A study of the variations of the population of Dysdercus fasciatus on cotton and on its wild host-plants is described.

The most important host of D. fasciatus in the Acacia savannah of Northern Rhodesia is Thespesia rogersii, and it is shown that at certain times a general flighting of stainer takes place from this host-plant caused by food shortage or defoliation. Generally a period of food shortage occurs during the earlier months of the year owing to rotting of the old crop and extensive destruction of the early new crop by the larvae of a Cossid moth. This causes an early influx into cotton, which leads to the establishment of a large population in the field at a critical period in the development of the bolls. When the early crop is retained on the trees a continuously increasing food supply is provided, and this first movement does not take place. A second movement from Thespesia, which is largely due to defoliation, begins generally in June. It has been shown that a second influx into cotton coincides with this later movement from Thespesia. A late influx such as this is of little practical importance since the greater part of the crop is picked in May and June.

The observations recorded indicate clearly that the movement of D. fasciatus into cotton is not due to any attraction exercised by the crop but is caused by a forced movement of stainers from Thespesia rogersii.