Genetics of resistance in the spider mite Tetranychus species

Abstract

This thesis comprises an investigation into the genetics of resistance to parathion in strains of T. urticae. The study involved, on the one hand, the breeding, isolation and crossing of spider mites in the French Dwarf beans, the latter grown in trays or pots and surrounded by water; and on the other, the testing of various generation of mite progeny using a topical application technique. A comprehensive survey of literature dealing with genetical studies of resistance in general, and with genetical studies of resistance in spider mites specifically, was presented. This was a pre-requisite for understanding and interpreting the shapes and slopes assumed by the dosage-mortality regression lines found for the various progeny tested. The results and conclusions were generally in accord with those of Helle (1962) for parathion resistance in the German strains, but not with those of Dittrich (1961, 1963). The latter author considers that resistance to meta-systox in the German strains is due to a number of dominant semi-lethal genes. These could be removed by inbreeding to reveal a single Mendelian recessive gene. Evidence was presented which showed that the contradictions in findings derive from genetic (probably allelic) differences in the mechanism of resistance to parathion and resistance to meta-systox. Also, it was considered that the quality of the resistance character had changed considerably during the period between the various investigations. In spite of this, however, there is a body of evidence which is at variance with, or fails to support, Dittrich's actual conclusions, that resistance to systox is polygenic. With the development of genetic markers in T. urticae, the study of resistance in this species is likely to become more directed and less speculative. So basic in its life history and genetics, so easy to manipulate, and so important as a pest in agriculture, this spider mite could well assume the role of an object of universal research. In the field of entomology, Drosophila has played this role over the last four or five decades.