Management of Riparian Woodlands for Control of Pierce’s Disease in Coastal
Removing and subsequent control of those plants that serve as BGSS breeding hosts and replacing them with native trees and shrubs continues to dramatically reduce BGSS abundance. Sticky traps and sweep net sampling used to monitor BGSS estimated the degree of reduction of BGSS from vegetation management. Over a three year period (1996-98) at our experimental site at Conn Creek, near Yountville, catches of BGSS on sticky traps were reduced from 95 to 99% in the plot where vegetation had been removed and replaced compared to an adjacent undisturbed control. A second experimental site, along the Napa River near Yountville, that had been cleared in 1996 also had large reductions (more than 70%) in activity of the BGSS; catches at a single trap near adjacent undisturbed vegetation accounted for a majority of the BGSS activity that we detected in the vegetation management plot. The most common major breeding hosts of BGSS in Napa and Sonoma riparian zones were California mugwort, Himalayan or California blackberry, wild grape, elderberry, and large periwinkle. Less commonly occurring but highly preferred breeding hosts were Mexican tea, mulefat, stinging nettle, California Brickellbush, and cocklebur. Trees planted in areas where mature trees had been removed in 1995 greatly exceed the growth of trees planted in competition with established trees. Planting shrubs such as wild rose, spice bush, and snowberry and leaving poison oak appears to replace understory growth of removed breeding hosts. The natural infectivity of BGSS from experimental plots ranged from 5 to 45% over a three year period at three sites. Very few BGSS could be tested for infectivity in the treatment plots because of their scarcity, so we could not assess the impacts of vegetation management on levels of natural infectivity of BGSS with the Pierce’s disease bacterium. The experimental management methods of removing relatively few species of plants drastically reduced populations of the primary vector (BGSS). The replacement plantings of riparian woodland species should increase biodiversity and long term environmental quality of the managed sites. Reductions of BGSS activity should reduce the spread of Pierce’s disease, but the circumstances and time available for the project have not yet demonstrated effects on disease. We intend to evaluate these effects by following the spatial patterns of Pierce’s disease that occur adjacent to the boundaries between our treatment and control plots in at least 2 of the 3 sites used in our study. Vineyards along the riparian study sites have been recently replanted, so it was not possible to collect adequate data on the effects of reducing BGSS activity in riparian zones on activity in adjacent vineyards. Studies are in progress by other researchers to evaluate environmental impacts of riparian vegetation management.