BIOCHEMICAL EFFECTS OF SOME INSECT GROWTH REGULATORS ON FIELD STRAINS OF THE COTTON LEAFWORM, Spodoptera littoralis

Abstract

Different strains of cotton leafworm, Spodoptera littoralis larvae were collected from heavily sprayed fields or recently cultivated fields in different areas in Alexandria, and El-Boheira, proviance governorate. Chitinase activity was determined in laboratory susceptible strain of cotton leafworm and compared with enzyme activity for other collected strains which cleared that highest level of chitinase activity was found in Alexandria strains. The sensitivity of chitinase activity to chlorfluazuron and teflubenzuron was measured by I50 values, values of I50 in the case of chlorfluazuron were 0.23, 0.34, 0.41, 0.60, and 0.69 µM for lab strain; Borg El-Arab; West of Nobaria; Abou El-Matamir, and Edko strains of Spodoptera 2nd larvae respectively, while I50 values were 0.31, 0.40, 0.46, 0.66, and 0.74 µM for lab strain and four field strains of Spodoptera 4th larvae respectively, similarly, the teflubenzuron were 0.40, 0.50, 0.57, 0.74, and 0.82 µM for lab strain and four field strains of Spodoptera 2nd larvae respectively, the I50 values were 0.47, 0.59, 0.65, 0.88, and 0.93 µM for lab strain and four field strains of Spodoptera 4th larvae respectively. Also, the inhibition constant (Ki) values were determined, the obtained data proved that compounds competitive inhibition of chitinase activity. The significant high mortality percentages were observed at all tested concentrations with chlorfluazuron than teflubenzuron and the result clearly showed that the 2nd instar larvae were more sensitive to the compounds tested, compared to those of the 4th instar, so when IGRs used for S. littoralis larvae control, dosage and timing of application should be carefully considered. The results of the present study may add some forward steps to use IGRs as alternative to conventional insecticides especially against this insect, so, the IGRs can be involved in important steps necessary for successful IPM programs applied against S. littoralis.