It is now more than ten years since the publication of Harris' list of insects injurious to native food crops in Tanganyika (Harris 1937). It may be well to recall the view expressed in his introduction “… these crops … have achieved, within the limits of their present distribution, an equilibrium with their insect pests. Where there is no reasonable return for his labours, the native has ceased to grow a particular crop. This equilibrium is unstable, since pests increase in numbers at intervals owing to a variety of causes …”.