Insecticidal activity of chitosans of different molecular weights and chitosan-metal complexes against cotton leafworm Spodoptera littoralis and oleander aphid Aphis nerii

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 × 105,3.60 × 105,5.97 × 105,and9.47 × 105 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 × 105 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. littoralis were evaluated at 4 g (a.i.) chitosan/kg diet. Chitosan of 2.27 × 105 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 × 105 and 5.97 × 105 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 × 105,3.60 × 105,and5.97 × 105 g/mol showed the highest efficacy at all concentrations tested; however, chitosan-Cu was significantly the most active among the complexes.