131 Plant Protect. Sci. Vol. 48, 2012, No. 3: 131–141 Insecticidal Activity of Chitosans of Different Molecular Weights and Chitosan-metal Complexes against Cotton Leafworm Spodoptera littoralis and Oleander Aphid Aphis nerii Mohamed E. I. BADAWY and Ahmed F. EL-ASWAD Department of Pesticide Chemistry and Technology, Faculty of Agriculture, Alexandria University, El-Shatby, Egypt Abstract Badawy M.E.I., El-Aswad A.F. (2012): Insecticidal activity of chitosans of different molecular weights and chitosan-metal complexes against cotton leafworm Spodoptera littoralis and oleander aphid Aphis nerii. Plant Protect. Sci., 48: 131–141. As an alternative to synthetic pesticides, chitosan has received much attention as a biopolymer active against some agricultural pests. The, insecticidal activity of chitosans of four molecular weights ( 2.27 × 10 5 , 3.60 × 10 5 , 5.97 × 10 5 , and 9.47 × 10 5 g/mol) was investigated against two species of arthropod pests: oleander aphid Aphis nerii and cotton leafworm Spodoptera littoralis . In addition, the most active chitosan of 2.27 × 10 5 g/mol was chemically modified with metals of Ag(I), Cu(II), Ni(II), and Hg(II) to give corresponding chitosan-metal complexes. Larval mortality, growth inhibition, and antifeedant activities for third instar larvae of S. littora- lis were evaluated at 4 g (a.i.) chitosan/kg diet. Chitosan of 2.27 × 10 5 g/mol and its complexes with Ni and Hg were the most active compounds. The results against A. nerii indicated that chitosans of 3.60 × 10 5 and 5.97 × 10 5  g/mol showed high activity among the different molecular weights in leaf-dip bioassay after 24 h of treatment with 48 and 49% mortalities, respectively, at 1000 mg/l. All compounds had a systemic effect against A. nerii . Chitosans of 2.27 × 10 5 , 3.60 × 10 5 , and 5.97 × 10 5 g/mol showed the highest efficacy at all concentrations tested; however, chitosan-Cu was significantly the most active among the complexes. Keywords: arthropod pests; larval mortality; biopolymer Chitosan is derived by deacetylation of chitin, the second most abundant natural biopolymer. Chitosan is a nontoxic copolymer consisting of β-(1,4)-2-acetamido-2-deoxy- d-glucopyranosyl and β-(1,4)-2-amino-2-deoxy- d-glucopyranosyl units. It is a large family of compounds with dif- ferent properties depending on its structure (No & Meyers 1997). Th e degree of N-acetylation (DA) and molecular weight are important factors that have an impact on properties such as solu- bility (Kubota & Equchi 1997) and biological activity (Rabea et al . 2003; Gerasimenko et al . 2004; Badawy 2008, 2010). Thus, the free amino groups on chitosan molecule are important for many applications. Recently, chitosan-metal complexes have at- tracted great interests for their potential use in agriculture, medical industry, and food industry (Nieto et al . 1992; Shigemasa & Minami 1996; Wang et al . 2004; Mekahlia & Bouzid 2009; Higazy et al . 2010). It is well known that both chitosan and metals such as Ag, Cu, Ni, Hg, and Zn have the properties of disinfection and bactericide (Wang et al . 2005). After chitosan binds to metal ions through nitrogen, oxygen or a combination, the bindings are likely to leave some potential donor atoms free and these free donor atoms enhance biological activity (Wang et al . 2005). However, the influence of the property of metal ions, mo- lecular parameters and environmental factors on