1. Department of Biochemistry and Microbiology, Rhodes University, P.O. Box 94, Grahamstown, 6140 South Africa
2. Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown, 6140 South Africa
3. Citrus Research International, P.O. Box 20285, Humewood, Port Elizabeth, 6013 South Africa