Testing the Efficacy of Barrier Plantings for Limiting Sharpshooter Spread

Results and Discussion

Effective implementation of a barrier program should be favored in the event of clearly definable vector source habitats. That is the case for G. atropunctata, which is well known to reside primarily in riparian habitats for much of the year, with short flights into vineyards occurring typically during a limited time during the season (Purcell 1974, 1979). Moreover, previous studies suggest that sharpshooters tend to fly near the ground (Blua and Morgan 2003) and artificial screens may limit their flights (Blua et al. 2005). Collectively, these factors should favor the efficacy of a barrier planted between vineyards and adjacent riparian habitat.

In the current study, the proportion of censuses for which there were any vectors caught on the vineyard side showed substantial variability among treatments and years. The main effect of barrier treatment was not significant, but there were significant interactions between treatment and year, treatment and riparian number, and a three-way interaction between treatment, year, and riparian number (Table 1). The open treatment had the highest catch frequency in 6 of the 8 years (Figure 2), significantly more than the trees, collectively, in three years: 2002, 2003, and 2006. This effect is largely attributable to the redwood treatment, which averaged approximately half that of the open plots in these years. However, the tree species did not differ significantly from each other in any of the years (Figure 2). Analysis of G. atropunctata cumulative density yielded similar results. The main effect of barrier treatment was not significant, but there was a significant interaction between barrier treatment and year in the final model (Table 1). Densities in 2003, and to a lesser extent 2004, were markedly higher than the other years (Figure 2). In 2003 the trees, collectively, resulted in significantly lower G. atropunctata density in the vineyard than the open treatment; the redwood treatment averaged less than half that of the open treatment. Densities did not differ significantly among the treatments in the other years (Figure 2). These results suggest that effects of barrier trees were inconsistent from year to year.

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Figure 2

Mean (±SE) (A) proportion of dates over the year with at least one G. atropunctata caught in a given plot and (B) density on the vineyard side (#/census) among treatments and years. Different letters above years denote significant overall differences in vector catches among years. * denotes significant differences between open plots and those with trees collectively. Pine, redwood, and Casuarina treatments did not differ significantly from each other in any years.

A time-by-treatment interaction might be expected in this type of study, because tree growth over the eight years could influence the permeability of the barrier plantings. Although the current study did not quantify barrier size and structure, anecdotally, it is clear that the trees grew (>five-fold) to upward of 15 m by the end of the study. Yet the nature of the treatment effects did not support the prediction that barrier effectiveness was accentuated over time. Rather, the treatment effects seem to be associated with overall vector abundance, which varied greatly among years. The strongest effects of the barriers were seen in three of the four years with the highest overall vector catches (i.e., 2002i.e., 2003i.e., 2006, but not 2004). If barriers reduce vector incursion proportionately, then effect sizes are likely to be small in years of low vector abundance and documenting them would require additional effort compared to years of high vector abundance.

In addition to polyetic effects on sharpshooter incursion, it is plausible that barriers could affect the time of year that sharpshooters disperse into vineyards. Graphocephala atropunctata abundance throughout the growing season has been demonstrated to increase substantially with rising temperatures of the spring and summer (Feil et al. 2000). The seasonal timing of vector incursion is potentially important for Pierce’s disease epidemiology, because the persistence of infections over the winter depends on when grapevines first become infected (Feil et al. 2003). Grapevines infected in spring have a far greater chance of staying infected than those later in the season. Within-season vineyard G. atropunctata dynamics in 2003 and 2004 were significantly affected by census date, but not barrier treatment or number of riparian G. atropunctata (Table 1). Both years showed peak sharpshooter abundance in late spring to early summer but without any clear seasonality in the relative differences among treatments (Figure 3). In 2003 G. atropunctata was consistently more abundant in open plots, whereas in 2004 all treatments were fairly comparable over time, suggesting that barriers are not likely to delay sharpshooter dispersal into vineyards. Given that the three trees evaluated were evergreen, great differences in barrier structure and sharpshooter movement over the season might not be expected.

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Figure 3

Mean number of G. atropunctata caught in vineyard plots among treatments over the (A) 13 censuses in 2003 and (B) 26 censuses in 2004. Error bars not shown to improve figure clarity. No significant within-season effects of barrier treatment occurred in either year.

Studies of aphid-borne viruses provide the best examples of how barrier plantings can be effective cultural control measures for limiting disease incidence. In a study of green peach aphid (Myzus persicae) and cotton aphid (Aphis gossypii) transmission of cucumber mosaic virus, pepper (Capsicum annuum) plots surrounded by sorghum (Sorghum vulgare) barrier plantings had 13% lower virus prevalence and nearly three-fold higher yield than open pepper plots despite no significant effect on vector movement (Fereres 2000). In these systems an important element of the barrier effect is that the pathogen is transmitted in a nonpersistent manner, meaning that the barrier plants can act as a pathogen sink (Hooks and Fereres 2006). Given that adult sharpshooters transmit X. fastidiosa in a persistent manner (Almeida and Purcell 2003), the pathogen sink effect of barrier plants is not likely to be epidemiologically significant in this system. Thus, although the current study did not quantify disease in the field, barriers effects on pathogen spillover into vineyards are likely to be weak in most years.

The results of the current study found inconsistent and limited effects of barrier plantings on sharpshooter incursion. However, there are at least two caveats worth considering before dismissing altogether barrier plantings as a sharpshooter control measure. First, the current study explored the effect of only three tree species, chosen primarily for their ability to achieve well over 5 m in height. Of these, redwood proved the most effective barrier, even though it did not reduce significantly sharpshooter incursion in most years. It is plausible that other plant species, even those of a shorter stature, may form a barrier that is less permeable to sharpshooter movement. Second, the current study investigated the effect of individual treatments in isolation. Other studies suggest that barriers are most effective when used in tandem with other management strategies (Hooks and Fereres 2006). Specifically, insecticide application on barrier plants can improve their ability to serve as physical barriers to vector movement (Anandam and Doraiswamy 2002). Thus, incorporating barrier crops into existing chemical control programs may reduce sharpshooter movement into vineyards without necessitating application directly on the vines.