INHIBITORY EFFECT OF THURINGIENSIN AND ABAMECTIN ON DIGESTIVE ENZYMESAND NONSPECIFIC ESTERASES OF SPODOPTERA-LITTORALIS (BOISD) (LEP, NOCTUIDAE) LARVAE

The effect of abamectin (Avermectin B-1) and thuringiensin (beta-exoto xin of Bacillus thuringiensis) on the main digestive enzymes and non-s pecific esterases in Spodoptera littoralis (Boisd.) larvae was determi ned. The fifth-instar larvae were allowed to feed for only 24 h on lea ves treated with sublethal concentrations of each compound, i.e. 5, 20 and 200 ppm through three successive generations, respectively. In en zyme assays, at F1-generation, abamectin (5 ppm) caused a remarkable d ecrease in invertase, amylase and trehalase activities by 81, 76 and 5 4%, respectively, compared to those recorded in the control larvae at day 3 after treatment. The body weights of larvae fed on abamectin-tre ated leaves (5 ppm), generally decreased continuously during feeding p eriods; this decrease was directly proportional to the lower enzymatic activity. At F3-generation, abamectin (200 ppm) reduced amylase activ ity by 21 and 46% at 2- and 3day feeding periods, respectively. Thurin giensin, at F1-generation (5 ppm), also caused a pronounced decrease i n digestive enzyme activities, especially amylase and invertase, at wh ich maximum inhibition, about 77 and 76%, respectively, was reached 3 days after treatment. Feeding at concentration of 200 ppm (F3-generati on) also reduced amylase and trehalase activities by 53% after 2 days of treatment. In addition, abamectin at 5 ppm caused a reduction in th e activities of both alpha-esterases and beta-esterases by 28.30% and 58.66%, after 1-day feeding on leaves treated with 5 ppm (F1-generatio n) and 200 ppm (F3-generation), respectively. The inhibitory effect of thuringiensin on the enzymatic activities of both alpha-esterases and beta-esterases seemed to be less than that caused by abamectin.