THOMPSON SEEDLESS GRAPEVINE VIGOR AND ABUNDANCE OF PACIFIC SPIDER-MITE (TETRANYCHUS-PACIFICUS MCGREGOR) (ACARI, TETRANYCHIDAE)

Spider mite outbreaks have been frequently related to factors that red uce plant vigour. We conducted a held study to determine the relations hip between the abundance of Pacific spider mite, Tetranychus pacificu s McGregor, and abiotic factors related to the vigour of 'Thompson See dless' grapevines, Vitis vinifera L. Our study supports previous obser vations on the relationship between Pacific mite abundance and vine vi gour, and partially explains these differences in terms of the previou sly proposed hypotheses of differences in leaf N and leaf temperature on vines of different vigour. Pacific mite densities were 5.5-fold hig her on low-vigour compared with high-vigour vines. Total leaf N was we akly correlated with vine vigour and Pacific mite abundance. Average d aily and afternoon leaf temperatures were higher on low-vigour vines r esulting in 45.3 degrees D greater accumulation on low-vigour vines co mpared with high-vigour vines during the increasing phase of Pacific m ite population growth. This difference represented 37.4% of the degree s 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 a re discussed and an additional hypothesis is proposed to further expla in the relationship between vine vigour and Pacific mite abundance.