RESULTS AND DISCUSSION

Estimates of imidacloprid concentrations in planta match the classes for management practices. All but one of the untreated sites had no detectable imidacloprid in leaf tissue samples. Conversely, both the regularly treated and those treated intermittently (i.e. “mixed” treatment history) had markedly higher insecticide concentrations (Fig. 2A). These results support the classification of most sites into the three treatment categories. Notably, however, several of the mixed treatment sites tended to have equivalent (or even higher) average imidacloprid concentrations to those of consistently treated sites in some years, indicating recent treatments occurred at those sites.

Over the two years of sticky trap monitoring more than 150 sharpshooters were caught in vineyards, the vast majority of which were the invasive glassy-winged sharpshooter. For example, we collected more than 75 sharpshooters, both H.

vitripennis and H. liturata, among all sites between November 2011 and May 2012. In both years, mean sharpshooter catch depended on treatment history grouping.

Untreated sites had the highest sharpshooter catches, consistently treated sites were intermediate, and mixed sites had the lowest catches over this period (Fig. 2C). It plausible that regularly treated sites had relatively higher catches than mixed treatment sites because their location exposes them to inherently more sharpshooters (i.e.

proximity to citrus); hence encouraging more frequent applications by growers.

Regardless, these results overall support the idea that imidacloprid application reduces vector pressure, through mortality or by having anti-feedant effects⁽⁵⁾, which may underlie any differences in disease spread among the treatment categories.

Fig. 2. A) Mean imidacloprid concentration, B) mean estimated Pierce’s disease prevalence, C) mean sharpshooter catch on sticky traps, and D) mean natural enemy catch on sticky traps among 34 Temecula vineyards based on treatment category:

untreated, intermittently treated (i.e. “mixed”) or consistently treated with imidacloprid.

Data shown for only 2012.

Estimated Pierce’s disease prevalence varied substantially among fields from several sites that had no detectable cases of disease to a single site with on the order of 10% infection. Yet, overall prevalence among the three years of disease surveys was consistently low, with an overall mean of approximately 1.3%. The results suggest that at least part of the variability in prevalence is attributable to systemic insecticide treatment history (Fig. 2B). Specifically, Pierce’s disease prevalence tended to be higher in untreated sites, was lowest in the consistently treated sites, and was intermediate in intermittently treated sites. Though not significant, this trend is at least consistent with the expectation of beneficial, albeit only slightly, effects of imidacloprid application on reducing vector pressure and pathogen spread.

Based on sticky trap catches there were little differences among the treatment categories with respect to natural enemy abundance in vineyards. We found similarly high catches of several natural enemy taxa, especially spiders, at all three types of sites (Fig. 2D). Tap sampling results showed a slightly different pattern. Generalist natural enemies were more common in untreated sites than mixed or consistently treated sites (Fig. 3A). However, this result does not appear to stem from disruption of natural enemy activity at treated sites. Rather, the abundance of all non-predatory arthropods (including several pest species such as grape leafhopper) were up to 7-fold higher on average at untreated sites compared to treated sites (Fig. 3B). Collectively these two sources of data indicate that imidacloprid applications do not dramatically upset natural enemy activity in vineyards. Rather, if anything, natural enemy populations may track pest populations, which are strongly affected by whether fields were treated with imidacloprid.

Fig. 3. A) Mean natural enemy abundance, and B) mean non-predatory arthropod abundance (+/- SE) based on tap sampling in Temecula Valley vineyards in 2012.

CONCLUSIONS

For many Southern California grape growers vector control is seen as a critical component to managing Pierce’s disease within vineyards. Yet epidemiological theory suggests that disease management via vector control is only practical for pathosystems in which vectors are of limited efficiency. In such situations, as seems to be the case for H. vitripennis, the expectation is that reducing vector populations should limit the potential for pathogen spread.

Broadly, the work presented here suggests that within-vineyard chemical control

has the potential to reduce vector pressure and curb pathogen spread. Moreover, at least in this system, there do not appear to be strong non-target effects of the preferred systemic insecticide on natural enemies that would contribute to secondary pest outbreaks. However, three aspects of these results are worth exploring further. First, the observed differences in prevalence among years appear to be based on chemical control strategies, with untreated vineyards having the highest average prevalence, but whether those differences are due to recent management, historical artifacts, or differences in vector pressure at individual vineyards remains unclear. Ultimately analyses that grapple with year-to-year changes in prevalence are needed (i.e. disease incidence), which are ongoing. Second, it is interesting to note that there were little to no differences in Pierce’s prevalence between the intermittently and consistently treated sites – especially in 2010 and 2011 surveys (MP Daugherty, unpublished data).

This suggests that it may not be necessary to treat vineyards every year to effectively manage sharpshooter populations. Rather, in systems such as this one where there is substantial interannual variability in vector populations, targeting only the “outlier”

years may be sufficient; assuming such years can be identified prior to treatment decisions. Finally, it is worth noting that such modest levels of observed disease are likely attributable to very low vector populations that exist currently relative to conditions during the peak of the H. vitripennis outbreak in the Temecula Valley region 15 years ago. The apparently slight differences in disease incidence among treatment categories might be expected to be substantially greater should the effective areawide control of sharpshooters be discontinued.

ACKNOWLEDGEMENTS

Thanks to B. Drake for help in identifying field sites, and the numerous Temecula Valley vineyard owners for their cooperation in allowing access to their fields. Funding for this project was provided by the California Department of Food & Agriculture Pierce’s Disease and Glassy-Winged Sharpshooter Board to MPD.

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Habitat Effects on Population Density and Movement of