Effect of Bacillus thuringiensis kurstaki on mid-gut structure of the beet web moth, Spoladea recurvalis F. larvae (Lepidoptera: Crambidae) reared on semi-synthetic diet

Abstract

Larvae of the beet web moth, Spoladea recurvalis F. (Lepidoptera: Crambidae), were reared on a semi-synthetic diet under laboratory temperature of 25 °C. The microbial bioinsecticide Dipel 2X (Bacillus thuringiensis kurstaki) was bio-assayed versus the 2nd instar larvae (L2) of S. recurvalis by mixing in the semi-synthetic diet at different concentrations (5, 2.5, 1.25; 0.6 and 0.3% w/w). The concentration 1.25% of Dipel 2X mixed in the semi-synthetic diet was fed to larvae (L2) to prolong the pre-death period for 5 days in order to follow its effect on the structure of mid-gut. Treated larvae as well as those of the control were fixed in Bouin’s fixative at 1, 2, 3, 4, and 5 days post treatment and processed for paraffin serial sectioning at 6–8 μm with double staining by eosin and hematoxylin. Cross sections in the mid-gut were examined with a light microscope in both treated larvae and those of the control. The gradual alterations in mid-gut structure till death of the treated larvae were described.