The present experiments were carried out at the plant protection research
station, Kaha, Qalubia Governorate, Egypt to study the efficiency of the bioagent
Bacillus thuringensis var. Kurstaki against the 2nd instar larvae of the lesser cotton
leafworm, Spodoptera exigua (Hb). Results revealed that the 2nd instar larvae were
more susceptible to B. thuringiensis towards 4th instar larvae. Accumulative mortality
percentage of the 2nd instar larvae of S. exigua treated with the different
concentrations of B. thuringeinsis kurstaki, increased gradually with increasing the
time elapsed after treatment. There was a stronger larvicidal effect on the 2nd instar
larvae giving 2.889x106 and 6.267x1013spore/ml for LC50 and LC90 respectively. The
mean larval duration for both LC50 and LC90 of B. thuringeinsis var. kurstaki was
elongated compared to the control treatment. Pupal duration was insignificantly
affected when 2nd instar larvae of S. exigua were pretreated with LC50 and LC90 of B.
thuringeinsis kurstaki. The percentage of pupation was significantly decreased than in
case of the control for both treatments, respectively. On the other hand, the
percentage of pupae that succeeded in surviving and reaching the adult stage
decreased in case of treated larvae than in case of the control for both treatments.