Ecology, behaviour and integrated control of cabbage insect pests in Tasmania

The cabbage white butterfly (CWB), diamondback moth (DM) and cabbage aphid (CA) are the most important pests of brassica crops in Tasmania. The basic biology and ecology of these pests were studied in laboratory and field experiments and commercial cabbage crops. Key biotic and abiotic factors influencing the seasonality and abundance were identified by regular sampling. Populations of CWB and DM were markedly seasonal with maximum densities recorded in December-January. In contrast, CA persisted in cabbage fields throughout the year and was the most abundant in spring and autumn. Number of generations of each species was related to the amount of heat they experienced as measured by degree-days and were 5, 5 and 13 for CWB, DM and CA respectively. Direct counts of insects per plant were the most reliable measure of abundance as conventional trapping techniques sampled insects in general flight rather than the population on the crops. Natural enemies were insignificant factors in population regulation. In the examination of the insect-plant interaction, the cabbage plant was classified into 6 readily identifiable growth stages, the development of which required a specific number of degree-days. The cabbage plant was able to compensate for insect damage however, attack by CWB at cupping, DM at early cupping, and pre heading and CA at post seedling, cupping and pre heading resulted in irreversible losses in vegetative growth and final marketable product. Plant sensitivity to defoliation is discussed in relation to the growth and development pattern of cabbage plant. Regular insecticide sprays promoted pest resurgence while lack of sanitation e.g. non-removal of crop wastes and residues, inappropriate insecticides and time of applications were found to be common features in commercial fields that aggravated pest status. A beneficial consequence of this study was that regular monitoring of crop and destruction of stubbles and crop residues became part of the commercial grower's programme. Criteria for spraying decisions were developed based on the kind and frequency of chemicals employed, the plant growth stage and the density and stage of the respective pests. Integrated control schedules including chemical insecticides and bacterial, fungal and nematode pathogen formulations were compared to recommended spray schedules. Although less damaging to natural enemies these alternative treatments were unreliable being dependent on appropriate plant growth stage and environmental conditions for effectiveness. Resource partitioning in multipest infestations was observed and the unilateral impact of infestation on plant economy was quantified. Spray application decisions based on plant stage and minimum damaging pest levels provided economic control for a lower cost. Oviposition and larval damage of lepidopterans were directly related to the degree of waxiness of cabbage cultivars. In contrast, CA was not affected by waxes but utilizes an alternative strategy involving direct testing (probing) and plant water status. Non-preference was the dominant mode of cabbage plant resistance to pest infestation. Experimental disruption of the leaf wax bloom by solvent sprays or systemic wax inhibitors was found to suppress oviposition and larval feeding in CWB and DM and alate colonization and larviposition by CA. The physiological and chemical basis of this phenomenon was investigated and it is hypothesized that CWB and DM behaviour is modified by changes in levels of wax components notably alkanes, ketones, alcohols, aldehydes and the triterpenoids a and (3 amyrin while CA is directly influenced by water status of plant as determined by probing. In summary, this dissertation provides : (i) a practical appraisal of cabbage pest control in terms of materials employed and cultural practices and makes recommendations for decision making related to pest infestation levels and plant growth stage and ; (ii) an alternative explanation to the mechanism of host plant selection by insect pests.