During a routine winter pruning of 15-year-old apple trees, an unsettling discovery was made—large, brown, hard, swollen lumps clinging to the branches., Even the espaliered apple trees in the front yard were not spared. These growths appeared concerning, almost as if they were harming the trees, but removing them immediately seemed risky. Would scraping them off cause further damage?
Consulting UC Integrated Pest Management (UC IPM) for expert guidance was the best course of action.. Lo and behold, a closer look revealed the culprit: the tiny woolly apple aphid (Eriosoma langligerum). At just 1/20 of an inch, these pests are easy to overlook, especially since they hide beneath a coating of white wax. Despite their size, they can cause harm by feeding on limbs, trunks, roots, and shoots, forming colonies in bark cracks and pruning wounds.
Woolly apple aphids rarely cause serious harm to the overall health of apple trees. However, their feeding leads to unsightly swollen, warty galls on the bark. While older trees are generally less affected by this invasion, these unsightly galls can still be a nuisance.
Unfortunately, the only real solution, is pruning the damaged branches. Since the damage is considered merely an "aesthetic issue," removal isn’t essential, but for those who prefer a cleaner look, cutting away the galls is the best option. The challenge arises when the damage appears on the main limbs, making removal more difficult. Some apple varieties are less susceptible to woolly apple aphids, offering an alternative for those looking to prevent future infestations. However, switching varieties means starting the espalier process from scratch—a daunting task for any gardener.
Moving forward, a more vigilant approach will be essential. Preventive measures like horticultural oil and insecticidal soap may curb further damage and keep these pests in check. Once the trees leaf out again in the coming months, these unsightly galls may no longer be visible, but their presence will still be felt. Staying proactive is the best defense against these hidden invaders, ensuring healthier trees for seasons to come.
