Overwintering survival of grapevine scale Parthenolecanium persicae (Hemiptera: Coccidae) in the Canberra region of Australia

Scales of the genus Parthenolecanium have become pests in some grapevine growing regions of Australia but not in the wine region in and around Canberra, Australian Capital Territory, Australia. Previous work suggests that population size of scales on certain cultivars varied, with one potential reason for observed differences being the number of scales surviving winter. Parthenolecanium persicae (Fabricius, 1776) overwinters as a third-instar nymph under the bark of grapevines. We investigated whether differences in bark thickness and thermal insulation contributed to survivorship and the extent to which the scales could tolerate cold temperatures. The thickness of bark and the temperature difference across the bark varied among cultivars studied (Shiraz, Chardonnay, Tempranillo, Riesling, Viognier), but these parameters were not correlated among cultivars. Third-instar scales could survive −15°C in dry conditions but froze and were killed when wet and temperatures were cooled to −10°C. Neither the differences in bark thickness nor scale cold tolerance could explain differences in infestations among cultivars observed previously. Grapevine scale outbreaks in various Australian grape-growing regions may coincide with changes in climate as a result of increased winter survival.