Seasonal movements of swarms of Locusta migratoria migratorioides (R. & F.) in western Africa in 1928 to 1931

Abstract

Cartographical analysis of the distribution of recorded swarming populations of Locusta migratoria migratorioides (R. & F.) in western Africa for the first four years of the 1928–41 plague of this species has shown seasonal latitudinal displacements in each year with a nroth-to-northweastward movement of swarms during the monthe February-March to August-September, and a south-to-south-westward return during the priod October until the end of the year or January or Ferbruary of the following year.Changes in the position of the northern limit of distribution of swatms in western Africa from month during 1931 have indicated a probable relationship between its position and that of the Intertropical Front, in its seasonal displacement, the position of the surface front being inferred from that of the 10-mm. isohyet. From February to September 1931, following the northward displacement of the Intertropical Front and the northward penetration of south-westerly winds, a general northward displacement of swarms occurred. Similarly, following the southward advance of northerly winds, there was a southward displacement of swarms.There was an eastward movement in the main latitudinal displacement. This was particularly marked in 1929 and 1930. By September 1929, the infestation that in April had been restricted to parts of south-western Guinea and norther Sierra Leone had spread north-eastwards to cover the greater part of the Voltaic Republic and southern Mali; from December 1929 to January-February 1930, following the south-westward movement from late September to November 1929 which confined swarms to the countries bordering the Atlantic and the Gulf of Guinea. there was a spread eastwards along the coastal areas from south-eastern Ghana to southern and eastern Nigeria; and following the main northward movement of swarms which began in western AFrica in April 1930, there was, from May-June onwards, a pronounced eastward migration to the central and eastern parts of the continent. Each of these eastward movements occurred in areas where, at that particular time of year, the prevailing winds would be from the south-west.The deduced movements suggest that swarms of Locusta in western Africa move downwind, and that the principal seasonal changes in the distribution of swarms in that area occur in response to the latitudinal displacement of the Intertropical Front and its associated wind field.