Grape growers in Mendocino/Lake County are experiencing outbreaks of the Virginia creeper leafhopper (Erythroneura ziczac) [Hemiptera: Ciccadellidae]. Feeding by E. ziczac causes leaf stippling, loss of photosynthetic capacity and can ultimately reduce crop yield and quality. This leafhopper is also thought to transmit the newly discovered grapevine virus “RedBlotch Disease”. The primary egg parasitoids of the Virginia creeper leafhopper (VCLH) are Anagrus daanei and Anagrus tretiakovae [Hymenoptera: Mymaridae]. A related vineyard pest, the Western grape leafhopper (Erythroneura elegantula, WGLH) is also parasitized by A. daanei as well as Anagrus erythroneurae. VCLH and WGLH are commonly found together in many North Coast vineyards. In California, A. daanei is the parasitoid species of most importance for VCLH control, as A. tretiakovae has never been found in California.
Over the past year we focused on determining parasitism levels and parasitoid species present in vineyards infested with VCLH and WGLH. Mendocino County surveys found that VCLH parasitism was practically non-existent while parasitism of WGLH eggs occurred with relatively high frequency. We isolated and reared the Anagrus species attacking WGLH eggs in these vineyards and found 87%A. erythroneurae and 13%A. daanei. While A. daanei is known to attack both WGLH and VCLH eggs, they are only attacking WGLH in Mendocino County. We subsequently reared Anagrus specimens from parasitized VCLH eggs from a vineyard in Yolo County. These specimens were identified as A. daanei. This finding brings into question the A. daanei populations found in these two counties – why is A. daanei attacking VCLH in Yolo, but not in Mendocino County? We will address this with our work in 2014.
We sampled for Anagrus and leafhopper species in the natural and cultivated habitats surrounding North Coast vineyards. While A. erythroneurae could be found on many host plants, we found A. daanei was very restricted in host diversity and overall in low abundance, which could explain the lack of VCLH parasitism. While we did find small populations of VCLH and WGLH on a variety of non-crop plants during the growing season, both pests appeared to overwhelmingly prefer cultivated grapes during the growing season and in the winter reside in vineyard leaf litter. The most common non-crop host was wild grape and VCLH actually appears to be reproducing on it. Work in 2014 will further evaluate VCLH use of wild grapes as refugia and reproductive sites.
We conducted a spray trial to determine effectiveness of OMRI approved products for VCLH control. Three insecticides were tested: Pyganic®, Mycotrol® and Grandevo™. Applicationtiming was scheduled to target young leafhopper nymphs (mid-June). Pyganic® significantly reduced nymph populations compared to the control while Mycotrol® and Grandevo™ were not significantly different from the control after the first or the second application. Further trials are planned in 2014 to evaluate application timing and frequency for non-OMRI products.