Effect of thermal stress on antioxidant responses of the biocontrol agent Galerucella placida (Coleoptera: Chrysomelidae)

Galerucella placida Baly feeds on the rice-field weed Polygonum orientale L. (Polygonaceae) in India and Bangladesh during autumn and winter seasons. The insect is widely available during the winter season when the temperature fluctuates between 9 and 24 degrees C. Hence, it is of considerable interest to study how the adults cope with the stress resulting due to temperature fluctuations. Consequently, we analysed the levels of H2O2, antioxidant enzymes [catalase (CAT), superoxide dismutase (SOD), peroxidases (POD), guaiacol peroxidase (GPX), pyrogallol peroxidases (PPX), ascorbate peroxidases (APOX) and glutathione-S-transferases (GST)] and malondialdehyde (MDA) at 9, 12, 15, 18, 21 and 24 degrees C in adult G. placida. H2O2 was highest at 24 degrees C followed by 9 degrees C and lowest at 18 and 21 degrees C. CAT and SOD were highest at 9 degrees C followed by 24 degrees C, while POD was highest at 24 degrees C followed by 9 degrees C. CAT and POD were lowest at 18 and 21 degrees C, but SOD was the lowest at 21 degrees C. APOX were three-fold higher at 9 degrees C compared to 12 degrees C, which was the lowest at 18 and 21 degrees C, implying that APOX had strong detoxification function at 9 degrees C. GST was highest and lowest at 9 and 21 degrees C, respectively. MDA was highest at 9 degrees C and lowest at 18 and 21 degrees C, which did not differ significantly at 15 and 24 degrees C, suggesting that lower and higher temperature stress was accompanied by lipid peroxidation. This finding provides useful information for mass rearing of G. placida for predicting population dynamics and understanding the potential for G. placida as biocontrol agent under varying environmental conditions.