Are your boxwood plant growing slowly, dropping leaves prematurely, loosing their canopy? You may have Boxweed Leafminer -- Monarthropalpus flavus.
Boxwood leafminer adults are tiny, delicate mosquito-like flies that lay eggs in new leaves during spring. Larvae are orange to greenish maggots that feed within leaves. Mines initially look like yellowish leaf blotches. Older larvae feed in groups. Leaf mines merge and leaves appear blistered or overall patchy yellow. Leaf margins may remain green.
Consider replacing susceptible species with resistant plants, including B. sempervirens 'Argenteo-variegata', B. sempervirens 'Pendula', or B. sempervirens 'Suffruticosa'. Rake and dispose of infested leaves in fall and winter; leafminers overwinter as pupae inside mined leaves.
If these measures fail, please contact the Marin Master Gardeners help desk by emailing HelpDesk@marinmg.org further advice.