Characterize virulence and life cycles of new phylloxera strains


  1. Characterize virulence and life cycles of new phylloxera strains ‘Non A/non B’ strains were tested in the laboratory and shown to have increased aggresivity on some rootstocks. Based on the population growth data, overall aggresivity was too low for us to predict that these strains would cause economic losses to strongly resistant rootstocks with no V. vinifera parentage. A ‘Harmony’ adapted strain (strain 4) from Napa Co. had high population growth in the laboratory tests and was observed to cause damage to field vines. This strain represents a new biotype and this finding supports recommendations not to use this rootstock for phylloxera control. A German strain, reported to be damaging to 5C there, has been established in the laboratory and the first tests indicate that it is probably not virulent enough to cause field losses. The tests completed to date were plagued by technical problems and will be repeated. We can not guarantee the permanent stability of phylloxera resistance in rootstocks; understanding variation in phylloxera aggresivity from California and the world will help us evaluate the risks for the future. People from Andy Walker’s and our laboratory, made comparisons of phylloxera DNA. Results indicate that at least several strains of biotypes A and B exist. Our work failed to identify markers for biotypes or for geographical origin of strains. One interpretation of these data is that biotype B was selected more than once (rather than spreading from a single epicenter). We conclude from this interpretation that AXR#1 is not safe to use anywhere in California and quarantines will not prevent new occurrences of biotypes.
  2. To determine the potential for control tactics other than rootstocks Mocap tests in large planter boxes failed to control phylloxera or protect vines. Greenhouse studies demonstrated that Fusarium and Pythium fungi contribute to vine damage associated with phylloxera feeding. Although vine damage is caused by phylloxera alone, presence of secondary fungi increased damage about two-fold. The two most common fungi observed in the greenhouse studies were also found to be present in phylloxera-caused feeding wounds on field vines. The contribution of fungi to phylloxera based vine damage makes economic injury levels difficult to establish for this insect. This work also suggests that curative insecticide treatments in the field will not result in rapid reversal of vine-damage symptoms. We are collecting field data in a Sonoma County experiment to determine whether prophylactic insecticide treatments will slow spread of phylloxera or prevent damage. The first year’s data are encouraging. We have been screening new chemical and biological control agents in the laboratory. Some manufacturers are encouraged by the tests and are planning to test some with field trials. Others will not.
  3. To evaluate the factors that affect phylloxera populations, damage and spread Work on temperature thresholds for grape phylloxera suggests that temperature dependency of population growth is complex. Temperature thresholds vary with the insect stage and change with pulses of unseasonable temperatures. These results argue against use of a day-degree model for this insect.