Doctoral Degrees (Conservation Ecology and Entomology)

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Browsing Doctoral Degrees (Conservation Ecology and Entomology) by Subject "Agricultural pests"

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    Biology of subterranean populations of woolly apple aphid, Eriosoma lanigerum (Hausmann)(Homoptera:Aphididae), in apple orchards
    (Stellenbosch : Stellenbosch University, 2000-03) Damavandian, M. R.; Pringle, K. L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: A study was made of the basic biology of subterranean Eriosoma lanigerum (Hausmann) populations in apple orchards in the Western Cape Province of South Africa. The absence of cornicles and the presence of a vulva could be used to identify 1st instar nymphs and adults respectively. Body length, body width and length of the hind femur are useful for separating 2nd , 3rd and 4th instars. However, separation of 2nd from 3rd instar nymphs was very unreliable. Maximum population growth rate was at 23°C while at 30 °C population growth was zero. The estimated minimum and maximum threshold temperatures for development were 4.32 and more than 30°C respectively. Numbers of underground E. lanigerum in soil samples taken using mechanical and hand augers were similar. However, numbers of aphids in samples were influenced by the distance from the trunk at which the samples were taken and the presence and the type of root material in the samples. More aphids were recorded close to the trunk, and at a given distance from the trunk more aphids were recorded if there was root material in the sample, especially if the roots were galled. early autumn (February, March) and declined during winter, especially if the winter rainfall was high. These cycles coincided with the nitrogen cycles in the roots. Embryos were also present in all instars throughout the year. There were more embryos in the 4th instar and adult aphids than in the other instars. The highest number of embryos in the 4th instar and adult aphids occurred during spring, which coincided with peak nitrogen levels in the roots of apple trees. Nitrogen levels in root material adjacent to galls and in ungalled roots were higher than in root galls. A number of entomopathogenic fungi, including species of Conidiobolus, Hirsufella and Beauveria were found. Their present contribution to biological control is not known. A straw mulch suppressed subterranean E. lanigerum population levels at least as well as the soil insecticide, imidacloprid, currently in use.