Browsing by Author "De Villiers, Marelize"
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- ItemDevelopment of a pest management system for table grapes in the Hex River Valley(Stellenbosch : University of Stellenbosch, 2006-03) De Villiers, Marelize; Pringle, K. L.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.A study was performed to develop a generic pest monitoring system for sampling the main table grape pests in vineyards in the Hex River Valley, Western Cape Province of South Africa. The presence of phytophagous and predatory mites on cover crop plants was also investigated as this may contribute to biological control of the phytophagous mites in vines. Life table studies for Epichoristodes acerbella (Walker), an important phytosanitary pest, were conducted to determine whether or not this pest was sensitive to high temperatures. Information gained from the latter can also be used for breeding purposes in the possible future development of a sterile insect technique (SIT) programme to control this pest. The sampling system consisted of inspecting 20 plots of five vines per plot per one to two hectares. The top fork of each of the five vines per plot was examined for Planococcus ficus (Signoret) to a distance of within 30 cm of the stem, as well as the distal 15 cm of one cane per vine for the presence of P. ficus and damage caused by Phlyctinus callosus Boh. One bunch per vine was examined for insect damage or presence, and one leaf per vine for the presence of leaf infesting arthropods, such as Tetranychus urticae Koch, P. ficus and Frankliniella occidentalis (Pergande). Corrugated cardboard bands, tied around the stem of one vine per plot, were used to monitor activity of P. callosus. Blue sticky traps, at a density of four to five traps per one to two hectares, were used to monitor activity of F. occidentalis. Pheromone traps, at a density of one trap per one to two hectares, were used to monitor activity of P. ficus, E. acerbella and Helicoverpa armigera (Hübner). All the above-mentioned inspections were done at two-weekly intervals, except traps for E. acerbella and H. armigera, which were inspected weekly. In each of the rows in which the sample plots were situated, one leaf of each of the cover crop plant species was examined for the presence of phytophagous mites and their predators. The abundance and distribution of cover crop plants were determined using a co-ordinate sampling system. Cover crop sampling was done at monthly intervals. The current threshold for P. ficus is 2% stem infestation, which is reached when more than 65 males per pheromone trap are recorded. Counting mealybugs on the sticky pads in the pheromone traps is time consuming. However, the number of grid blocks on the sticky pad with males present can be counted. When P. ficus males are found in 27 blocks on the sticky pad, stem inspections should commence. Due to the spatial association between P. ficus bunch and stem infestation, stem infestation could give an indication of where bunch infestation could be expected. The use of blue sticky traps for predicting halo spot damage, caused by F. occidentalis, is not recommended. The presence of thrips on the vine leaves could not give an indication of where to expect bunch damage, since thrips on the leaves and halo spot damage were not spatially associated. A suitable sampling method for F. occidentalis still needs to be developed. The monitoring system described here can only provide information on the infestation status of the vineyard. For E. acerbella, H. armigera and P. callosus, the traps and cardboard bands could be used to identify vineyards where these pests are present and therefore, where phytosanitary problems may arise. The presence of P. callosus under the bands was spatially associated with P. callosus damage and could be used as an indicator of the latter. The presence of drosophilid flies in the bunches could not be used as an indicator of the presence of E. acerbella in the bunches. If 5% bunch damage is used as an economic threshold for E. acerbella and P. callosus, there will be a good chance of not under spraying if control measures are applied at 1% bunch damage. Epichoristodes acerbella favoured more moderate constant temperatures, with constant temperatures of 28°C and above being unfavourable for development. The economic threshold for Tetranychus urticae Koch is six mites per leaf, or if presence-absence sampling is used, 11 to 29% leaf infestation. Three important predatory mites, that kept T. urticae under control, were found in the Hex River Valley, namely Euseius addoensis (Van der Merwe & Ryke), Neoseiulus californicus (McGregor) and an undescribed phytoseiid in the genus Typhlodromus. Various cover crop plants served as hosts for T. urticae and predatory mites. The presence of these plants created suitable conditions for the survival of these mites and may have influenced their presence on the vine leaves. In the case of phytosanitary pests, both field and pack shed inspections can be used to conclude with a 99% degree of certainty that infestation levels in the pack shed will be 10% or less, since similar results for both methods were obtained. However, more than 20 plots will have to be inspected.
- ItemIncluding irrigation in niche modelling of the invasive wasp Vespula germanica (Fabricius) improves model fit to predict potential for further spread(Public Library of Science, 2017) De Villiers, Marelize; Kriticos, Darren J.; Veldtman, RuanThe European wasp, Vespula germanica (Fabricius) (Hymenoptera: Vespidae), is of Palaearctic origin, being native to Europe, northern Africa and Asia, and introduced into North America, Chile, Argentina, Iceland, Ascension Island, South Africa, Australia and New Zealand. Due to its polyphagous nature and scavenging behaviour, V. germanica threatens agriculture and silviculture, and negatively affects biodiversity, while its aggressive nature and venomous sting pose a health risk to humans. In areas with warmer winters and longer summers, queens and workers can survive the winter months, leading to the build-up of large nests during the following season; thereby increasing the risk posed by this species. To prevent or prepare for such unwanted impacts it is important to know where the wasp may be able to establish, either through natural spread or through introduction as a result of human transport. Distribution data from Argentina and Australia, and seasonal phenology data from Argentina were used to determine the potential distribution of V. germanica using CLIMEX modelling. In contrast to previous models, the influence of irrigation on its distribution was also investigated. Under a natural rainfall scenario, the model showed similarities to previous models. When irrigation is applied, dry stress is alleviated, leading to larger areas modelled climatically suitable compared with previous models, which provided a better fit with the actual distribution of the species. The main areas at risk of invasion by V. germanica include western USA, Mexico, small areas in Central America and in the north-western region of South America, eastern Brazil, western Russia, north-western China, Japan, the Mediterranean coastal regions of North Africa, and parts of southern and eastern Africa.