Study of destruxin B and tea saponin, their interaction and synergism activities with Bacillus thuringiensis kurstaki against Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)

Abstract

This study reveals the individual and synergistic effects of destruxin B (DB) (mycotoxin from Metarhizium anisoplae), tea saponin (Ts) and Bacillus thuringiensis (Bt) against Spodoptera exigua. DB, Ts and Bt reduced the growth of neonate larvae by up to 91.30, 89.17 and 77.17%. EC50 values of DB against 4th and 5th instars were 0.17 and 0.22 mg ml-1, 0.35 and 0.41 mg ml-1 against 4th and 5th instars for Ts and 0.0031 and 0.0035 mg ml-1 after being treated with Bt. The synergism of DB and Ts with Bt resulted in the increased efficiency of these chemicals as mortality percentages significantly increased up to 94.2% with DB+Ts+Bt followed by DB+Bt with 91.99% and were significantly higher than individual treatments of Bt 65.81% and Ts 76.66%. Nutritional analysis revealed the increased toxicity of DB and Ts in combination with Bt, but DB+Bt showed higher efficiency with minimal relative consumption rate (RCR) 2.69 mg/mg/day, relative growth rate (RGR) 1.33 mg/mg/day and efficiency of conversion of ingested food (ECI) 35.77%, respectively. Changes in antioxidant enzymes, superoxidase dismutase (SOD) and catalases (CAT), were noticed to some extent over different exposure times in all treatments. The highest SOD activity was observed in individual DB treatment, while individual Bt treatment caused the highest variation in CAT values. In all treatments, SOD and CAT values fell sharply on the final day except for individual Bt treatment, with which the values were relatively stable on the final day.