Insecticidal activity of Brevibacillus brevis (Paenibacillaceae) bacterial culture filtrate on Spodoptera littoralis (Lepidoptera: Noctuidae) larvae

Abstract

Spodoptera littoralis (Lepidoptera: Noctuidae) larvae caused severe damage and reduction in crops yield. The bacterium Brevibacillus brevis elicited antifungal activity against some plant pathogens. The current study ascertained the insecticidal activity of B. brevis filtrate (Thin-film) against S. littoralis larvae. The LC50 value was 28.66µl cm-2 (95% F.L. = 12.22µl cm-2 – 56.34µl cm-2, Slope = 3.32 ± 0.658) after 20h post-treatment. Ten percentages of larvae exposed to 31.25µl cm-2 of filtrate developed into pupa-adult intermediate and died. The bacterial filtrate at ratio of 46.87µlcm-2 caused 100% reduction in the adult emergence. Comparing to control, TEM observation of the midgut epithelium showed: disappearance of the columnar cell boundaries; irregular shape of nuclear envelop, condensed chromatin, deformed endoplasmic reticulum and mitochondria as well. Some cells began to bleb into gut lumen while other necrotic cells were burst and their content fall down into gut lumen. The viability of the circulating haemocytes of the treated larvae was determined using Acridin orange-Ethedium bromide stain. Using fluorescence microscope, the death of granulocytes increased by 79.8%. The current results showed that filtrate B. brevis triggers apoptosis in the midgut epithelium and larval haemocytes, which could be used as a biocontrol agent against S. littoralis.