A population study of Phaulacridium vittatum Sjost (Acrididae)

Abstract

Phaulacridium vittatum is a very abundant grasshopper in pastures in the Southern Tablelands area of New South Wales. It has a single generation per year, the active stages of which are present from late spring until mid-autumn of the following year. In grazed pastures female grasshoppers lay their egg pods in bare spaces between plants and there is a close correlation between the density distribution of egg pods and that of adults of the parent generation. An egg diapause occurs and mortality of eggs is relatively low. The first-instar nymph of P. vittatum feeds on prostrate and rosette-forming plants. However, irrespective of their abundance it is unable to locate these plants where grasses form the dominant plant cover and so fails to survive. Heavy spring rains which produce an abundance of annual and perennial grasses limit the amount of favourable space where the young nymphs can locate suitable food plants and thus result in high mortality. Heavy grazing and the introduction of mat-forming Trifolium subterraneum (L.) maintain open low pastures and favour survival of first-instar nymphs. When hatching has been relatively late, the seasonal drying off of annuals, particularly T. subterraneum, which is a favoured food plant of P. vittatum, results in heavy mortality of the early-instar stages. In most instances, populations have reached the fourth-instar stage by the time that the annuals dry out and dispersal then occurs. Frequently, dispersal consists merely of movement from sites where survival was high after hatching into areas in which the cover was initially unfavourable for post-hatching survival. However, where conditions were uniformly favourable for hatchling survival, mass movement of the grasshoppers to trees may occur. Under average rainfall and evaporation rates in summer very little plant growth occurs, so that to develop from the fourth instar to the sexually immature adult stage, the grasshoppers depend on the foliage accumulated by broad-leafed plants during the spring. As the numbers of fourth-instar nymphs are usually excessive in relation to the amounts of food accumulated during the spring growing period, heavy depletion of food occurs and numbers fall simultaneously. However, development of the grasshoppers is not interrupted by food shortage and, on reaching the sexually immature adult stage, numbers become stable. The persistence of populations under conditions of limited food is attributed to poor discrimination between favourable plants and those unfavourable for development or survival, the restriction of the movements of individuals to ambits or "home ranges" of limited area, an apparent inability of individuals to locate food plants when they wander away from their ambits in search of food and cannibalism of weakened individuals. In contrast to nymphs, adults can survive for prolonged periods in the sexually immature condition on a diet consisting exclusively of the fresh growth of those shallow-rooted perennial grasses which respond to light falls of rain. They do not become sexually mature under these conditions, but reproduce when sufficient rain falls to induce renewed growth of broad-leafed plants.