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Comparison of Efficiency and Propensity as Measures of Vector Importance in Zucchini Yellow Mosaic Potyvirus Transmission by Aphis gossypii and A. craccivora. Caiyao Yuan, graduate research assistant, Department of Plant Pathology, University of Hawaii at Manoa, Honolulu 96822; Diane E. Ullman, professor, Department of Entomology, University of California, Davis 95616. Phytopathology 86:698-703. Accepted for publication 19 February 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-698.

A comparison of efficiency and propensity as measures of vector importance in zucchini yellow mosaic potyvirus (ZYMV) transmission by Aphis gossypii and A. craccivora was made. Efficiency was measured in the laboratory with single aphids exposed in sequence to an infected plant and then four noninfected zucchini test plants. Individual A. craccivora, a species that does not colonize zucchini, had a significantly higher ZYMV transmission efficiency than did A. gossypii, a species that colonizes zucchini. Similar assays for ZYMV transmission efficiency with groups of aphids resulted in significantly higher incidence of infection for both aphid species. Propensity was measured by arena tests in which aphids could move between plants and feed without interference. A. craccivora had a significantly higher propensity to transmit ZYMV (52.77%) than A. gossypii (11.73%). Thus, both tests of efficiency and propensity showed that A. craccivora is responsible for more infections per aphid than A. gossypii. No significant variation in propensity was found between several A. gossypii populations from different Hawaiian islands. Duration of virus retention did not vary significantly between aphid species; however, the duration of time required for initiation of the first probe was significantly shorter for A. craccivora than for A. gossypii. In addition, A. craccivora dispersed to significantly more plants than A. gossypii. These characteristics may contribute to A. craccivoraís higher transmission efficiency and propensity. In contrast, the increased time required for A. gossypii to initiate the first probe and the reduced dispersal to surrounding plants may relate to its lower transmission efficiency and propensity. These findings highlight the potential importance of noncolonizing, transient vector species in the epidemiology of nonpersistently transmitted viruses and suggest that measures of propensity are most accurate in determining the importance of various vector species in epidemics.