Effects of Tetranychus lintearius (Acari: Tetranychidae) on the structure and water potential in the foliage of the invasive Ulex europaeus (Fabaceae) in Australia

Ulex europaeus, a native of Western Europe and the Mediterranean coast, is a serious agricultural and environmental weed in many parts of the earth, including Australia. Tetranychus lintearius attacks U. europaeus in its natural range and is one of the four biological control agents introduced into Australia to regulate populations of U. europaeus. Tetranychus lintearius is specific to different species of Ulex, which enhances its status as a key biological control agent. Little is known on the physiological effects and structural changes induced by T. lintearius on U. europaeus. This article describes anatomical changes 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 a result of feeding on mesophyll parenchyma by inserting its stylets through the stomatal aperture. Unfed parenchyma cells display signs of stress, either exhibiting deformed cell organelles aligned along the walls of vacant cells or with cells with no well-defined cell organelles. Subsequent to T. lintearius feeding, leaves externally manifest bleaching. After feeding by a population of T. lintearius, leaf water potential is decreased by 30% and tender shoots lose their turgor, indicating water-related stress. Tetranychus lintearius has the potential to decelerate vigour and retard the competitiveness of U. europaeus by inflicting stress. Our findings reinforce the use of T. lintearius in the biological management of populations of U. europaeus in temperate Australia.