A population study of Phaulacridium vittatum Sjost (Acrididae)
Authors: Clark DP 
Published: June 2005

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.