Heliothis armigera (Hübner) and Heliothis punctigera Wallengren are "arguably the most important pests, insect or otherwise, of field crops in Australia". Their pest status broadly derives from a suite of characteristics - polyphagy, high mobility, high fecundity, and a facultative diapause - that enables them to survive in unstable habitats. These also make the species ideally suited to exploit the conditions of continuous change within agroecosystems. Over the last forty years in Australia, agricultural development has increased vastly the expanse and favourability of the temporary habitats available for Heliothis development. In effect, the probability that migrants will survive movements among habitat patches has increased.
From a Heliothis management perspective, where immigration is important to local population dynamics control strategies are limited to either discouraging prospective immigrants or increasing local mortality rates. Significant migration events in early spring, especially by H.punctigera, apparently initiate Heliothis infestation of many agricultural areas, including the Darling Downs. This influx from "unknown" sources (possibly central Australia) is less common in H.armigera, but the later emergence of local individuals from diapausing pupae assures this species is well represented each season. As there is little practical way in which field workers can distinguish between the eggs of immigrant and local moths of either species, a pragmatic first step towards reducing the local Heliothis problem is to study local mortality factors to determine if, and how, increases in mortality rates might be achieved.
A three-year study was undertaken to provide both qualitative and quantitative information on the natural mortality of Heliothis individuals infesting fields of agriculturallyimportant plant species during the summer growing season on the Darling Downs of south-east Queensland.
In-field variation in mortality was more important than variation in natality (the number of eggs laid) in determining the numbers entering subsequent Heliothis developmental stages; except on sunflower, where natality was the more important influence. On all plant hosts, agespecific life tables showed mortality affected the Heliothis eggs and first instar larvae most severely, the latter being the critical stage influencing total intra-generational mortality and hence the subsequent larval infestation levels of particular fields. The causes of this mortality generally were not identified during direct observation.
Analysis showed that intra-generational Heliothis survival varied significantly among cohorts on a common plant host, and across plant hosts according to plant "traits". Infrequently, first instar larval mortality was density-dependent, but usually in association with higher larval densities created via artificial manipulation. In these cases, the density-dependence appeared to be mediated from the trophic level of the plant. However, the mortality of eggs and first instars generally was independent of their density (measured as number per plant). The Weibull distribution proved an appropriate model for describing the cohorts' survival curves. While parasitoids potentially may cause significant Heliothis egg mortality, the life tables indicated that parasitoids and pathogens were not important causes of death in first instar larvae. Medium to large larvae (instars HI-VI) which were collected from fields and incubated in the laboratory usually emerged as moths, reflecting a higher survival of these later stages. Collected larvae that died during incubation more likely did so due to disease (NPV) or unspecified causes rather than parasitism. Again, this larval mortality predominantly occurred independently of the larval density (nr2) from which larvae were collected.
Experimental cohorts within cage exclosures were used to gauge the stage-specific effects of predation on Heliothis individuals. Measured predation was highly variable across insect stages and plant hosts, and often there was a confounding effect of the cage on such measurements. Methodological problems aside, predation of Heliothis eggs and first instar larvae was best described as limited and patchy; perhaps because of the depletion of predator abundance caused by pesticide application within precincts of intensive agriculture such as the study area.
Significant effects of predator-exclusion cages alone on survival rates demonstrated the existence of important weather-induced mortality. Predictive analysis revealed particular weather factors (temperature, relative humidity, solar radiation, wind, rain) were significantly correlated with mortality of Heliothis eggs and first instars, but regression relationships accounted for very little of the variation within the stage-specific survival rates. Early stage survival generally improved in the absence of the physically-impacting factors, wind and rain, and the eggs and larvae of both Heliothis species were resilient to short exposures to extremes of temperature and relative humidity in combination. Weather possibly was more important than was apparent because recorded conditions may be an inadequate description of those actually experienced by the Heliothis individuals.
As action by natural enemies was relatively unimportant in the observed mortality of early Heliothis stages, and since the variation within weather factors reliably explained less than 40% of the observed variation in stage-specific survival rates, the plant host was the residual factor for providing an explanation of the major part of observed Heliothis mortality. While not specifically investigated, there was ample evidence in the literature to suggest that plant characteristics and effects of microclimate, the plant's interaction with weather, could cause losses of such scale.
Several recommendations for immediate and future improvements in Heliothis management are made. Of particular interest would be the identification of compounds which, sprayed on the plant surface, would relatively increase the mobility of first instar larvae or make them more likely to drop from the plant host.