[As of summer 2021, mites' presence in N. America was limited to California's greater San Francisco Bay Area and to Western Oregon. A 2020 study of the mites present in N. America concluded that only Mariposa plum trees formed galls--although almonds, peaches, and even cotoneaster have been effected in Europe and the Middle East.]

[F]eeding activity causes cells to grow and envelop the mites, leading to the formation of galls, which range in diameter from 1.3 to 1.8 mm, around buds on almond, apricot, plum, and peach trees. Mites may also lead to the deformation of fruit spurs .. susceptibility ... is highly variety dependent.

Adult females ... emerge in the spring, when galls crack open. Mites then migrate toward new buds ... [where t]he presence of feeding mites leads to the formation of new galls ... These galls enclose the recently-arrived mites and will expand to accommodate all of their descendants.

- Diekmann, Lucy et al.: (2021 (rev.)) Plum Bud Gall Mite: An Emerging Pest in the Greater Bay Area^©