The Impact of Biotic Factors on Chortoicetes terminifera (Walker) in Invasion Areas of Southeastern Australia

Abstract

Abiotic factors, principally rainfall, drive the population dynamics of the Australian plague locust, Chortoicetes terminifera (Walker), in semi-arid source areas. However, with the possible exception of the low summer rainfall southern invasion areas, biotic factors-especially scelionid, sarcophagid and nemestrinid insect parasitoids and mermithid nematode parasitoids- become increasingly important in reducing survival in successive generations following outbreaks, while the abiotic factors assume a more indirect role through their influence on biotic factors. Parasitoids of acridids have a relatively stable population of acridid hosts, principally eastern plague grasshopper, Oedaleus australis Saussure, on the western plains and wingless grasshopper, Phaulacridium vittatum (Sj6stedt), on the western slopes and tablelands, because of the high summer rainfall in these invasion areas. The host-parasitoid relationship of each parasitoid is distinctive and exhibits temporal variation in relation to the outbreak phase of C. terminifera and alternative host abundance. Typically the unlimited availability of hosts during outbreaks allows full realization of the reproductive capacity of the parasitoids. In the future parasitoid induced recessions may be initiated at an earlier stage in the outbreak cycle because of the increasingly efficient control of C. terminifera in semi-arid source areas and consequent reduction in the density of populations invading the Southeast. However, parasitoid-accommodating control strategies, including the demarcation of districts in which a do nothing strategy would be appropriate in some seasons, need to be adopted so that the supplementary control of C. terminifera afforded by parasitoids can be maximized.