Insect defoliation could reduce winter hardiness of young grapevines, but such effects have not previously been quantified for field-grown vines. The impact of Japanese beetle (JB) defoliation on midwinter primary bud cold hardiness of Norton, Chambourcin, and Cabernet Sauvignon grapevines was measured during the first two years of vineyard establishment. Effect on shoot length and periderm browning by late autumn was also evaluated for first-year vines. Three treatments were used to manipulate levels of defoliation: carbaryl applied either every 7 or every 14 days during the JB flight period (mid-June to mid-August) or no insecticide spray. Cuttings from dormant canes were subjected to a controlled freezing stress each February, and the temperature causing 50% lethal injury (LT50) to primary buds was compared among cultivars and treatments. All three cultivars sustained similar levels of JB defoliation, which ranged from light (3 to 8%), to moderate (13 to 26%), to severe (38 to 48%) under the 7-day, 14-day, and no-spray regimes, respectively. Norton and Chambourcin primary buds were more cold tolerant (i.e., had lower LT50) than Cabernet Sauvignon in both years. Japanese beetle defoliation of nonsprayed vines significantly reduced cold hardiness of all cultivars in one or both winters. Notably, biweekly cover sprays were as effective as weekly sprays in mitigating the adverse impact of JB defoliation on vine cold hardiness. Defoliation also reduced shoot length of first-year vines, and may be associated with earlier termination of late-season vine growth. The previously undocumented potential for JB injury to reduce winter hardiness of young grapevines exacerbates its impact as a serious vineyard pest.
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