Some aspects of the population ecology of the grasshopper Acrida conica Fabricius

Michael Charles Calver

The population ecology of the grasshopper Acrida conica Forskal was investigated at sites near Perth, Western Australia, and interpreted in the light of data collected on the species' predators, growth and development, fecundity and reproductive behaviour. Basic Biology A. conica has a univoltine life history with eggs hatching in mid-November and most individuals maturing by eight weeks later. Females pass through a total of seven instars and males six. A marked sexual dimorphism develops with females measuring 1.5 times the length of the male and weighing two to three times as much. The species is polymorphic in colour, and individuals may range from yellow and brown through to green. Laboratory experiments indicated that the polymorphism was under environmental control, with humidity and diet being the regulating factors. Population Dynamics Data on hatching, senescence, moulting rates and survival rates in juveniles were collected by applying Read and Ashford's maximum likelihood models to sequential sampling data, because capture-recapture data are inappropriate where animals lose marks through moulting. A second analysis was made for juveniles using a new model which combines capture-recapture data with the Read and Ashford model. Capture-recapture techniques were applied for the adults, and analysed using the Jolly-Seber model. Both sexes hatched in similar proportions. However, either the third or fourth male instar was extended considerably in each season compared to the females, and this led to synchronization of adult emergence despite the difference in initial hatching times and the extra female instar. Survival rates between the sexes were dissimilar, with juvenile female survivorships being less than those of males, although adult females had higher survivorships than adult males. The differential survival produced unequal adult sex ratios which varied between 2 : 1 and 13 : 1 males to females at different sites. Predation Observations of A. conica in the field indicated that flight, startle displays and crypsis were the principal defences, and were supported by the adoption of an aggregated distribution. The incidence of regurgitation was insignificant. Laboratory experiments indicated that grasshoppers were capable of matching their backgrounds. Birds took longer to catch a grasshopper on a matching background, and long backgrounds conferred protection irrespective of their colour. Similarly, capture times were increased when larger numbers of live grasshoppers were presented, but not when the grasshoppers were dead, suggesting that appropriate behaviour in aggregation has a defensive value. Calculations based on handling time and biomass for bird predators indicated that specific instars gave an optimum return of biomass/unit time. In the case of magpies, Gymnorhina tibicen, which are predators in the field, fifth and sixth instar females represented the optimum, coinciding with the increased mortality rates of these instars observed in the field. Predation in the field was estimated by serological analysis of invertebrate predators, examination of prey in spiders' webs and examination of droppings from vertebrate predators. Of the invertebrates spiders took the most grasshoppers, these being mainly green females from the earlier instars. Larger grasshoppers were attacked primarily by birds, and disproportionate numbers of green grasshoppers were caught. Growth, Development and Fecundity Grasshoppers reared in the laboratory on well-watered grasses of Avena fatua (wild oats) and Zea mayes (maize) showed improved growth rates compared to other grasshoppers which were reared on poorly watered plants of wheat and A. fatua. Better nourished females also laid more eggs/pod. Comparison of condition factors among female grasshoppers collected in the field showed that sixth instar individuals from areas with lusher grass were in better condition thari others, and that females tended to aggregate in such areas. Male densities were higher in drier zones. Reproductive Behaviour In laboratory trials heavier males were found to be more successful in gaining access to females, although females did reject males who were slow in establishing copulatory union. Copulation varied in length from 30 minutes to over two hours, and was followed by a period of mate guarding of similar length. Conclusion It is suggested that female life histories are designed to maximize the fecundity of individuals at a possible cost in higher juvenile mortalities. By contrast, males aim to accentuate the sexual dimorphism to increase their survivorship relative to the females. They also regulate their development to ensure maturation at the optimum time to maximize mating success.

Some aspects of the population ecology of the grasshopper Acrida conica Fabricius

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