The potential effect of population outbreaks on within and between genetic variation of populations in pest species has rarely been assessed. In this study, we compare patterns of genetic variation in different sets of historically frequently outbreaking and rarely outbreaking populations of an agricultural pest of major importance, the migratory locust, Locusta migratoria. We analyse genetic variation within and between 24 populations at 14 microsatellites in Western Europe, where only ancient and low-intensity outbreaks have been reported (non-outbreaking populations), and in Madagascar and Northern China, where frequent and intense outbreak events have been recorded over the last century (outbreaking populations). Our comparative survey shows that (i) the long-term effective population size is similar in outbreaking and non-outbreaking populations, as evidenced by similar estimates of genetic diversity, and (ii) gene flow is substantially larger among outbreaking populations than among non-outbreaking populations, as evidenced by a fourfold to 30-fold difference in FST values. We discuss the implications for population dynamics and the consequences for management strategies of the observed patterns of genetic variation in L. migratoria populations with contrasting historical outbreak frequency and extent.
Red locusts frequently cause crop damage in Madagascar. This problem has worsened in recent years, likely due to the intensive deforestation that is under way. Little is known about this locust In Madagascar, contrary to continental Africa. Several studies have been carried out in southwestern Madagascar, where locust damage is regularly reported, with the aim of determining the cause of recent outbreaks and improving control strategies-they included surveys of rural inhabitants and an analysis of over 25 years of archival data at the Centre national antiacridien (Malagasy locust control centre). The results revealed several zones between which the red locust-a single-generation species that undergoes imaginal diapause in the dry season-seasonally migrates on a northeast to southwest track, and vice versa, depending on rainfall and wind patterns. The migration dynamics and interannual variations associated with rainfall irregularity certainly partially account for the recurrence of red locust outbreaks in Madagascar.Résumé. Ecologie et migrations du criquet nomade, Nomadacris septemfasciata (Serville) (Orthoptera : Acrididae) dans sa phase solitaire dans le sud-ouest de Madagascar. Le Criquet nomade est un problème récurent de protection des cultures à Madagascar. Il s'est aggravé récemment, très certainement en partie sous l'influence d'une déforestation intensive. Contrairement au continent africain, cette espèce est peu connue dans la Grande Ile. Afin d'établir les causes des récentes pullulations et d'améliorer la stratégie de lutte, plusieurs études ont été entreprises dans le sud-ouest du pays où des dégâts sont régulièrement rapportés : en particulier des enquêtes en milieu rural et l'analyse de 25 années de données d'archives du Centre national antiacridien. On ainsi a pu mettre en évidence plusieurs zones entre lesquelles le Criquet nomade, espèce monovoltine à diapause imaginale de saison sèche, effectue des migrations saisonnières, du nord-est vers le sud-ouest et inversement, en fonction des pluies et de l'alternance des vents. La dynamique de ces déplacements et leurs particularités interannuelles liées à l'irrégularité des pluies sont certainement une cause importante des pullulations de cette espèce à Madagascar.
Little is known about phase polymorphism in the red locust, Nomadacris septemfasciata (Serville) in Madagascar, despite its economic importance. Only solitary and transiens forms have been reported from the island earlier, but not the gregarious form. Important recent outbreaks have allowed for the studying of the phase polymorphism under experimental and field conditions. Morphometric differences (pronotum size and shape, and F/C and E/F ratios) were noted among hoppers and adults caged for several generations and at various densities, in a field laboratory. These criteria were later used to study adults collected in different areas between 1996 and 2002, from populations with various densities (, 1000 to . 25,000 imagos/ha). Morphometric measurements revealed that gregarious red locust populations have existed naturally in Madagascar and that these were erroneously classified as transiens forms. The upper threshold density for the solitary phase was estimated at 5000 adults/ha; beyond this limit, locusts develop either into transiens or into highly gregarious forms. The southern part of the island, known as a red locust outbreak area, has harboured gregarious phases despite previous reports that prevailing ecological conditions are not suitable for its complete phase transformation. Deforestation in the northern part of the island has probably led to new favourable biotopes for serious outbreaks and gregarious populations development.phase grégaire existe à Madagascar et que tous les individus grégaires ont été depais longtemps considérés abusivement comme des transiens. Le seuil maximal des populations solitaires a été estimé à 5000 imagos/ha, au-delà duquel commence la transformation phasaire. Le sud de l'île a abrité depuis longtemps des populations grégaires quoique, les conditions écologiques dans cette zone Acrididae étaient traditionnellement considérées comme impropres à favoriser la grégarisation et les pullulations du criquet. Dans le Nord, des pullulations sont aussi récemment survenues probablement suite à la déforestation qui a permis l'apparition de nouveaux biotopes très favorables à la grégarisation.
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