Individual and synergistic toxicity of solanaceous glycoalkaloids against two coleopteran stored-product insects

Toxicity of solanaceous glycoalkaloids against stored-grain insects was investigated under laboratory conditions. The total glycoalkaloids (TGAs), alpha-chaconine and alpha-solanine from potatoes, Solanum tuberosum L. and alpha-tomatine from tomatoes, Lycopersicon esculentum Mill. were isolated and tested in this bioassay. Their acute and residual toxicity were assessed against the rust red flour beetle, Tribolium castaneum Herbst (Coleoptera: Tenebrionidae), and the rice weevil, Sitophilus oryzae L. (Coleoptera: Curculionidae). All compounds were tested either individually or as binary mixtures. Results revealed considerable toxicity of the tested glycoalkaloids against the target insects. When adults of S. oryzae were exposed to a dry-film residue of these phytochemicals, the total glycoalkaloids (TGAs) fraction was the most toxic, followed by alpha-solanine, alpha-chaconine, and alpha-tomatine with LC50s of (38.6 and 22.1), (48.2 and 38.9), (52.00 and 41.6), and (82.3 and 67.00) mu g/cm(2) at 24 and 48 h post-treatment, respectively. The order of toxicity against T. castaneum in a descending order was TGAs > alpha-chaconine > alpha-solanine > alpha-tomatine. All compounds were more toxic when insects were fed grains treated with these phytochemicals (LC50s of TGAs were 7.4 and 16.2 mg/kg grains at 48 h post-treatment against S. oryzae and T. castaneum, respectively. All compounds, particularly the TGAs, exhibited promising residual toxicity effects. Toxicity of glycoalkaloids was exceeded when tested as binary mixtures indicating their synergistic interaction. The study recommends the use of glycoalkaloids of Solanaceae as biorationals and natural leads to protect stored grains from insect infestation.