Ulex europaeus, a native of Western Europe and the Mediterranean coast, is a serious agricultural and environmental weedin many parts of the earth, including Australia. Tetranychus lintearius attacks U. europaeus in its natural range and is one ofthe four biological control agents introduced into Australia to regulate populations of U. europaeus. Tetranychus linteariusis specific to different species of Ulex, which enhances its status as a key biological control agent. Little is known on thephysiological effects and structural changes induced by T. lintearius on U. europaeus. This article describes anatomicalchanges and evaluates the physiological effects in U. europaeus consequent to feeding by T. lintearius for more than 100 d.This study demonstrates that feeding by T. lintearius reduces the quantity of photosynthetic tissue in U. europaeus, as aresult of feeding on mesophyll parenchyma by inserting its stylets through the stomatal aperture. Unfed parenchyma cellsdisplay signs of stress, either exhibiting deformed cell organelles aligned along the walls of vacant cells or with cells withno well-defined cell organelles. Subsequent to T. lintearius feeding, leaves externally manifest bleaching. After feeding by apopulation of T. lintearius, leaf water potential is decreased by 30% and tender shoots lose their turgor, indicating water-relatedstress. Tetranychus lintearius has the potential to decelerate vigour and retard the competitiveness of U. europaeus byinflicting stress. Our findings reinforce the use of T. lintearius in the biological management of populations of U. europaeusin temperate Australia.