Alex Franc

Outbreaks, gene flow and effective population size in the migratory locust, Locusta migratoria: a regional-scale comparative survey.

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.

Phase polymorphism in the red locust, Nomadacris septemfasciata (Orthoptera: Acrididae) in 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. Fhe 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.RésuméLe polymorphisme phasaire du criquet nomade, Nomadacris septemfasciata (Serville), a été peu étudié à Madagascar, malgré son importance économique. On considérait jusqu’à présent que cette espèce n’y avait jamais atteint la phase grégaire et que seules les populations solitaires et transiens s’y développaient. Des pullulations enregistrées ces dernières années ont conduit à étudier le phénomène phasaire chez cette espèce en milieu contrôlé et sur terrain. Un élevage en cages du masse à differentes densitis et sur plusieurs générations a permis de mettre en évidence les modifications des caractères morphométriques (taille et forme du pronotum et rapports F/C et E/F), induites par le groupement chez les juveniles et les adultes. Ces critères ont ensuite été utilisés pour étudier les adultes des populations naturelles de densités variées (<1000 à >25 000 imagos/ ha), collectés sur l’ensemble du territoire, entre 1996 et 2002. Les données morphométriques ont prouvé que la 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.

Ecology and migration patterns of solitary red locusts, Nomadacris septemfasciata (Serville) (Orthoptera: Acrididae) in southwestern Madagascar

Abstract 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.

Life Cycle, Reproductive Maturation, and Wing Color Changes in Nomadacris septemfasciata (Orthoptera: Acrididae) in Madagascar

ABSTRACT Red locusts, Nomadacris septemfasciata (Serville, 1838), frequently damage crops in Madagascar, and this problem has worsened in recent years, likely because of intensive deforestation. Little is known about this pest in Madagascar, contrary to southern Africa. We studied the reproductive maturation in relation to climate and vegetation in northwestern Madagascar. Our results show that adults overwinter, with females undergoing a 6- to 7-mo-long reproductive diapause, followed by a variable quiescent period of 2–4 wk, depending on ecological conditions, during which vitellogenesis is delayed,. Food availability in response to good rains at the end of spring seems to be the key factor for triggering the end of quiescence and the beginning of rapid vitellogenesis (= reproductive maturation). Hind wing color in adults changes throughout the year and was correlated with age and reproductive state in females and seasonal climate change.

Red locust Nomadacris septemfasciata (Serville) upsurges in northern Madagascar between 1998 and 2004

Abstract The Red locust (Nomadacris septemfasciata Serville) is commonly found in southern Africa and the Indian Ocean islands. In Madagascar until 1998, only infested crop fields were controlled. However, since 1998 the Red locust has caused considerable crop damage in northern Madagascar, where gregarious individuals were identified for the first time in Madagascar in 2002. In this study, an accurate history of the outbreaks which occurred between 1998 and 2004 is drawn up on the basis of field surveys and anecdotal data. A total area of more than 60,000 ha was infested between 2001 and 2003, at the peak of the outbreak. With these results, we can make out a first biogeographical synthesis for this locust.