Weather, food and plagues of locusts

Species of acridids throughout the world are shown to have very similar habitats and food plants, and to increase and decrease in abundance in response to very similar patterns of weather. Changes in the rate of survival of the very young hoppers from unexplained causes would appear to be the major factor influencing these changes in abundance; parasites and predators apparently have little if any significant effect.It is postulated that variation in survival of the very young is brought about by changes in the abundance and nitrogen content of their food plants, and that this in turn is largely influenced by changes in patterns of rainfall.The hypothesis-already proposed for other phytophagous insects-states that the environment of these insects is basically inadequate in that there is normally a relative shortage of nitrogenous food for the rapidly growing young. Species have adapted to this inadequate environment by producing large "surpluses" of young.When chance combinations of weather stress the food plants, making them a richer source of nitrogen, there is a greatly increased rate of survival of the very young, rapidly leading to explosive increases in abundance-to outbreaks.Some of the implications of applying this hypothesis more generally to the interpretation of the ecology of herbivores are discussed.