Studies on the insecticidal and IGR activity of red algal seaweeds of Rameswaram coast on a polyphagous pest, Spodoptera litura (Fab.)

Investigations on the insecticidal activity of red algal seaweeds viz., Liagora ceranoides J.V. Lamouroux, Acanthophora spicifera (M. Vahl) Borgesen, Asparagopsis taxiformis (Delile) Trevisan, Chondrococcus hornemannii (Lyngbye) F. Schmitz, Gracilaria corticata J. Agardh, Gracilaria opuntia J. Agardh, Hypnea panosa J. Agardh, Jania rubens (Linnaeus) J.V. Lamouroux were carried out against leaf eating caterpillar, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae). The prepared seaweed powder of Red algae was evaluated for their toxicity and IGR activity on third instar larvae of S. litura as per the following treatments viz., 1, 3, 5, 7, 10, 20% and untreated control and the experiments were conducted with three replications under completely randomized design. Data on the larval mortality every 12 hrs, revealed no anti-feedent and repellent action but pronounced larvicidal action. Significant larvicidal action was noticed only after 72 hrs of treatment in all the seaweeds except G. corticata where the action began at 36 hrs of treatment. There was an increase in mortality up to 96 hrs in all the seaweeds except G. corticata wherein the mortality was 100% in all the concentrations tested. In other seven seaweed treatments the larvicidal action initiated only at 72 hrs and extended further. The larvicidal activity may be due to the presence of 13 phytochemicals identified in G. corticata through GCMS. The G. corticata's larval activity needs to be further explored with aqueous and solvent extracts for confirmation.