'Thompson Seedless' grapevine vigour and abundance of Pacific spider mite (Tetranychus pacificus McGregor) (Acari, Tetranychidae)
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Spider mite outbreaks have been frequently related to factors that reduce plant vigour. We conducted a field study to determine the relationship between the abundance of Pacific spider mite, Tetranychus pacificus McGregor, and abiotic factors related to the rigour of 'Thompson Seedless' grapevines, Vitis vinifera L. Our study supports previous observations on the relationship between Pacific mite abundance and vine vigour, and partially explains these differences in terms of the previously proposed hypotheses of differences in leaf N and leaf temperature on vines of different vigour. Pacific mite densities were 5.5-fold higher on low-vigour compared with high-vigour vines. Total leaf N was weakly correlated with vine vigour and Pacific mite abundance. Average daily and afternoon leaf temperatures were higher on low-vigour vines resulting in 45.3D greater accumulation on low-vigour vines compared with high-vigour vines during the increasing phase of Pacific mite population growth. This difference represented 37.4% of the D requirements for the development of 1 Pacific mite generation and could be partially responsible for the observed difference in Pacific mite abundance in relation to vine vigour. Other confounding factors are discussed and an additional hypothesis is proposed to further explain the relationship between vine vigour and Pacific mite abundance.
