Aspects of the ecology of black beetle (Heteronychus arator (F.), Coleoptera: Dynastinae)

Abstract

The study included laboratory and field studies of the ecology of the black beetle (Heteronychus arator F.) pre-adult and adult stages and field studies of population dynamics. The objectives were to understand population change and regulation, particularly in relation to host plant species and explore the possibilities of the prediction of outbreaks in order to rationalise the use of control strategies. Whereas eggs were relatively tolerant to a range of soil moisture conditions, first instar larvae were very susceptible to extremes in soil moisture. In comparison to temperate scarabs, relatively high temperatures were necessary for optimum egg viability, larval feeding, utilisation, growth and survival. Adult oviposition preference, fecundity and survival, and larval food consumption, growth and survival were all influenced by the pasture plant species eaten. All grass species tested with the exception of phalaris were favourable for population growth while phalaris and the legumes tested were unfavourable. The dynamic nature of the life cycle, the relatively low population densities and dispersal of adults made estimation of populations with reasonable levels of precision difficult. Life tables were constructed which quantified individual and generation mortalities, and identified the key factors causing population change. These were variation in natality, summer mortality and/or winter.- spring disappearance. Significant relationships were found between key mortalities and the level of population development which in turn was related to spring and summer temperatures. Two very different types of population dynamics were evident. Populations in paspalum pastures which are tolerant to feeding damage were relatively stable. These pastures act as refuge areas during adverse climatic conditions. In contrast, population development was more rapid and large population decreases occurred between autumn and spring in ryegrass based pastures which were less tolerant of damage. Ryegrass based pastures appear to be favourable habitats during spring and summer only, and adult immigration in the spring is necessary for populations to reach high levels. Since spring migration only occurs in warm years, outbreaks in ryegrass based pasture are sporadic. A population model was constructed which used mean daily air temperatures in September, October and November to simulate population densities over a 23 year period from temperature records. The prediction of high population numbers showed good correlation with years of previous outbreaks. Some density dependence was found to be necessary in the model for paspalum pasture indicating the occurrence of population regulation. It was concluded that except in years of high population numbers, population regulation does not occur in ryegrass based pastures. Rationalisation of control strategies was discussed in the light of the differences in the two types of population dynamics and the prediction of outbreaks. This research has revealed the necessity for a well designed population study in order to understand the manner in which mortalities operate and the way they affect numbers. As the first attempt at a quantitative ecological study of black beetle, the results have provided some understanding of black beetle ecology and suggested further areas for investigation.