The impact of Japanese beetle (Popillia japonica Newman, JB) defoliation on vine growth and mid-winter primary bud cold hardiness with three different carbaryl spray regimes (non-sprayed control, 7 d spray, and 14 d spray) was evaluated on first year vines of six economically important grape cultivars including: ‘Concord’, ‘Norton’, ‘Cabernet Franc’, ‘Cabernet Sauvignon’, ‘Chambourcin’, and ‘Frontenac’. ‘Concord’ grapes, which have significantly tougher and thicker leaves, with a lower water content and nitrogen level, show resistance to JB attacks with <10% total defoliation for all three spray treatments. The other five cultivars exhibit high susceptibility to JB defoliation with significant levels (p<0.0001) of defoliation between the three spray treatments. A significant level of reduction (p<0.0058) in total growth of the dominant trunk was observed between the non-sprayed and 7 d spray treatments for susceptible cultivars. As defoliation levels increased with the decrease in frequency of carbaryl spray regimes, the LT50 of the primary buds increased. The LT50 of ‘Norton’, ‘Chambourcin’, and ‘Cabernet Sauvignon’ increased by 19%, 23%, and 10%, respectively, when grapevines were defoliated 44%, 46%, and 48%, respectively. This study provides sustainability information for one-year old vines under increased herbivory pressure by JB. Keywords: Japanese beetle, grapevines, defoliation, LT50