Physiological Responses of Cotton to Two-Spotted Spider Mite Damage

Spider mites are important pests of cotton ('Gossypium hirsutum L.'), capable of dramatically affecting growth, yield, and fiber quality. This study investigated the physiological response of cotton leaves to feeding damage by the two-spotted spider mite ('Tetranychus urticae Koch') in two experiments in the field over two seasons. Mite colonies initially established and developed in the basal areas of leaves, where the leaf blade joins the petiole. These infestations caused rapid and severe reductions in photosynthetic rate, stomatal conductance, transpiration, transpiration efficiency (TE), and chlorophyll content. In basal areas, a peak of 68 adult female mites per leaf caused photosynthesis to decline to zero, while undamaged leaves averaged 33 µmol CO₂ m⁻² s⁻¹. Differences in plant responses to mites occurred between seasons despite similar infestation levels, possibly related to later timing of infestation and harder leaves in the second season. Compensation for mite damage was not apparent at the leaf level, since photosynthesis was reduced on undamaged portions of damaged leaves. The sequence of mite damage events on the undamaged leaf portions was determined to be: first, reduction of stomatal conductance; second, reduction of transpiration; third, reduction of photosynthetic rate; and finally, reduction of transpiration efficiency. At the leaf level, the overall effect of mite damage on photosynthesis was greater than expected because undamaged areas surrounding those visibly damaged were also affected.