Dispersal strategies of phytophagous insects at a local scale: adaptive potential of aphids in an agricultural environment

BACKGROUND: The spread of agriculture greatly modified the selective pressures exerted by plants on phytophagous insects, by providing these insects with a high-level resource, structured in time and space. The life history, behavioural and physiological traits of some insect species may have evolved in response to these changes, allowing them to crowd on crops and to become agricultural pests. Dispersal, which is one of these traits, is a key concept in evolutionary biology but has been over-simplified in most theoretical studies. We evaluated the impact of the local-scale dispersal strategy of phytophagous insects on their fitness, using an individual-based model to simulate population dynamics and dispersal between leaves and plants, by walking and flying, of the aphid Aphis gossypii, a major agricultural pest, in a melon field. We compared the optimal values for dispersal parameters in the model with the corresponding observed values in experimental trials. RESULTS: We show that the rates of walking and flying disperser production on leaves were the most important traits determining the fitness criteria, whereas dispersal distance and the clustering of flying dispersers on the target plant had no effect. We further show that the effect of dispersal parameters on aphid fitness depended strongly on plant characteristics. CONCLUSION: Parameters defining the dispersal strategies of aphids at a local scale are key components of the fitness of these insects and may thus be essential in the adaptation to agricultural environments that are structured in space and time. Moreover, the fact that the effect of dispersal parameters on aphid fitness depends strongly on plant characteristics suggests that traits defining aphid dispersal strategies may be a cornerstone of host-plant specialization

Bridgehead Effect in the Worldwide Invasion of the Biocontrol Harlequin Ladybird

Recent studies of the routes of worldwide introductions of alien organisms suggest that many widespread invasions could have stemmed not from the native range, but from a particularly successful invasive population, which serves as the source of colonists for remote new territories. We call here this phenomenon the invasive bridgehead effect. Evaluating the likelihood of such a scenario is heuristically challenging. We solved this problem by using approximate Bayesian computation methods to quantitatively compare complex invasion scenarios based on the analysis of population genetics (microsatellite variation) and historical (first observation dates) data. We applied this approach to the Harlequin ladybird Harmonia axyridis (HA), a coccinellid native to Asia that was repeatedly introduced as a biocontrol agent without becoming established for decades. We show that the recent burst of worldwide invasions of HA followed a bridgehead scenario, in which an invasive population in eastern North America acted as the source of the colonists that invaded the European, South American and African continents, with some admixture with a biocontrol strain in Europe. This demonstration of a mechanism of invasion via a bridgehead has important implications both for invasion theory (i.e., a single evolutionary shift in the bridgehead population versus multiple changes in case of introduced populations becoming invasive independently) and for ongoing efforts to manage invasions by alien organisms (i.e., heightened vigilance against invasive bridgeheads)

Local dispersal pathways during the invasion of the cactus moth, Cactoblastis cactorum, within North America and the Caribbean

Cactoblastis cactorum, a species of moth native to Argentina, feeds on several prickly pear cactus species (Opuntia) and has been successfully used as a biological control of invading Opuntia species in Australia, South Africa and native ruderal Opuntia species in some Caribbean islands. Since its introduction to the Caribbean its spread was uncontrolled, invading successfully Florida, Texas and Louisiana. Despite this long history of invasion, we are still far from understanding the factors determining the patterns of invasion of Cactoblastis in North America. Here, we explored three non-mutually exclusive explanations: a) a stepping stone model of colonization, b) long distance colonization due to hurricanes, and/or c) hitchhiking through previously reported commercial routes. Genetic diversity, genetic structure and the patterns of migration among populations were obtained by analyzing 10 nuclear microsatellite loci. Results revealed the presence of genetic structure among populations of C. cactorum in the invaded region and suggest that both marine commercial trade between the Caribbean islands and continental USA, as well as recurrent transport by hurricanes, explain the observed patterns of colonization. Provided that sanitary regulations avoiding humanmediated dispersal are enforced, hurricanes probably represent the most important agent of dispersal and future invasion to continental areas.Fil: Andraca Gómez, Guadalupe. Universidad Nacional Autónoma de México. Instituto de Ecología; MéxicoFil: Lombaert, Eric. Université Côte d'Azur; Francia. Centre National de la Recherche Scientifique; FranciaFil: Ordano, Mariano Andrés. Fundación Miguel Lillo; Argentina. Universidad Nacional de Tucumán. Instituto de Ecología Regional. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto de Ecología Regional; ArgentinaFil: Pérez Ishiwara, Rubén. Universidad Nacional Autonoma de Mexico. Departamento de Ecología Evolutiva; MéxicoFil: Boege, Karina. Universidad Nacional Autonoma de Mexico. Departamento de Ecología Evolutiva; MéxicoFil: Domínguez, César A.. Universidad Nacional Autonoma de Mexico. Departamento de Ecología Evolutiva; MéxicoFil: Fornoni, Juan. Universidad Nacional Autonoma de Mexico. Departamento de Ecología Evolutiva; Méxic

Utilization of microcosms to test invasion biology hypotheses

Comprendre les facteurs déterminant le succès ou l’échec des processus invasifs est un objectif majeur en biologie de l’invasion. De nombreux travaux théoriques se sont intéressés aux composantes écologiques et évolutives de ces facteurs. Cependant, les tests d’hypothèses associés à une démarche expérimentale restent rares. La plupart des résultats empiriques sont issus de l’analyse a posteriori d’invasions fortuites et ne permettent donc, au mieux, que des approches corrélatives. Dans cet article, nous discutons de la pertinence des microcosmes, i.e. des environnements simplifiés, contrôlés et reproductibles, comme alternatives aux introductions expérimentales en milieu naturel. En nous basant sur une synthèse de la littérature, nous présentons les avantages et limites des approches en microcosmes pour l'étude des invasions biologiques. Notre analyse se concentre sur les publications impliquant des populations en dynamique transitoire après un goulot d’étranglement et/ou soumises à un challenge adaptatif, deux caractéristiques clés des processus invasifs. Malgré le nombre encore réduit de telles études, leur intérêt a été montré pour explorer les influences des caractéristiques de l’aire envahie (les conditions environnementales ainsi que leur hétérogénéité spatiale ou temporelle). Dans une moindre mesure, les microcosmes ont également permis de tester l’influence des caractéristiques des populations introduites et de la communauté envahie. Cependant, ils doivent être utilisés avec précaution car ils ne permettent pas de reproduire la complexité des milieux naturels. Les expériences en microcosmes sont donc complémentaires aux études théoriques et à celles menées en populations naturelles et contribuent à renforcer la valeur prédictive de la biologie de l’invasion en liant théorie et expérimentation.Understanding the factors underlying establishment and spread of exotic species in order to predict invasion risks is a major goal in invasion biology. Many theoretical studies investigated the ecological and evolutionary components of these factors and their impact on the invasive process. Yet, hypothesis tests through experimental approaches are still scarce because of the practical and ethical difficulties associated with the introduction of exotic species in nature. Thus, most empirical results come from a posteriori analyses of fortuitous invasions, which allow correlative approaches at best and give no information about invasion failures. In this paper, we propose microcosms, i.e. reproducible controlled simplified environments, as an alternative to experimental introductions in natura. From a review of the literature, we discuss the distinctive features of microcosms to test theoretical predictions about invasion. Our analysis focuses on studies involving populations in transitory dynamics after a demographic bottleneck and/or subject to an adaptive challenge, two key characteristics of invasive processes. Despite their small number, these studies have been used successfully to explore the influences of various factors, mainly related to the introduction site characteristics (its abiotic conditions and their spatial and temporal heterogeneity), and to a lesser extent to the introduced individuals themselves (propagule pressure, genetic diversity and adaptations in the introduced population) or the invaded community. We argue that microcosms, as model systems, can be powerful tools to test theoretical hypotheses. They must however be used with care, as they do not account for the same complexity as natural systems. They are thus complementary to theoretical studies and field surveys, and contribute to reinforce the predictive value of invasion biology by linking theory and experimentation

Global invasion history of the agricultural pest butterfly revealed with genomics and citizen science.

The small cabbage white butterfly, , is a major agricultural pest of cruciferous crops and has been introduced to every continent except South America and Antarctica as a result of human activities. In an effort to reconstruct the near-global invasion history of , we developed a citizen science project, the "Pieris Project," and successfully amassed thousands of specimens from 32 countries worldwide. We then generated and analyzed nuclear (double-digest restriction site-associated DNA fragment procedure [ddRAD]) and mitochondrial DNA sequence data for these samples to reconstruct and compare different global invasion history scenarios. Our results bolster historical accounts of the global spread and timing of introductions. We provide molecular evidence supporting the hypothesis that the ongoing divergence of the European and Asian subspecies of (∼1,200 y B.P.) coincides with the diversification of brassicaceous crops and the development of human trade routes such as the Silk Route (Silk Road). The further spread of over the last ∼160 y was facilitated by human movement and trade, resulting in an almost linear series of at least 4 founding events, with each introduced population going through a severe bottleneck and serving as the source for the next introduction. Management efforts of this agricultural pest may need to consider the current existence of multiple genetically distinct populations. Finally, the international success of the Pieris Project demonstrates the power of the public to aid scientists in collections-based research addressing important questions in invasion biology, and in ecology and evolutionary biology more broadly

Secondary contact and admixture between independently invading populations of the Western corn rootworm, diabrotica virgifera virgifera in Europe

The western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), is one of the most destructive pests of corn in North America and is currently invading Europe. The two major invasive outbreaks of rootworm in Europe have occurred, in North-West Italy and in Central and South-Eastern Europe. These two outbreaks originated from independent introductions from North America. Secondary contact probably occurred in North Italy between these two outbreaks, in 2008. We used 13 microsatellite markers to conduct a population genetics study, to demonstrate that this geographic contact resulted in a zone of admixture in the Italian region of Veneto. We show that i) genetic variation is greater in the contact zone than in the parental outbreaks; ii) several signs of admixture were detected in some Venetian samples, in a Bayesian analysis of the population structure and in an approximate Bayesian computation analysis of historical scenarios and, finally, iii) allelic frequency clines were observed at microsatellite loci. The contact between the invasive outbreaks in North-West Italy and Central and South-Eastern Europe resulted in a zone of admixture, with particular characteristics. The evolutionary implications of the existence of a zone of admixture in Northern Italy and their possible impact on the invasion success of the western corn rootworm are discussed

Colonization history of the western corn rootworm (Diabrotica virgifera virgifera) in North America: insights from random forest ABC using microsatellite data

First described from western Kansas, USA, the western corn rootworm, Diabrotica virgifera virgifera, is one of the worst pests of maize. The species is generally thought to be of Mexican origin and to have incidentally followed the expansion of maize cultivation into North America thousands of years ago. However, this hypothesis has never been investigated formally. In this study, the genetic variability of samples collected throughout North America was analysed at 13 microsatellite marker loci to explore precisely the population genetic structure and colonization history of D. v. virgifera. In particular, we used up-to-date approximate Bayesian computation methods based on random forest algorithms to test a Mexican versus a central-USA origin of the species, and to compare various possible timings of colonization. This analysis provided strong evidence that the origin of D. v. virgifera was southern (Mexico, or even further south). Surprisingly, we also found that the expansion of the species north of its origin was recent—probably not before 1100 years ago—thus indicating it was not directly associated with the early history of maize expansion out of Mexico, a far more ancient event

Colonization of the Mediterranean Basin by the vector biting midge species Culicoides imicola: an old story

Understanding the demographic history and genetic make-up of colonizing species is critical for inferring population sources and colonization routes. This is of main interest for designing accurate control measures in areas newly colonized by vector species of economically important pathogens. The biting midge Culicoides imicola is a major vector of Orbiviruses to livestock. Historically, the distribution of this species was limited to the Afrotropical region. Entomological surveys first revealed the presence of C. imicola in the south of the Mediterranean basin by the 1970's. Following recurrent reports of massive bluetongue outbreaks since the 1990s, the presence of the species was confirmed in northern areas. In this study, we addressed the chronology and processes of C. imicola colonization in the Mediterranean basin. We characterized the genetic structure of its populations across Mediterranean and African regions using both mitochondrial and nuclear markers, and combined phylogeographical analyses with population genetics and approximate Bayesian computation. We found a west/east genetic differentiation between populations, occurring both within Africa and within the Mediterranean basin. We demonstrated that three of these groups had experienced demographic expansions in the Pleistocene, probably because of climate changes during this period. Finally, we showed that C. imicola could have colonized the Mediterranean basin in the late Pleistocene or early Holocene through a single event of introduction; however we cannot exclude the hypothesis involving two routes of colonization. Thus, the recent bluetongue outbreaks are not linked to C. imicola colonization event, but rather to biological changes in the vector or the virus