Organic grape growers in Mendocino and Lake County have been experiencing severe outbreaks of the Virginia creeper leafhopper (Erythroneura ziczac) for the past 3 years. Feeding by E. ziczac causes leaf stippling and reduced photosynthesis which can impact crop yield and quality. The primary natural enemies of E. ziczac are the small ‘mymarid’ egg parasitoids Anagrus daanei and Anagrus tretiakovae. A related pest, the Western grape leafhopper (Erythroneura elegantula) is also parasitized by A. daanei as well as Anagrus erythroneurae. Erythroneura ziczac and E. elegantula are commonly found together in North Coast vineyards. Anagrus daanei is the parasitoid species of most importance for E. ziczac control, whereas A. tretiakovae is rarely found in California.
Our approach to improving E. ziczac control involves a combination of short- and long-term strategies. Short-term work focuses on the evaluation of Organic Materials Review Institute (OMRI) approved pesticides. In 2014, we tested Stylet oil and DeBug® Turbo (applied twice) and Pyganic® (applied once) on the development of the first leafhopper brood. All of these products significantly reduced E. ziczac nymph populations relative to an untreated control.
Long-term strategies are focused on the identification and evaluation of Anagrus parasitoids to improve biological control. A survey in Mendocino, Lake, Napa (Pope Valley), Yolo and El Dorado County vineyards found that E. ziczac parasitism was consistently low (0-2%) with the exception of Yolo County, where rates reached 10-15%. Surprisingly, A. daanei was attacking E. elegantula in all of the surveyed vineyards, but only in Yolo County was it attacking both E. elegantula and E. ziczac. Therefore, we questioned whether or not the A. daanei in Yolo County are the same species as the A. daanei that don’t attack E. ziczac in other regions. Molecular comparison of the A. daanei from different Californai regions is still in progress, but to date morphological evaluations have not shown any differences among the A. daanei populations tested. We conducted a trial in which we forced A. daanei from Mendocino County onto E. ziczac eggs in order to see whether or not, in the absence of their preferred E. elegantula host, they would attack the E. ziczac eggs. Findings from this study indicated they would not.
In another trial, we separately inoculated potted grape vines with E. ziczac eggs from Mendocino, Lake and Yolo County and then exposed sets of these vines to the A. daanei in each of these regions. Results showed fairly consistent parasitism of all three E. ziczac populations by the A. daanei in Yolo County. Having verified that the A. daanei in Yolo County will readily attack the E. ziczac population in Mendocino and Lake County, we now feel that there is adequate evidence to support a collection and re-release program in which A. daanei from Yolo County are introduced into Mendocino and Lake County vineyards. This redistribution of California parasitoid material would be carried out in conjunction with an area-wide IPM program to promote additional best management practices to further reduce E. ziczac outbreaks.