Pathogenicity of Bacillus thuringiensis against three important date palm insect pests

The larval stage of the following three insect pests Ephestia kuehniella Zeller (Lepidoptera: Pyralidae), Plodia interpunctella (Hübner)(Lepidoptera: Pyralidae) and Batrachedra amydraula Meyrick (Lepidoptera: Batrachedridae) cause serious yield loss to stored date palm fruits that can reach 50 to 70%. This study investigated the pathogenic effect of the bacterium Bacillus thuringiensis (Bt) kurstaki on larval stage of the three moth species. The bacterial concentrations used in the bioassay were selected and prepared logarithmically spaced between the minimum dose of 107 CFU/ml and the maximum dose of 109 CFU/ml. Results showed that the pathogenic bacterial isolate had the ability to cause disease on the moth larva. The first symptoms observed were brown rot and black discoloration of larval cuticle. The external symptoms of the disease on larvae appeared 2 to 3 days after the death of larvae. The Bt LD50 on B. amydraula, E. kuehniella and P. interpunctella were 2.15×10(8), 5.18×10(8) and 2.71×10(8) CFU/ml, respectively. The shortest LT50 was 5.79 days when the concentration of 109 spores/ml of Bt was used against B. amydraula, and the longest was 11.57 days when the concentration of 108 spores/ml of Bt was used against E. kuehniella.