Detection of twospotted spider mite (Tetranychus urticae Koch) damage on strawberries, using digital imagery and leaf spectroscopy

Twospotted spider mites (TSSM), Tetranychus Urticae Koch, are the most prominent and economically damaging mite pests that affect field-grown and greenhouse strawberries. At high infestation levels, TSSM can significantly affect plant growth and markedly reduce fruit yield. Traditional monitoring techniques can be labor intensive, expensive to implement in large scale production, and ineffective due to the highly clustered distribution patterns of TSSM in strawberry fields. Emergent imaging and spectral technologies could potentially improve TSSM monitoring and assessment strategies in strawberries, by creating a real-time detection and treatment system. The potential for this image based detection of TSSM is possible due the observable change in leaf color and texture associated with mite feeding. The feasibility of simple visual imagery techniques was examined by performing post-processing analysis, to accurately correlate red, green, blue, and near infrared color band reflectance values to varying mite infestation levels. Color images were obtained for two different varieties of strawberries in 2014 and four different varieties in 2015. Linear regression analysis was used to determine the correlation between color bands as well as derived vegetation indices, and mite infestation levels. Reflectance values generated from different single and multiple band combinations of red, green, and near infrared were significantly correlated with mite infestation levels across variety, though Florida Festival and Sensation varieties showed no significant correlation between light reflectance and mite infestation levels. Leaf spectroscopy and partial least squares regression were also used to determine important wavelengths associated with mite infestation levels in greenhouse grown strawberries.