Phenological resistance of grapes to green June beetle damage

Changes in fruit characteristics associated with ripening increase the vulnerability of crops to insect depredation, making it difficult for growers to protect cultivated fruits from pest injury close to harvest. This study evaluated phenological resistance, the use of cultivars that ripen before or after peak pest activity, for reducing injury to grapes (Vitis spp.) by the green June beetle [GJB] (Cotinis nitida L.), an obligate feeder on soft, ripe fruits. Accumulation of sugars, softening of berry skins, and recruitment of GJB feeding aggregations were monitored on replicated vines of early-, mid-, and late-season ripening cultivars that require from 85–125 growing days from bloom to harvest. Green June beetle flight peaked in late July and early August coinciding with later stages of veraison of early-season ripening cultivars which recruited numerous GJB feeding aggregations resulting in > 95% crop loss. Small (1–2 wk) phenological differences between mid-season ripening cultivars and peak GJB flight translated to marked differences in injury, whereas cultivars that ripened in mid-August or later, after GJB flight had waned, sustained little or no damage. Trapping experiments confirmed that the tougher berries and low sugar content of less-ripe fruit clusters inhibited beetle feeding and induction of yeast-mediated volatiles responsible for GJB host-location. Implications of these findings for sustainable or organic management of GJB and other near-harvest fruit pests are discussed.