Grape phylloxera damage, ecology, variability, and management.

Grape phylloxera (Daktulosphaira vitifoliae Fitch) originated on North American native Vitis species. Phylloxera’s feeding and damage to wild American Vitis is distinct from its feeding and damage to V. vinifera (L.). Its feeding on leaves of the American Vitis is common and stunts cane growth, but feeding on the leaves of V. vinifera is rare. Phylloxera feed on roots less than one year old of both Vitis types without causing substantive vine damage. However, feeding on V. vinifera mature roots>1 year old results in vine death. Because phylloxera generally do not feed on mature roots of American Vitis species, vine damage does not occur allowing these plants (and their hybrid combinations) to be used as phylloxera resistant rootstocks for more than 100 years. Rootstocks that are hybrids between American Vitis and V. vinifera can exhibit resistance, but some after a period of efficacy, succumb. Such an occurrence was seen in California with the hybrid AXR#1. We developed a laboratory bioassay based on phylloxera demography to explain the selection of phylloxera virulence associated with AXR#1 and other V. vinifera containing rootstocks. This bioassay can also be used to ask whether the pure American rootstocks might also eventually select for phylloxera virulence. Such virulence has been seen in