What is the specific purpose of applying horticultural oils when treating visible, above-ground woolly aphid sites during dormancy?

To smother overwintering eggs or young stem mothers

Horticultural oils are a primary tool for managing above-ground woolly aphid populations, but their effectiveness is maximized when applied during the dormant season. This timing is strategic because it targets the life stages that are vulnerable before the heavy waxy covering is fully developed or before massive spring reproduction occurs. The oil works by creating a physical barrier that suffocates, or smothers, any overwintering eggs that have been laid on the bark or twigs, or any young stem mothers that have recently emerged to begin establishing the new season's colonies. This treatment method is less effective against heavily established, fully woolly colonies during the growing season because the dense protective flocculent material shields the aphids beneath from the oil's suffocating action.