Measuring resistance to armoured scale insects in kiwifruit (Actinidia) germplasm

Abstract

A laboratory bioassay is described for measuring the resistance of kiwifruit vines to armoured scale insects (Hemiberlesia lataniae Signoret and Hemiberlesia rapax Comstock) using excised canes. The method measures the area of the scale cap using a template and compares relative cap sizes between genotypes. The method was tested for two species of kiwifruit (Actinidia deliciosa [A. Chev.] C.F. Liang et A.R. Ferguson and Actinidia chinensis Planch.) using a range of experimental genotypes and two commercial varieties. Large differences in the susceptibility of the genotypes to H. lataniae were observed, but much smaller differences were observed for H. rapax. The bioassay method was tested by comparing results obtained with observations of armoured scale insects on kiwifruit vines in the field. Sampling of simulated scale populations showed that differences in population mean scale cap areas of 0.3–0.5 mm² could be detected with sample sizes of 50–200. A bioassay of H. lataniae growth on an experimental genotype, comparing it with the commercial kiwifruit cultivars ‘Hayward’ and ‘Hort16A’ and using a template to categorize scale cap size into 0.2 mm² size categories, showed that the technique was quick, reliable and able to discriminate between kiwifruit varieties. This method could be easily adapted to test the susceptibility of germplasm from a variety of different horticultural plants to a wide range of armoured scale insect pests.

Keywords:

• Diaspididae
• bioassay
• kiwifruit
• host plant resistance

Acknowledgements

We thank Doug Allan for assistance with the first experiment. Cathy McKenna and Dave Rogers made valuable comments on an earlier draft. The work was funded by the New Zealand Foundation for Research Science and Technology (project CO6X0301, Insecticide Risk Reduction) and ZESPRI Group Ltd.