Abstract
When Oncometopia nigricans (Walker) and Homalodisca coagulata (Say) were collected from grapevines in vineyards and tested for Pierce’s Disease (PD) natural infectivity, the bacterium was transmitted only after the 3rd week in Apr. 1976 and the 3rd week in June 1977. O. nigricans and H. coagulata collected from wild weed hosts in winter and early spring were never found to be naturally infective prior to the time that the PD bacterium could be acquired from cultivated grapevine. O. nigricans adults were first observed on cultivated grapevines in early Mar., peaked in late Mar., and slowly declined through the summer, whereas the earliest H. coagulata adults were observed on grapevines in May and increased slowly until Sept. or Oct. O. nigricans would be expected to have the greater epidemiological impact in the spread of PD due to its earlier arrival and higher populations on grape.
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