In this study the susceptibility of different larval instars of Colorado potato beetle to Bacillus thuringiensis var. tenebrionis (Ber.) alone and in combination with henna was investigated. For this purpose the larvae were reared on potato plant in laboratory. Different larval instars were separated through head capsule diameter measurements. The experiments were conducted on 1st, 2nd, 3rd and 4th instar groups LC50 values being found to be 261.539,586.696, 1585.521 and 4093.131 ppm, respectively. To evaluate the synergistic effect of henna in combination with B. thuringiensis a known amount of henna was added to a certain concentration of B. thuringiensis. (500ppm) for 3rd instar larvae.The results revealed that there is a striking synergistic effect of this material on increasing efficiency of the strain. The larval mortality rate for a mixture of B. thuringiensis (500 ppm) with henna (5000 ppm), after six days, was 72.5%, whereas it was 17.5% and 2.5% for B. thuringiensis and henna each alone, respectively.
)