Red locust phases in Madagascar (Nomadacris septemfasciata Serville, Orthoptera, Cyrtacanthacridinae)

Franc, Alex; Rabesisoa, Lalaina Fenomanana; Luong-Skovmand, My-Hanh; Lecoq, Michel

Red locust phases in Madagascar (Nomadacris septemfasciata Serville, Orthoptera, Cyrtacanthacridinae)

2005

Franc, Alex | Rabesisoa, Lalaina Fenomanana | Luong-Skovmand, My-Hanh | Lecoq, Michel

Little is known about phase polymorphism of the red locust, Nomadacris septemfasciata (Audinet-Servilla, 1838) in Madagascar despite its economic importance. Gregarious forms of this locust had never been recorded until now on the island, only solitarious and transiens forms. The worsening of outbreaks provided an opportunity to study phase polymorphism under experimental and field conditions. Morphometric changes were noted when hoppers and adults were caged at different densities. Changes in different parameters (size, pronotum shape, and E/F ratio) were recorded under crowded conditions. These criteria were then used to analyze adults collected in different areas from amongst populations of various densities (from less than 1000 adults/ha to more than 25000/ha). In the past, gregarious red locusts were likely classified as transiens locusts. The results of the present study revealed that true gregarious populations do exist in Madagascar. The phase transformation threshold density was determined: adults living in low density populations, Le. less than 5000/ha, showed solitarious traits; whereas beyond this density the locusts were either transiens or even highly gregarious. Gregarious populations were observed in the northern part of the island, where serious outbreaks recently occurred after deforestation and the creation of new favorable biotopes. True gregarious populations were also observed in the south, a common outbreak area where, in the past, it was assumed that ecological conditions were not suitable for complete phase transformation to occur. (Texte intégral)

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