Insecticidal Potential of Botanicals from Red Seaweeds against Stored Grain Pests, Rice Weevil (Sitophilus oryzae L.) and Cowpea Weevil (Callosobruchus maculatus Fab.)

Abstract

Marine botanicals are enriched with natural bioactive compounds have been utilized in the current study to alleviate the stored grain pests infestation. Four species of red seaweeds Asparagopsis taxiformis, Laurencia karachiana, Gracilaria foliifera, and Jania rubens found abundantly along the Karachi coast, Pakistan were collected in the year 2015-18. Samples were extracted using a Soxhlet extraction method with solvents of varying polarity (hexane, dichloromethane, and methanol). Five different concentrations of the extracts were subjected to the toxic assessment against two species of stored grain pests (Sitophilus oryzae and Callosobruchus maculatus). Results obtained from experimental trail proved that the C. maculatus were susceptible with higher adult mortality than S. oryzae. The highest toxic effect was induced by dichloromethane obtained extract from A. taxiformis (DA) against C. maculatus (LC50 1.15 mg/cm2) after 24 h exposure and S. oryzae although resist for one day showed (LC50 1.44 mg/cm2) after 48 h. Neurotoxic effects were also determined after 12 and 24 h. L. karachiana was the second in toxicity against C. maculatus and S. oryzae population. In addition, all treatments of A. taxiformis and L. karachiana significantly reduced the eggs laying by C. maculatus counted after 96 h of treatment. More than 70% mortality was also obtained after 96 h exposure at a dose of 2.2 mg/cm2 with most of the seaweed extracts against S. oryzae and C. maculatus. While treatment with J. rubens and G. foliifera provided only low to moderate toxicity to both tested species.