Where do Woolly Apple Aphid colonies frequently cluster on aerial portions of apple trees?

Along twigs and in branch crotches

Woolly Apple Aphid colonies, when established above ground on their host trees, show a distinct pattern of congregation in protected and sheltered areas. They are frequently found clustered along twigs, where they can feed relatively undisturbed, and critically, within branch crotches—the junctions where smaller limbs meet the larger limbs or the main trunk. These sheltered spots provide physical protection from wind, rain, and some levels of direct exposure to sunlight, allowing the waxy, flocculent material to remain effective as camouflage and defense. Infestations are also often initiated or heavily concentrated around areas where the tree has sustained wounds, such as old pruning cuts or breaks in the bark, as these represent points of weakened plant defense.