Nymphs
In 2018, we conducted weekly nymph counts at three vineyard locations in Mendocino County from May 24- September 6. The first emergence of VCLH nymphs occurred during the week of May 14th (Table 1, Figure 1) and densities peaked on June 8, July 6, August 12 and September 4. We didn't observe WGLH nymphs until June 1, about 2 weeks after VCLH nymphs. WGLH nymph densities peaked on June 7, July 5, August 9 and August 30. In response to growing leafhopper populations, cooperating vineyards sprayed for leafhoppers at various times of the year (see black arrows in Fig. 1).
Parasitism Surveys
Leafhopper parasitism surveys were conducted in July and September across 9 vineyard locations from Lake and Mendocino Counties. This includes some sites where, in 2015-2017, we released Anagrus daanei, the key parasitoid of VCLH.
Parasitism data from both periods in 2018 were relatively similar, with relatively low rates of VCLH parasitism and high rates of WGLH parasitism (Table 2, Figure 2). This follows trends observed earlier in the season (Figure 3), and in previous years of study (2014-2017). VCLH parasitism does tend to increase towards the end of the season, but still remain <5% at most sites, although it is notable that the two sites with parasitism rates >5% were both vineyards where we released Anagrus daanei in previous years.
Anagrus daanei is present in the North Coast, but this regional population does not attack VCLH. In 2014, an alternate population that readily attacks VCLH was identified in the Sacramento Valley. Between 2015-2017 more than 30,000 of these Sacramento Valley Anagrus daanei were released into Mendocino and Lake County vineyards, yet it remains unclear whether or not these parasitoids established in the region. While parasitism of VCLH certainly occurs now in the North Coast, higher rates of attack are needed to adequately control VCLH. Genetic evaluations of Anagrus daanei reared from North Coast leafhoppers are currently underway in an attempt to better understand whether or not parasitized VCLH are being attacked by the local or introduced population of Anagrus daanei.
Table 2. Average parasitism rate of VCLH and WGLH during the two sample periods.
| VCLH | WGLH |
July | 0.6% | 38.5% |
September | 3.7% | 43.9% |
Conclusions
It is important to note the difference between the emergence time of VCLH and WGLH. Since VCLH emerge about 2 weeks earlier than WGLH, timing of sprays for VCLH will be earlier than what is typically expected for WGLH.
Due to the lack of VCLH biological control, effective early-season control of VCLH is critical. Based on monitoring, if control measures are needed, the optimum application period would be when the majority of the first-generation population is still in the earlier nymphal stages (1st-3rd instars, approximately May/June). Depending on population levels at your site, follow-up applications may be required depending on previous treatments and leafhopper population thresholds, so it is important to continue to monitor for nymphs throughout the season.
Finally, it is also important to notice how late season WGLH densities are generally lower than VCLH, even after multiple sprays. This is likely due to increased biological control of WGLH later in the season (see Figs. 2-3).
Acknowledgements
Ryan Keiffer and Lucia Varela, UCCE, conducted the 2018 leafhopper monitoring. Houston Wilson analyzed the parasitism survey data and contributed to blog posts. We would like to thank our cooperating growers, PCAs, and vineyard managers for access to vineyards in Lake and Mendocino Counties. For more information, please visit the Areawide Virginia Creeper Leafhopper website: http://ucanr.edu/sites/vclh/