Evaluation the effect of two products of Bacillus thuringinsis on Lymantria dispar L. (Lep., Lymantriidae) larvae in the Arasbaran forests, Iran

The gypsy moth, Lymantria dispar L. (Lep., Lymantriidae) is one of the most destructive pests of forest shrubs and trees throughout the Arasbaran region (East Azarbaidjan, Iran). The pest causes serious periodical damage on its host plant species, especially oak trees. In this study, the effects of two formulations of Bacillus thuringiensis containing Btk (commercial product Dipel) and BtH (made in Iran) were evaluated on gypsy moth larvae. Results showed that there was a significant difference between Dipel and BtH. Dipel (1.2 g/L) and BtH (6 g/L) treatments caused maximum mortality 29.23% and 23.05%, respectively and 1.2 g/L dosage of BtH with average 13.61% caused the less mortality on the pest larvae. Mean comparison of L. dispar larvae mortality in differenttimes after treatments (for both formulations) indicated that there was no significant difference between 1 and 3 days, but difference between those two mentioned days with 7 and 14 days was significant, so that the treatment effect increased with time. By comparing the number of parasitoid wasp and nematode emerged from treated survived larvae, the treatments effects on natural enemies was quite evident. Natural enemies emerged from larvae treated by 1.2 g/L, 3 g/L and 6 g/L dosage of BtH and Dipel were reduced 12.3, 34.7, 59.8 and 81.6 percent, respectively in comparison to control. In other words, by increasing the effectiveness of Bt treatments, the possibility of naturalenemies emergence was reduced. In this regard, high concentrations of BtH and Dipel have the most impact in reduction of natural enemies emerging. Assessment of larval feeding on the treated leaves revealed that because of the rapid effects of Bt to 2nd instar larvae, there was no possibility to continue their feeding. But in the 4th instar larvae, because of slow effects of Bt, their feeding on leaves in some treatments continued for up to 100 percent. Thus, application of Dipel and high dosage BtH against 2nd instar larvae of the pest will be more effective than other treatments in Arasbaran forests.