Differences in host utilization by California strains of grape phylloxera

Grape phylloxera, Daktulosphaira vitifoliae (Pitch), were collected from phylloxera-resistant rootstocks as part of a survey of the diversity of the insect in California. In laboratory bioassays on root pieces, four strains (lineages descending from a single founder) differed from colonies classified as California biotypes A and B. Population growth of all four strains was slower than both biotypes in bioassays on Cabernet Sauvignon. On AXR#1 root pieces, ail strains were weaker than biotype B, and three were more vigorous than biotype A. There was greater survival to adulthood of the four strains on pieces of many other rootstocks, mostly on callus or newly formed roots. One strain grew well and formed tuberosities on Harmony rootstock. That strain, collected from Harmony, is inflicting severe damage to vines at the collection site. No other rootstocks that are thought to be resistant to biotypes A and B appear to be susceptible to the new strains.