Toxicity and growth inhibitory activities of methanol extract and the β-carboline alkaloids of Peganum harmala L. against two coleopteran stored-grain pests

Methanol extract and the β-carboline alkaloids were extracted from the seeds of Peganum harmala (Zygophyllaceae). Their toxicity, growth inhibitory and effects on the progeny production of Tribolium castaneum and Rhyzopertha dominica was studied. To assess any additive effects among the extracted β-carbolines, they were tested as binary mixtures (1:1) or as a crude alkaloid fraction. All extracts exhibited a considerable adulticidal effect with increasing activities in response to increased exposure period. Using the contact toxicity bioassay, the crude β-carboline fraction was the most effective (LC50’s were 20.1 and 36.7) μg/cm2, 48 h post-treatment against R. dominica and T. castaneum, respectively. LC50’s of (harmaline + harmine), (harmaline + harmane), and methanol extract were (31.2, 39.4), (33.7, 47.2), and (39.8, 65.2) μg/cm2, 24 h post-treatment against R. dominica and T. castaneum, respectively. At 48 h post-treatment, LC50 of (harmaline + harmine) reached 22.4 μg/cm2 against R. dominica. When mixed with the insect’s diets, toxicity of all extracts were increased with the crude alkaloidal fraction the most toxic (LC50’s were 7.8 and 14.7) mg/kg grains, 48 h post exposure against R. dominica and T. castaneum, respectively. When the 2nd instar larvae were fed sub-lethal doses-treated grains, development and F1 progeny of both insects were significantly affected (P ≤ 0.001). At 3.5 mg/kg grains of the crude alkaloidal extract, percentages of malformed larvae and pupae of T. castaneum were 19.7 and 33.4%, respectively. In this case, a total life span of 81.3 days was recorded for the treated individuals compared to 44.2 for the control. A reduction in the adult progeny of 56.9, 44.0 and 43.6% was obtained with 3.5 mg/kg of the crude alkaloids, (harmaline + harmine) and methanol extract, respectively. Meanwhile, the reduction in adult progeny of R. dominica reached 79.2% with the same concentration of the crude alkaloid extract.

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► Insecticides and growth regulators of plant origin have potential values to combat harmful insects.
► They may reduce problems of insect resistance and detrimental effects on non-target organisms including users.
► Methanol extract and the β-carboline alkaloids of P. harmala L were prepared.
► Extracts were tested against R. dominica and T. castaneum.
► All extracts showed toxicity, growth inhibitory and significantly affect progeny production of both insects.