Larval susceptibility of Spilarctia obliqua against Bacillus thuringiensis isolates

First instar larvae, in general were found to be more susceptible to all the tested Bacillus thuringiensis isolates except dipel and Btk path-1, which caused low mortality. The third instar larvae, in comparison to fourth, fifth and sixth instar larvae exhibited greater mortality to all the tested B. thuringiensis isolates. Critical growth phase, responsible for changes in the susceptibility of the insect was noticed between hatch and third instar. Once this stage was over, little difference occured between treated and control larvae of Spilarctia obliqua. Irrespective of stage of the insect, Btk HD-73 was found superior over all tested B. thuringiensis isolates by registering 75.8% mean larval mortality however, Btk path-1 was noticed least effective. The lowest LC₅₀ value (0.0951) was observed in case of Btk HD-73 whereas lowest LC₉₀(0.25715), LC₉₅(0.34096) and LC₉₉(0.57877) were recorded for Bt aizawai against third instar larva of S. obliqua. In case of B. thuringiensis commercial products as well first instar larvae remained most susceptible and irrespective of larval stage dipel registered maximum larval mean mortality.