Caribbean region, presumably part of the native range, is inhabited by a clade of ants sharing very similar or identical mtDNA haplotypes, suggesting the possibility of multiple introductions or high levels of gene flow across that area. Most invasive populations in the Pacific were closely related to these ants. The invasive populations in Gabon and New Caledonia arise from another, relatively distantly related clade. Phylogenetically independent contrasts confirm McGlynn's (1999) observation that invasive W. auropunctata populations are smaller than native populations. Given the complex phylogeographical structure of W. auropunctata populations, future comparative work should correct for phylogenetic effects.     

Temporal habitat variability and the maintenance of sex in host populations of the pea aphid

The evolutionary maintenance of sex, despite competition from asexual reproduction, has long intrigued the evolutionary biologists owing to its numerous apparent short-term costs. In aphids, winter climate is expected to determine the maintenance of sexual lineages in the high latitude zones owing to their exclusive ability to produce frost-resistant eggs. However, diverse reproductive modes may coexist at a local scale where climatic influence is counteracted by microgeographical factors. In this study, we tested the influence of local habitat characteristics on regional coexistence of reproductive modes in the pea aphid, Acyrthosiphon pisum. In the laboratory, the induction of sexual morph production of many pea aphid genotypes from the local fields of annual (pea and faba bean) and perennial (alfalfa and red clover) crops in Western France indicated that A. pisum lineages from annual crops had a significantly higher investment in sexual reproduction than A. pisum lineages from the perennial hosts. We propose that temporal habitat variability exerts a selective pressure to maintain the sexual reproduction in A. pisum. The ecological and evolutionary consequences of the association between the mode of reproduction and the host population on gene flow restriction and on ecological specialization are discussed.     

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.     

Exotic plants increase and native plants decrease with loss of foundation species in sagebrush steppe

Dominant plant species, or foundation species, are recognized to have a disproportionate control over resources in ecosystems, but few studies have evaluated their relationship to exotic invasions. Loss of foundation species could increase resource availability to the benefit of exotic plants, and could thereby facilitate invasion. The success of exotic plant invasions in sagebrush steppe was hypothesized to benefit from increased available soil water following removal of sagebrush (Artemisia tridentata), a foundation species. We examined the effects of sagebrush removal, with and without the extra soil water made available by exclusion of sagebrush, on abundance of exotic and native plants in the shrub steppe of southern Idaho, USA. We compared plant responses in three treatments: undisturbed sagebrush steppe; sagebrush removed; and sagebrush removed plus plots covered with “rainout” shelters that blocked winter-spring recharge of soil water. The third treatment allowed us to examine effects of sagebrush removal alone, without the associated increase in deep-soil water that is expected to accompany removal of sagebrush. Overall, exotic herbs (the grass Bromus tectorum and four forbs) were 3–4 times more abundant in shrub-removal and 2 times more abundant in shrub-removal + rainout-shelter treatments than in the control treatment, where sagebrush was undisturbed. Conversely, native forbs were only about half as abundant in shrub removal compared to control plots. These results indicate that removal of sagebrush facilitates invasion of exotic plants, and that increased soil water is one of the causes. Our findings suggest that sagebrush plays an important role in reducing invasions by exotic plants and maintaining native plant communities, in the cold desert we evaluated.     

Relationships among native and introduced populations of the Argentine ant (Linepithema humile) and the source of introduced populations

The Argentine ant (Linepithema humile) is a damaging invasive species that has become established in many Mediterranean-type ecosystems worldwide. To identify likely sources of introduced populations we examined the relationships among native Linepithema populations from Argentina and Brazil and introduced populations of L. humile using mitochondrial cytochrome b sequence data and nuclear microsatellite allele frequencies. The mitochondrial phylogeny revealed that the populations in Brazil were only distantly related to both the introduced populations and the native populations in Argentina, and confirmed that populations in Brazil, previously identified as L. humile, are likely a different species. The microsatellite-based analysis provided resolution among native and introduced populations of L. humile that could not be resolved using the mitochondrial sequences. In the native range, colonies that were geographically close to one another tended to be genetically similar, whereas more distant colonies were genetically different. Most samples from the introduced range were genetically similar, although some exceptions were noted. Most introduced populations were similar to native populations from the southern Rio Parana and were particularly similar to a population from Rosario, Argentina. These findings implicate populations from the southern Rio Parana as the most likely source of introduced populations. Moreover, these data suggest that current efforts to identify natural enemies of the Argentine ant for biological control should focus on native populations in the southern Rio Parana watershed.     

Ecologically heterogeneous populations of the invasive ant Wasmannia auropunctata within its native and introduced ranges

Abstract 1. The biology of most invasive species in their native geographical areas remains largely unknown. Such studies are, however, crucial in shedding light on the ecological and evolutionary processes underlying biological invasions. 2. The present study focuses on the little fire ant Wasmannia auropunctata, a species native to Central and South America that has been widely introduced and which has become invasive throughout the tropics. We characterise and compare several ecological traits of native populations in French Guiana with those in one of its introduced ranges, New Caledonia. 3. We found ecologically heterogeneous populations of W. auropunctata coexisting in the species’ native geographical area. First, we found populations restricted to naturally perturbed areas (particularly floodplains) within the primary forest, and absent from the surrounding forest areas. These populations were characterised by low nest and worker densities. Second, we found dominant populations in recent anthropogenic areas (e.g. secondary forest or forest edge along road) characterised by high nest and worker densities, and associated with low ant species richness. The local dominance of W. auropunctata in such areas can be due to the displacement of other species (cause) or the filling‐up of empty habitats unsuitable to other ants (effect). With respect to their demographic features and ant species richness, the populations of native anthropogenic habitats were to a large extent similar to the invasive populations introduced into remote areas. 4. The results point to the need for greater research efforts to better understand the ecological and demographic features of invasive species within their native ranges.     

Biotic resistance on the increase: native predators structure invasive zebra mussel populations

1. Abundant native predators, parasites and pathogens that switch to consuming a hyper‐successful exotic species may be able to control the invasive population. Native predators may, however, need time to adapt to feed effectively on an exotic resource. In this case, mortality on an exotic population from native predators could increase over time even without a numerical increase in the predator population. 2. We measured mortality of zebra mussels (Dreissena polymorpha) in the Hudson River both in controls open to predation and in exclosures that excluded large predators to estimate mortality of zebra mussels from large predators and other causes. 3. We found that predation by the blue crab (Callinectes sapidus), and perhaps other predators, causes high mortality on zebra mussels in the Hudson River estuary. This predation apparently led to increased mortality and altered population structure in the invader over time. 4. Long‐term data from the Hudson River suggest that components of the invaded ecosystem, like rotifers, are recovering through predator‐caused release from zebra mussel grazing. Increased mortality on hyper‐successful exotic populations over time may be a common phenomenon with both ecological and management implications.     

Admixture between native and invasive populations may increase invasiveness of Mimulus guttatus

Self-fertilization and admixture of genotypes from different populations can have major fitness consequences in native species. However, few studies have addressed their potential roles in invasive species. Here, we used plants of Mimulus guttatus from seven native North American, three invasive Scottish and four invasive New Zealand populations to address this. We created seeds from self-fertilization, within-population outcrossing, between-population outcrossing within the same range, and outcrossing between the native and invasive ranges. A greenhouse experiment showed that native and invasive plants of M. guttatus suffered to similar degrees from inbreeding depression, in terms of asexual reproduction and biomass production. After outcrossing with plants from other populations, M. guttatus benefited from heterosis, in terms of asexual and sexual reproduction, and biomass production, particularly when plants from native and invasive populations were crossed. This suggests that, when novel genotypes of M. guttatus from the native North American range will be introduced to the invasive ranges, subsequent outcrossing with M. guttatus plants that are already there might further boost invasiveness of this species.     

Behavioral interactions of the invasive Argentine ant with native ant species

Summary: The Argentine ant, Linepithema humile, has invaded many areas of the world, displacing native ants. Its behavior may contribute to its competitive success. Staged and natural encounters were observed at food resources in the field, between Argentine ants and eight ant species native to northern California. There was no relation between the frequency of aggression by any ant species and the outcome of encounters, though Argentine ants were more likely than ants of native species to behave aggressively. When an ant of one species initiated an encounter of any kind with an ant of another species, the ant that did not initiate was likely to retreat. This was true of all species studied. Most encounters between ants were initiated by Argentine ants. Thus the native species tended to retreat more frequently than Argentine ants. Interactions between Argentine ants and native species at food resources, causing ants of native species to retreat, may help Argentine ants to displace native species from invaded areas.     

A native ant armed to limit the spread of the Argentine ant

Argentine ants (Linepithema humile) usually actively displace native ants through a combination of rapid recruitment, high numerical dominance and intense aggressive fights. However, in some cases, native ants can offer a strong resistance. In Corsica, a French Mediterranean island, local resistance by the dominant Tapinoma nigerrimum has been proposed as a factor limiting Argentine ant invasion. With the aim of evaluating the abilities of T. nigerrimum in interference and exploitative competition, this study tested in the laboratory the aggressive interactions between this native dominant ant and the invasive Argentine ant. We used four different assays between L. humile and T. nigerrimum: (1) worker dyadic interactions, (2) symmetrical group interactions, (3) intruder introductions into an established resident colony, and (4) a competition for space and food. This study confirms the ability of Argentine ants to compete with native species, by initiating more fights, using cooperation and simultaneously deploying physical and chemical defenses. However, despite Argentine ant fighting capabilities, T. nigerrimum was more efficient in both interference and exploitative competition. Its superiority was obvious in the space and food competition assays, where T. nigerrimum dominated food in 100% of the replicates after 1 h and invaded Argentine ant nests while the reverse was never observed. The death feigning behavior exhibited by Argentine ant workers also suggests the native ant’s superiority. Our study thus demonstrates that T. nigerrimum can offer strong competition and so may be able to limit the spread of Argentine ants in Corsica. This confirms that interspecific competition from ecologically dominant native species can affect the invasion success of invaders, notably by decreasing the likelihood of incipient colony establishment and survival.     

Historical land use and present-day canopy thinning differentially affect the distribution and abundance of invasive and native ant species

Disturbance may play a key role in affecting animal invasions; less appreciated is that past and present disturbances might interact to affect invasion. For example, although large portions of forest ecosystems worldwide are jointly affected by a history of past agricultural land use as well as contemporary timber harvest, it is unclear whether these disturbances facilitate biological invasions. We conducted a large-scale experiment that coupled adjacent non-agricultural and post-agricultural sites and a factorial canopy-thinning manipulation to understand how past agricultural land use might interact with contemporary canopy thinning to affect the distribution (i.e., presence/absence of mounds) and abundance of a highly invasive ant species, Solenopsis invicta, and common native ant species, Dorymyrmex bureni. Mounds of S. invicta were more likely to be present in unthinned post-agricultural habitats than unthinned non-agricultural habitats, but this legacy effect disappeared with canopy thinning. In unthinned habitats, the presence of D. bureni mounds did not differ based on past land use. However, presence of D. bureni mounds was greater in thinned non-agricultural habitats than thinned post-agricultural habitats. Once present in an area, mound abundance was largely related to temperature near the ground for both species, and negatively related to soil compaction for D. bureni. Our results provide large-scale evidence that an understanding of anthropogenic events that occur decades before present may be essential for interpreting contemporary invasion dynamics and the distribution of some species, and that local contemporary habitat characteristics play a key role in determining ant abundance once an area is colonized.     

Mosaic structure of native ant supercolonies

According to the inclusive fitness theory, some degree of positive relatedness is required for the evolution and maintenance of altruism. However, ant colonies are sometimes large interconnected networks of nests, which are genetically homogenous entities, causing a putative problem for the theory. We studied spatial structure and genetic relatedness in two supercolonies of the ant Formica exsecta, using nuclear and mitochondrial markers. We show that there may be multiple pathways to supercolonial social organization leading to different spatial genetic structures. One supercolony formed a genetically homogenous population dominated by a single mtDNA haplotype, as expected if founded by a small number of colonizers, followed by nest propagation by budding and domination of the habitat patch. The other supercolony had several haplotypes, and the spatial genetic structure was a mosaic of nuclear and mitochondrial clusters. Genetic diversity probably originated from long‐range dispersal, and the mosaic population structure is likely a result of stochastic short‐range dispersal of individuals. Such a mosaic spatial structure is apparently discordant with the current knowledge about the integrity of ant colonies. Relatedness was low in both populations when estimated among nestmates, but increased significantly when estimated among individuals sharing the same genetic cluster or haplogroup. The latter association indicates the important historical role of queen dispersal in the determination of the spatial genetic structure.     

Genetic variation and structure in native populations of the fire ant Solenopsis invicta: evolutionary and demographic implications

We studied population genetic variation and structure in the fire ant Solenopsis invicta using nuclear genotypic and mitochondrial DNA (mtDNA) sequence data obtained from samples collected throughout its native range. Geographic populations are strongly differentiated at both genomes, with such structure more pronounced in Brazil than in Argentina. Higher-level regional structure is evident from the occurrence of isolation-by-distance patterns among populations, the recognition of clusters of genetically similar, geographically adjacent populations by ordination analysis, and the detection of an mtDNA discontinuity between Argentina and Brazil coinciding with a previously identified landform of biogeographical relevance. Multiple lines of evidence from both genomes suggest that the ancestors of the ants we studied resembled extant northern Argentine S. invicta , and that existing Brazilian populations were established more recently by serial long-distance colonizations and/or range expansions. The most compelling evidence for this is the corresponding increase in F _K (a measure of divergence from a hypothetical ancestor) and decrease in genetic diversity with distance from the Corrientes population in northern Argentina. Relatively deep sequence divergence among several mtDNA clades, coupled with geographical partitioning of many of them, suggests prolonged occupation of South America by S. invicta in more-or-less isolated regional populations. Such populations appear, in some cases, to have come into secondary contact without regaining the capacity to freely interbreed. We conclude that nominal S. invicta in its native range comprises multiple entities that are sufficiently genetically isolated and diverged to have embarked on independent evolutionary paths.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 541–560.     

Quantifying the effects of predators on aphid populations

Four methods of analysis based on a simple population model and applicable to aphid and predator field density data are described. Two of the analyses determine the extent to which predators accounted for an observed change in aphid densities, and the other two estimate by how much the aphid population would have increased if the predators had not been present. The methods are illustrated with examples taken from cereal aphid research, and their advantages and limitations are discussed.

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Résumé Quatre méthodes d'analyse, basées sur un modèle de population simple, applicables aux données concernant les densités de pucerons et de prédateurs dans la nature, sont décrites. Deux des analyses déterminent l'importance de l'intervention des prédateurs lorsque l'on observe une modification de la densité de pucerons; les deux autres estiment de combien la population de pucerons aurait dû s'accroître en l'absence de prédateurs. Ces méthodes sont illustrées d'exemples tirés des recherches sur les pucerons des céréales. Possibilités et limites de ces méthodes sont discutées.

     

黑蚁对雪松长足大蚜集群时空变化的取食反应

我们研究了黑蚁 (Lasiusniger)如何开发距离其巢不同距离的多种食物资源 ,以及黑蚁的照看活动是否与雪松长足大蚜 (Cinaracedri)种群的增长有关 ,以及 /或者受空间分布的影响。笔者定期调查了Cinaracedri集群的空间分布 ,以及在雪松树枝上黑蚁取食的时间变化 ,发现只有 2 5 %的蚜虫集群在研究后数周内保持在同样的位置 ,这表明蚜虫集群的高度运动性迫使黑蚁改变活动路线以获取事物资源。实际上 ,黑蚁集群能选择最近的食物资源以及蚜虫数量最多的地点     

Experimental increase in fecundity causes upregulation of fecundity and body maintenance genes in the fat body of ant queens

In most organisms, fecundity and longevity are negatively associated and the molecular regulation of these two life-history traits is highly interconnected. In addition, nutrient intake often has opposing effects on lifespan and reproduction. In contrast to solitary insects, the main reproductive individual of social hymenopterans, the queen, is also the most long-lived. During development, queen larvae are well-nourished, but we are only beginning to understand the impact of nutrition on the queens'' adult life and the molecular regulation and connectivity of fecundity and longevity. Here, we used two experimental manipulations to alter queen fecundity in the ant Temnothorax rugatulus and investigated associated changes in fat body gene expression. Egg removal triggered a fecundity increase, leading to expression changes in genes with functions in fecundity such as oogenesis and body maintenance. Dietary restriction lowered the egg production of queens and altered the expression of genes linked to autophagy, Toll signalling, cellular homeostasis and immunity. Our study reveals that an experimental increase in fecundity causes the co-activation of reproduction and body maintenance mechanisms, shedding light on the molecular regulation of the link between longevity and fecundity in social insects.