RESIDUAL ACTIVITY OF METHOMYL AND RADIANT INSECTICIDES AGAINST THE LARVAE OF COTTON LEAFWORM, SPODOPTERA LITTORALIS (BOISD.)

Abstract

The bio-residual activity of two compounds, methomyl and radiant against the 2nd and 4th instars larvae of the cotton leafworm, Spodoptera littoralis was evaluated under laboratory and semifield conditions. The obtained larvae were fed for 24h on cotton leaves treated with methomyl compound and for 48h on the leaves treated with radiant at 0, 3, 7,9 and 12 days after the treatment. The 2nd instar larvae treated with radiant were more susceptible than the fourth ones. While the treatment of both 2nd and 4th larval instars with the methomyl at the three tested leaf residues 0, 3 and 7days had the highest effect, it caused 100 % mortality. Whereas, the larval treatment with radiant was more higher residual effect than the methomyl after feeding on leaves at 12 days post treatment, it caused 70 and 60% larval mortality of both 2nd and 4th instars, respectively, compared to 60 and 50% for the two instars, respectively, treated with methomyl. Methomyl and radiant treatments had a latent effect on the biological activities of this insect when the larvae fed on leaves at 12 days post treatment. The effect was varied according to the larval instar and tested compound. It was found that the treatment of 2nd and 4th instars larvae with radiant had the strongest effect. It caused an increase in the larval duration and pupal malformation and reduced the pupal weight. While, the treatment of 2nd and 4th instar with methomyl was higher reduced the adult emergence and increased the adult malformation percentages. The treatment of the 2nd instar larvae with the two compounds had the highest effect in decrease the pupation percentage to reach 50 and60% compared to control. While the treatment of the 4th instar with radiant at the 12day age leaf residues had the most potent in the fecundityinhibition to reach zero eggs/female in comparison to 569+113.2 eggs/female of control, and it reduced the eggs hatching to reach 0% as compared to 99% of control. While the larval treatment of the 4th instar with the both methomyl and radiant had significant decrease in the adult longevity .Whereas, the larval treatment of the same instar with methomyl had the highest effect in adult sex ratio shifting of both males and females as compared to that of control and the radiant had the next effect in this respect