Evaluation of some photosensitizers against the cotton leaf worm, Spodoptera littoralis (Lepidoptera: Noctuidae), in relation to spectral and thermal reflectance

Photosensitizing compounds are eco-friendly promising organic dyes for managing insect pests without facing the risk of resistance. The photodynamic efficacy of four Photosensitizing compounds (rose Bengal, rhodamine B, methylene blue and methyl violet) was monitored against the third larval instar of Spodoptera littoralis (Boisduval), after exposure to sunlight. The LC₅₀ values of the four compounds; rose Bengal, rhodamine B, methylene blue and methyl violet recorded 0.029 × 10^-5, 0.24 × 10^-5, 14.69 × 10^-5 and 15.59 × 10^-5 M, respectively. Therefore, rose Bengal was the most effective compound followed by rhodamine B, Methylene blue and methyl violet. Compared to similar previously documented photosensitizer compounds, these four tested compounds recorded higher mortality percentage. The effect of those compounds on the larval biochemical components was assessed by measuring larval spectral and thermal reflectance. Larvae fed on photosensitizing compounds showed distinct spectral reflectance patterns. Treated larvae had same reflectance manner; which were higher than that of control samples. Along the whole spectrum, the highest reflectance was after 60 min of exposure to sunlight in case of rose Bengal, rhodamine B and methylene blue. Methyl violet reflected sunlight mostly at zero time then the reflectance decreased after 30 min then increased again after 60 min. There was a noticeable absorption of IR wavelengths at ~ 1900, 1400, 1200 and 950 nm in all treated samples. Thermal imaging indicated abnormal larval warmth after treatment. Differences in reflectance were monitored immediately after feeding, confirming the speed and mode of action of those compounds.