Field studies were carried out in Mauritania in collaboration with the Integrated Biological Control of Grasshoppers and Locusts project of the GTZ from February to April 1998. The field site was located near Akjoujt (19o46'N; 14o25'W; 109 m above sea level), in the Inchiri district of Mauritania. According to climate and vegetation classifications, Akjoujt is located in the southern Sahara zone, just North of the boundary with the Sahel. It is surrounded by low hills which form part of the Mauritanides chain; bedrock of the region includes schists, granites and calcareous rock with important copper and gold deposits, overlaid in depressions with aeolian sands and silts Caruba & Dars (1991). Average annual temperature and rainfall are 28.8oC and 91 mm respectively. During the period of this study, temperatures typically reached a maximum near 40oC in early afternoon, and descended to between 15-20oC at dawn. Most of the rain falls in the autumn, but sporadic showers can occur year round. Rainfall is extremely variable and unpredictable: for instance, total rainfall in 1993 was 52 mm, followed by 22 mm in 1994 and 98 mm in 1995 el Hadj (1997).
The vegetation around Akjoujt falls in the category of what Monod Monod (1951) described as `contracted' vegetation, where plant growth is limited to wadis and depressions that concentrate and channel rainfall. Grasses (mainly Panicum turgidum) only provide partial cover, and annuals such as Fagonia olivieri and Schouwia purpurea dominate interdune hollows, interspersed with a few trees and shrubs (including Acacia spp., Maerua crassifolia, Balanites aegyptica, etc) Barry & Celles (1991); Ozenda (1977). Popov et al. Popov et al. (1991) give a detailed description of plant communities in different parts of the western Sahara, including the Inchiri.
The hills surrounding Akjoujt drain into a central depression, forming a semi-permanent body of water known as the Lac d'Akjoujt. Water levels rise in the autumn, then gradually fall, sometimes disappearing altogether during the summer months. As the water recedes, vegetation develops in the moist silts and clays that are uncovered (mainly Hyoscyamus muticus, Calotropis procera, Tamarix senegalensis, Glinus lotoïdes, Citrullus colocynthis, Chrozophora brocchiana). Natural plant cover is highly variable between years: totalling 57% in 1994 and 26% in 1995 el Hadj (1997). Many of these areas are also planted with millet. As the vegetation persists much longer in the depression than elsewhere (some green is available year round), it constitutes ideal habitat for Desert Locusts and has long been known as an area of winter breeding. This area is home to a solitarious population, which produces several generations annually and sometimes is present throughout the year. Solitarious locusts from the summer breeding zones invade the area in most autumns, thus renewing the population. Gregarisation only rarely occurs, and seems to be dependent upon the influx of exceptionally large numbers of individuals from summer breeding ground to the South and East. For instance, in 1994, exceptionally important immigration into the local population led to gregarisation and the beginnings of swarm formation el Hadj (1997).
The semi-field experiments described in chapters 7 & 8 were conducted in an area of native vegetation at the boundary of a millet plantation in the Lac d'Akjoujt depression. This a community dominated by Hyoscyamus muticus ( 25% cover), Citrullus colocynthis (7%), and Glinus lotoïdes (5%) on a sandy loam soil. Plant cover was measured according to a method developed by McAuliffe McAuliffe (1990) for use in discontinuous cover desert communities. Unfortunately the solitarious locust population was at too a low density to provide insects for the experiments. The solitarious adults used in chapter 8 were collected from another more distant wild population; the nymphs used in the present chapter and in chapter 7 were reared in isolation from eggs taken from the gregarious culture.