Phase polymorphism in the red locust, Nomadacris septemfasciata (Orthoptera: Acrididae) in Madagascar

Abstract

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.

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

“…This is obviously the best way to avoid observing gregarious individuals. Our results complemented recent studies (based on morphometric measurements) showing that the gregarious phase amongst the imagos was indeed present in Madagascar from the extreme south to the extreme north of the country [67]. A new gregarious area has moreover recently been identified following major outbreaks that occurred from 1999 to 2003 in the far north, surely as a result of intensive deforestation leading to the creation of new suitable biotopes [67,68].…”

“…To our knowledge, this is the first indication of this type in the Red locust. For adults, this threshold has been recently estimated to be around 5,000 individuals per hectare by Franc et al [67]. In comparison, the threshold is estimated at 2,000 adults/ha for the Migratory locust [69].…”

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

“…First, it was previously believed that the red locust never reaches the full gregarious stage in Madagascar (Tê tefort andWintrebert 1967, Scherer 1997), whereas in Africa, widespread invasions by fully gregarious forms are common (Morant 1947, Bahana andByaruhanga 1999). During our Þeld surveys, samples collected in hopper bands and swarms in northern Madagascar during the 1999 Ð2004 upsurge period were gregarious individuals (Franc et al 2005). Environmental modiÞcations, especially tree cutting and forest burn clearing, seem to be the main cause of this new gregarisation in Madagascar (Franc et al 2007).…”

“…Individuals were collected in their typical habitats, i.e., shrubby high savannas covered with tall grasses such as Hyparrhenia rufa, Panicum maximum, and Sorghum sp. The densities of the populations sampled were from 10 to 3,000 adults/ha, i.e., below the phase transformation threshold, which is estimated at ϳ5,000 adult/ha for the red locust (Franc et al 2005).…”

“…In the solitary phase, red locusts occur at very low densities and are harmless to crops, whereas gregarious individuals form enormous swarms, displace to new locations, and can devastate crops over vast areas (Uvarov 1977). In Madagascar, the Þrst gregarious specimen was described in the south in 1997 and then in 2002 in the SoÞa river basin (north) (Franc et al 2005). In this area, serious outbreaks damaged crops between 1999 and 2004 (Franc et al 2007).…”