The effect of an irrigated buckwheat cover crop on grape vine productivity, and beneficial insect and grape pest abundance in southern California

Abstract The effect of an irrigated buckwheat cover crop on populations of beneficial insects and grape pests, vine growth, grape yield, and berry quality was investigated over 1 year in a commercial organic vineyard in southern California, USA. Buckwheat was grown in the spring and summer with additional irrigation that supplemented prevailing vine watering regimens. Treatments replicated four, three and six times respectively were: (1) buckwheat cover crop with supplemental irrigation between vine rows; (2) supplemental irrigation with no buckwheat cover crop; and (3) control plots with no buckwheat cover crop or supplemental irrigation. Flowering buckwheat was extremely attractive to beneficial insects at the beginning of the trial, resulting in 27 times more insects captured from shake sampling, compared with grape foliage in control plots. Results from sticky trap and visual count data indicated that buckwheat may enhance the abundance of generalist predators at certain times. However, densities of pestiferous leafhoppers on grape leaves in August was significantly higher (129–240% greater) in irrigated buckwheat and irrigated plots lacking buckwheat when compared with control plots. This increase in leafhopper density may be attributed to these pests preferring well-irrigated, vigorously growing vines. Mean cane weight was 222% and 170% greater for vines in irrigated buckwheat and irrigated plots lacking buckwheat, respectively, compared with controls indicating that vine vigor increased with supplemental irrigation. An irrigated buckwheat cover crop increased berry size, on average, by 0.67 mm for berries harvested on the side of the row that contained buckwheat, and reduced sugar content of berries by 3.2° Brix, compared with non-irrigated controls. Additionally, the buckwheat cover crop was associated with reduced berry quality because of insect feeding damage. Under drought conditions and water shortages, supplemental irrigation to support cover crops may result in water use penalties. Irrigated cover cropping during summer months may not be a viable pest management option for grape growers in southern California.