Comparative Toxicity of Spores and Crystals from the NRD-12 and HD-1 Strains of Bacillus thuringiensis suhsp. kurstaki to Neonate Beet Armyworm (Lepidoptera: Noctuidae)

The toxicities of different components of the spore-parasporal body complex of the NRD-12 and HD-1 strains of Bacillus thuringiensis subsp. kurstaki (Berliner) to neonate Spodoptera exigua (Hübner) were determined using diet incorporation bioassays. The LC50’s of NRD-12 and HD-1 from lyophilized powders obtained from sporulated liquid cultures were 20.8 µg/ml diet and 49.3 µg/ml diet, respectively. Both strains produced a bi pyramidal crystal containing a 135-kDa (kilodalton) protein (PI) and a cuboidal crystal containing a 65-kDa protein (P2). Preparations of the NRD-12 PI crystal (LC50 = 63.0 µg/ml diet) were more toxic than similar preparations of HD-1 (LC50 = 153 µg/ml diet). Alternatively, the P2 crystal from HD-1 (LC50 = 34.2 µg/ml diet) was more toxic than preparations of the NRD-12 P2 crystal (LC50 = 72.4 µg/ml diet). Combinations of the P1 and P2 crystals of NRD-12 were more toxic (LC50 = 82.0 µg/ml diet) than HD-1 P1/P2 preparations (LC50 = 157 µg/ml diet). Spore preparations from HD-1 (LC50 = 117 µg/ml diet) were slightly more toxic than those from NRD-12 (LC50 = 166 µg/ml diet). Significant (P ≤ 0.005) potentiation resulted when the LC25 of spores was added (1:1 wt/wt) to the LC25 of the 65-kDa/I35-kDa protein combination from the same isolate. Significant (P ≤ 0.005) potentiation also was observed when NRD-12 spores were added to HD-1 crystal protein and when HD-1 spores were added to the NRD-12 crystal protein. However, there was no difference in toxicity between these two combinations. These data suggest that the comparatively high toxicity of NRD-12 to S. exigua is due at least in part to the 135-kDa proteins of this strain.