M.Sc. (Zoology) / This study centres around the findings on mites (Acari: Eriophyidae) (also referred to as "eriophyids") as potential biological control agents of weeds in South Africa. The use of mites in the biological control of weeds is briefly reviewed. Four species of the family Eriophyidae have been used against weeds in other countries. Seventeen eriophyid species, causing symptoms possibly of significance to biological weed control of 12 plant species regarded as weeds in South Africa, are discussed. Methods to rear and study eriophyids in the laboratory have not yet been fully developed and studying these practically invisible arthropods, very closely associated with their host plants, frequently presents difficulties. Reported methods, and those used for the present study, are reviewed and discussed. The eriophyid, Aceria lantanae (Cook), causing flower and leaf galls on Lantana camara L. could not be successfully established in a quarantine laboratory on potted plants propagated from cuttings of L. camara forms from South Africa. The results obtained during this attempt were inconclusive. The failure of establishment on local L. camara forms could be attributed to a variety of factors, of which rearing methods and host incompatibility are the most likely. Convolvulus arvensis is a troublesome agricultural weed and occurs in especially the Orange Free State and the eastern and southern parts of the Cape Province. Biological control may possibly contribute to curbing this weed in South Africa. The eriophyid, Aceria malherbae Nuzzaci, causing deformation and galling on C. arvensis is regarded as one of the most promising candidates for the biological control of this weed. It was imported and successfully established on potted plants in a quarantine laboratory on biotypes of the weed occurring in South Africa. The biology and host specificity of A malherbae are accounted for, by using reported information and data obtained during this study. The biology of A. malherbae broadly conforms to the biology of other eriophyid species. This species has a narrow host range, being restricted to species very closely related to C. arvensis. The symptoms caused by this eriophyid on C. arvensis are described, and seem especially harmful to the reproductive potential of the plant. Two eriophyid species from L. camara from Paraguay, Tegonotus stefneseri and Paraphytoptus magdalenae, are described as new species. An illustrated as well as a dichotomous key to species (Eriophyoidea) described from L. camara, including the two new species, are provided. A key, and a review of known Tegonotus species, incorporated in a compendium based on the original descriptions of 74 species, are included. Shevtchenkella Bagdasarian is considered to be a junior subjective synonym of Tegonotus Nalepa.

D.Sc. (Botany) / Please refer to full text to view abstract