Spatial distributions of grape root borer (Lepidoptera: Sesiidae) in Virginia vineyards and the development of a quantitative sampling scheme

Grape root borer, Vitacea polistiformis (Harris), is an oligophagous and potentially destructive pest of Vitaceae in parts of the eastern United States. In Virginia, larvae feed on grape roots for ~22 months then pupate beneath the soil surface around the vine base. Emerging adults leave empty pupal exuviae at the soil surface, the presence of which is the only unequivocal, non-destructive indication of vine infestation. Weekly collections of pupal exuviae (July – August) from a 1-m diam, vegetation-free zone around the base of 80 sample vines in commercial vineyard blocks in Virginia were used to determine the spatial distribution of borer infestations. The grid of sample vines in each block consisted of the first vine in each of 10 panels per row in 16 alternating rows. Ten of 40 blocks sampled were divided into two infestation level groups, based on total numbers of exuviae collected during the season; 1) heavily infested = 0.88 - 6.4 pupal exuviae/vine, and 2) lightly infested = 0.25 - 0.42 pupal exuviae/vine. Spatial autocorrelation of the presence of pupal exuviae among sample points in each block was determined using semivariograms, and interpolated distribution maps were developed using kriging. Best-fitted semivariograms with spatial dependence indicated aggregation of grape root borer in heavily infested blocks within an average linear distance of 12.4 m, while an aggregated distribution pattern was not evident in lightly infested blocks. These results are discussed in relation to the development of a quantitative sampling scheme for assessing grape root borer infestations in commercial vineyards.