Variability of responses of grape phylloxera (Homoptera: Phylloxeridae) to bioassays that discriminate between California biotypes

Two bioassays were developed to discriminate between laboratory colonies of two California grape phylloxera biotypes (A and B). These tests, adapted from previous life table studies, contrasted potential population growth of two biotypes on the grape rootstock 'Ganzin 1', a Vitis vinifera X V. rupestris hybrid. Responses to bioassays of 118 other grape phylloxera collections from viticultural regions of California were like those of one laboratory colony and unlike those of the other, enabling each to be classified as biotype A or B. Geographic variation in responses to the bioassays and variability associated with the type of root from which the collection originated were not significantly greater than the variation seen in repetitive tests on the laboratory colony of the same biotype. There was no evidence that grape phylloxera from roots of cultivated or wild grapes in California are of more than two biotypes. The apparently low variability of grape phylloxera in California suggests adoption of strategies to minimize the occurrence of new biotypes.