Anti-Locust Bulletin (1955) 20, 1-31

D.F. Vesey-Fitzgerald (1955)
The vegetation of the outbreak areas of the Red Locust (Nomadacris septemfasciata Serv.) in Tanganyika and northern Rodesia
Anti-Locust Bulletin 20, 1-31
Abstract: Swarms of the Red Locust (Nomadacris septemfasciata Serville) are believed to originate in certain restricted places, but the ecological reasons for this have not been analysed. Indeed, the only criterion for the recognition of an outbreak area has been the actual production of swarms there. In Central Africa there are three areas, the Rukwa valley, the Mweruwa Ntipa depression and the Malagarasi Basin, where resident locust populations have been known to exist and from which there has been evidence that swarms have actually been produced. It was considered that a study of the vegetation would provide evidence of some specific ecological conditions prevailing in an outbreak area, and that if the vegetation in the three suspected areas could be shown to have some features in common, recognition of other areas potentially capable of producing locust outbreaks would become possible.
It is known that all the three areas are subject to the regime of one dry and one rainy season each year. During the six dry months, extremely arid conditions prevail, but during the six wet months the areas are liable to be flooded. These extremes imposed by the climate axe supplemented by the sluggishness of the drainage, which in all these areas is impeded by the low relief and in some cases is actually reversed. A study of the vegetation has demonstrated that the various types of grassland present are dependent on the type of drainage. In areas of free drainage the vegetation tends to be regularly zoned. Open woodland with a ground cover of grass and herbs grows on the raised land where soils are of the alluvial type. Along the sides of the valleys, a woodland-grassland formation is established in which trees are less regularly spaced and the ground cover is composed of tall grasses; the soil here is of alluvial type. In the valley bottoms, flanking the watercourses, there is open grassland on a black alluvial soil, with few trees except sometimes on the immediate fringe of the drainage line.
Where the drainage is impeded, the area of alluvial soils is greatly extended and it is here that the vast open plains occur which form the locust habitat. Under these conditions three additional types of herbage are found. In the areas of accumulation of land drainage, dense beds of the grass Echinochloa pyramidalis are characteristic. In lower places, where the water stands longer and deeper, semi-permanent fresh-water swamps prevail in which the watergrass Vossia cuspidator is usually dominant. Around lagoons and fresh-water lakes, permanent swamp is found, and here various species of Cyperaceae are characteristic. The Rukwa and Mweruwa Ntipa depressions have no outlet and the land drainage accumulates to form shallow saline lakes which are liable to dry up. On the brackish soils around these lakes, further types of grassland are found established and they fall into three main categories, namely, lake-shore grasslands characterised by Sporobolus robustus, grasslands liable to flood due to the lake rising, which are characterised by Diplachne fusca, and the dry bed of the lake, which is colonised by various grasses, notably Sporobolus spicatus.
During the rains or floods the herbage grows robustly, but during drought periods it dries off, and is burnt, so that the soil is exposed. The annual alternation of wet and dry seasons is supplemented by annual fluctuations of the precipitation and flooding, which tend to cause great quantitative variations in the herbage. As a result, the ground cover varies greatly from year to year, especially in the zones of contact between the various grassland types. Observation has shown that the Red Locust is resident only in some of these valley-grassland types and evidence has been obtained that its abundance in these and extension to other types of valley grassland are influenced by the state of the herbage. Although the more detailed ecological requirements of the locust are not discussed in the present paper, sufficient evidence is given for recognising the types of grassland which may be expected to form permanent habitats for it. They are grasslands characterised by a limited number of species, but usually by two which are co-dominants over the area as a whole though each tends to grow by itself. Often these co-dominants are of very different stature, so that a mozaic of long and short species occurs, or two different grass formations, each with its own dominant, contact each other with the result that a mosaic of the two species may occur in the zone of contact. Within the mozaic, the contact between the grass colonies may be close and well marked, but under conditions of drought an ecological hiatus, characterised by poor growth and patches of bare soil, may occur. These quantitative variations in the herbage are believed to influence the locust population. As a result of this study it should now be possible to recognise potential outbreak areas of the Red Locust, and also to assess the risks of swarms being produced in them in a particular season.
(The abstract is excluded from the Creative Commons licence and has been copied with permission by the publisher.)

Research topic(s) for pests/diseases/weeds:
environment - cropping system/rotation
population dynamics/ epidemiology

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Pest and/or beneficial records:

Beneficial	Pest/Disease/Weed	Crop/Product	Country	Quarant.
	Nomadacris septemfasciata