Food-Plant Distribution and Density and Larval Dispersal as Factors Affecting Insect Populations

It is generally agreed that insects whose larvae feed on plants do not increase to the larval food limit except in sporadic and unusual cases. In the few detailed studies which have been made, as, for example, Varley's (1947) study of the knapweed gallfly, the evidence suggests that the insects are held in check by parasites and predators. Andrewartha and Birch (1954) are of the opinion that shortage of food is probably the least important factor in limiting the numbers of animals in a natural population. On the other hand, they have pointed out (p. 489) that while it is unusual for an extensive population to consume all or most of the stock of food in its area, it does not follow that the amount of food is rarely of major importance in determining the numbers of a natural population. Several examples (p. 492) were given in which shortage of food was chiefly responsible for failure of population increase even though the absolute supply seemed adequate. In these cases local supplies were exhausted and the chance of finding fresh supplies depended upon the distribution and abundance of food and the powers of dispersal of the animals. In other words, the food was inaccessible relative to the animals' capacities for dispersal and searching.