Seasonal dimorphism of the desert locust in agricultural areas in the Sahara

摘要

The development of Saharan agriculture in recent decades by increasing irrigated areas for vegetable production in central Sahara has led to significant outbreaks of locusts. This insect, which is note deserticola type, has succeeded in developing a series of behavioural, morphological and physiological characteristics to adapt to this environment which has become one of its areas of reproduction and dispersal This work aims at evaluating the morphometric variation of locusts in two different habitats located in the southern Sahara, natural and cultivated environments. The results of biometric analysis on male and female populations, conducted from 1993 to 2004, primarily on the elytra (E), femur (F) and head capsule (C), revealed the existence of differences between locusts from two locations. Population densities were low and did not exceed 500 individuals per hectare, except in April 2004 when a maximum of 10 000 individuals/ha was reached in Adrar. Morphometric ratios (E/F and F/C) of the studied populations oscillate between the solitary and transiens-dissocians phases, except for populations in June 2004 in Tamanrasset and April 1998 in Adrar, which were gregarious. Analyses of variance both with and without population density as a covariate, showed the effect of density on measured traits. Separate analysis of variance of males and females emphasized a marked seasonal effect on the femur and head capsule, especially in Adrar. The male population at this location In the dry season presented shorter femur and smaller head capsule measurements than those present in the wet season at the same location and those of Tamanrasset. This result shows the influence of seasons and weather conditions on morphometric traits. A seasonal dimorphism in size coupled with a contrast of phase status has been highlighted in male populations of Adrar. The relationship between human activities and the change in morphometric traits at the locations studied is discussed. These results are of significance in preventing the proliferation of the desert locust.