My-Hanh Luong-Skovmand

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.

Phase-Dependent Color Polyphenism in Field Populations of Red Locust Nymphs (Nomadacris septemfasciata Serv.) in Madagascar

Pigmentation of the Red locust hopper, Nomadacris septemfasciata Serv., was studied in natural conditions in Madagascar in relation to population density. More than one thousand hoppers were collected and described according to a semiquantitative method. A typology is proposed, strictly reflecting the increase in population densities. This correctly translated the progressive evolution of a solitary state into a gregarious state, while passing through several intermediate transiens stages. According to their density, hopper populations consist of a mixture, in various proportions, of several pigment types. The gregarization threshold is estimated at 100,000 hoppers/ha. A slight black spot on the hind femur is the first sign of gregarization. These results should improve the reliability of the information collected by the Malagasy National locust centre when surveying this major pest. They question the rapidity of the gregarization process in natural conditions as well as the stimuli involved.

Laboratory and field investigation of a mixture of Metarhizium acridum and Neem seed oil against the Tree Locust Anacridium melanorhodon melanorhodon (Orthoptera: Acrididae)

The in vitro compatibility of Metarhizium acridum strain IMI 330189 with different concentrations of Neem seed oil Azadirachta indica A. Juss. was investigated under laboratory conditions. Water, diesel and ground nut oil dilutions of M. acridum were inoculated into semi-synthetic culture medium with Neem oil and incubated for 10 days at 20, 28, and 34°C. Fungus vegetative growth and conidia production were estimated and compatibility calculated according to the in vitro classification model [T]. Field tests were also conducted during two successive years, where fourth instar nymphs of the Tree Locust, Anacridium melanorhodon melanorhodon, were sprayed with Metarhizium, Metarhizium/Neem mixture, Neem, or Malathion, on a plantation of Acacia senegal. Samples of the treated nymphs were taken 24 h after application, placed in cages and their mortality observed daily for 3 weeks. The compatibility test revealed that at 28°C, all Neem concentrations below 2% were compatible with M. acridum and concentrations of 2 and 2.5% were moderately toxic, while at 20°C, 1.0% Neem was not compatible with the fungus. In the field, Metarhizium+Neem resulted in 74 and 92% mortality during two successive years compared to 64 and 83% for Metarhizium alone. These findings clearly reveal that Neem concentration on the mixture could be increased to just under 1.0% Neem without negative impact on the fungus and that mixing Metarhizium with small quantities of Neem oil accelerates locust mortality and increases efficacy.