Four Years

Abstract

Since the recognition of Numicia viridis, Muir, as a pest of sugarcane (Dick 1963), annual surveys have been carried out with two main objectives. In the first place it has been considered important to study the geographical distribution of the insect within the area covered by the South African sugar industry, to notice any population trends which may occur and to have early warning of increases in particular areas. For example, the 1966 survey revealed the presence of a local infestation near Paddock on the Natal South Coast and, although it has not proved necessary to apply specific control measures, this area has been kept under observation since the survey. Secondly, it was hoped that inspection of several hundred sites under various ecological conditions might yield information on the type of environment in which outbreaks are most likely to occur. Four surveys have been carried out and this paper is an atteinpt at collating and generalizing upon the infor-matlion collected. Population studies in Swaziland are not included since these are carried out in a different way and form the subject of a special research project (Carnegie 1967). Methods In every selected site five counts are made, each representing the population found in one square yard of cane-field. The area is defined by placing a sheet of black plastic material between the cane rows, where it is kept in position by two strips of wood attached to opposite sides. Nymphs and adults of Numicia falling on to the plastic sheet are counted and recorded separately. During the 1963 survey, the sugarcane above and around the sheet was liberally dusted with a 5 per cent malathion powder and the sheet was left in position for five hours, after which the insects which had fallen on it were counted. This method enables fairly accurate counts to be made even when insects are present in large numbers , but it limits the sites which a team of two observers can examine per day to about five. Each site must be visited twice, first to apply the insecticide and later to collect and count the insects. In addition, the sites which can be examined in a day are limited by the number of plastic sheets which can be carried. Since populations found in 1963 were seldom high enough to make counting difficult, it was decided that subsequent surveys would be based on the insects landing …