These crops are all either indigenous or of early introduction, except in the case of special varieties and they have achieved within the limits of their present distribution an equilibrium with their insect pests. Where there is no reasonable chance of a 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. For example, the introduction of cassava into new areas to prevent famine during the locust invasions brought to light the stem-borer moth, Opogona chlorophanes Meyr. In some cases cassava could not be grown and sweet potato had to be substituted. Acridids such as Eupropacris uniformis suddenly appear in great numbers at long intervals and do. considerable damage, then disappear just as suddenly. Similarly the adverse effect of climate on the crop will render it susceptible to particular pests, especially Aphids, during periods of drought. Agricultural practice is also concerned in this equilibrium, as for example where the time of planting legumes is governed by the attacks of various Galerucid beetles, particularly Ootheca bennigseni, Wse., which are most active during the early rains.