Spatial and temporal dynamics of spider mites (Acari: Tetranychidae) in 'Thompson Seedless' vineyards
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Quantitative knowledge of beneficial and pest arthropod distribution patterns in relation to plant vigor and habitat structure is essential for the development of reliable and cost-effective methods for the assessment and management of arthropod populations in agroecosystems. The spatial and temporal distributions of the spider mites, Tetranychus pacificus McGregor, Pacific spider mite, and Eotetranychus willamettei (McGregor) were determined in 3 'Thompson Seedless' grape, Vitis vinifera L., vineyards located in the central San Joaquin Valley of California. T. pacificus was the dominant species in 2 of the vineyards, whereas E. willamettei was dominant in the 3rd vineyard. T. pacificus infestations were most severe in vineyard areas with low vine vigor compared with areas with high vigor. In contrast, E. willamettei appeared to be less sensitive to differences in vine vigor in the vineyards where the effect of vine vigor could be tested. Spatial variations in between- and within-vine distributions of spider mites were significant during most of the growing season in all vineyards. T. pacificus and E. willamettei distributions overlapped considerably on the south and north zones of vine canopy but differed between top and interior zones. The majority of T. pacificus and E. willamettei were found on leaves near the base of shoots during spring, and on midshoot leaves during summer. Aggregation indices indicated that both spider mite species were aggregated during most of the season, with T. pacificus being more aggregated between vines and within vines. The biological significance of T. pacificus and E. willamettei spatial dynamics are discussed along with implications for population assessment.
