Browsing by Author "Torrance, Leigh Ami Isbell"

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    The bio-ecology of the Cape grapevine leafminer, Holocacista capensis (Lepidoptera: Heliozelidae), in the Western Cape
    (Stellenbosch : Stellenbosch University, 2016-03) Torrance, Leigh Ami Isbell; Addison, Pia; Geertsema, Henk; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The Cape grapevine leafminer, Holocacista capensis Nieukerken & Geertsema (Lepidoptera: Heliozelidae), occurring on Vitis vinifera L., has recently become of economic importance in the Western Cape, South Africa. The leafminer is of quarantine importance as cocoons are often attached to grape bunches intended for export when grapevine infestation is severe. Fortnightly monitoring efforts have indicated the occurrence of several overlapping generations within a growing season which are likely to affect potential management strategies. Adult and larval abundances tend to increase as the grapevine growing season progresses and as temperatures increase. A peak in adult and live larval abundance is reached between February and March, usually after harvest. The severity of leafminer infestation may be affected by the location of a vine within a cultivated block and strong evidence suggests that structural aspects (i.e. the trellis system used) affect the invasion potential of H. capensis. The leafminer tends to overwinter in the larval or pupal life stage within a cocoon which is often sheltered under the bark on the stem of a grapevine, in leaf litter or in a small crevice of a trellis post. Male moths have been detected throughout the colder winter months suggesting that small fluctuations may affect the developmental rate of overwintering pupae depending on the shelter experienced by cocoons. It is a possibility that adults produce offspring in surrounding foliage or are sourced by such refuges, although it is likely that eclosed adults undergo diapause and contribute to the first generation of leafmining larvae as soon as conditions become favourable. Pheromone baited traps, placed throughout the Western Cape, have indicated the presence of the moth in two of the largest table grape producing regions, namely the Berg River and Hex River Valley. The overall distribution of the moth has proven to be far more extensive than previously thought and may be present throughout southern Africa based on herbarium samples that yielded symptoms of leafminer damage. Monitoring strategies should be conducted as early as possible within a grapevine growing season to avoid severe infestations and unnecessary chemical applications depending on the bunch infestation tolerance. At an economic threshold of 5% bunch infestation, the commencement of management strategies is recommended when 442 adult male moths are caught per trap per fortnight. Results suggest that bunch infestation could be prevented altogether if management strategies were implemented when less than 87 adult male moths are caught. In cases where bunch infestation has occurred and post-harvest cold treatments are considered, a cold-treatment exposure of no less than 26 days at -0.5°C should be considered if 100% mortality is desired. Parasitoid wasps associated with the leafminer have been identified and could potentially be used in the area-wide control of H. capensis. A variety of chemical compounds have been used to control leafminer infestations in the past, although preventative applications of spinetoram seem to have had the most success in preventing infestation. Management strategies in line with the ideals of current Integrated Pest Management strategies should be explored in future research endeavours.