Research Article Contact Toxicity Effects of Selected Organic Leaf Extracts of Tithonia diversifolia (Hemsl.) A. Gray and Vernonia lasiopus (O. Hoffman) against Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae) Stephen Maina Gitahi , 1 Mathew Piero Ngugi , 1 David Nganga Mburu , 1 and Alex Kingori Machocho 2 1 Department of Biochemistry, Microbiology and Biotechnology, Kenyatta University, P. O Box 43844-00100, Nairobi, Kenya 2 Department of Chemistry, Kenyatta University, P. O Box 43844-00100, Nairobi, Kenya Correspondence should be addressed to Stephen Maina Gitahi; gitahism@gmail.com Received 21 October 2020; Revised 22 February 2021; Accepted 4 September 2021; Published 16 September 2021 Academic Editor: Edson Gandiwa Copyright © 2021 Stephen Maina Gitahi et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Maize weevil (Sitophilus zeamais) infestation results in a substantial reduction in the quantity and deterioration of the quality of stored maize. Most farmers control weevils using conventional pesticides which are usually associated with several human health risks as well as intoxication of the fauna and flora. However, bioinsecticides form an alternative intervention since they possess fewer side effects on human health, are ecofriendly, and are readily available. is study sought to validate and document, in a systematic way, the pesticidal properties of the species Tithonia diversifolia and Vernonia lasiopus used for many years by many people of the world on S. zeamais. e plant leaf samples were obtained from Embu County, Kenya. Dichloromethane and ethyl acetate solvents were used to extract active phytochemicals from the dried plant sample powder. e GC-MS analysis of the obtained extracts was performed at ICIPE laboratories to identify their phytochemical compositions. Twenty grams of maize grains were put in 50 ml plastic vials and admixed with different treatments. e positive control group was treated with Actellic Super ™ , while the negative control was treated with the respective extracting solvent only. e experimental groups were separately treated with the plant leaf extracts at doses of 25, 50, 75, and 100%. After treatment, each of the six groups was infested with 10 male S. zeamais and weevil mortality as a result of contact toxicity of the treatments was assessed at 6, 24, 48, 72, and 96 hours after the insects were exposed to the extracts. Results of the study indicate that the selected organic leaf extracts of T. diversifolia and V. lasiopus possessed significant contact insecticidal effects that ranged between 1.67 to 99.98%. Furthermore, the GC-MS analysis revealed several active biocompounds in T. diversifolia and V. lasiopus extracts, which are known for their considerable insecticidal effects. Our data suggest that the organic leaf extracts of T. diversifolia and V. lasiopus have considerable insecticidal properties and would, therefore, be a valuable bioprotective agent for stored maize grains against S. zeamais. 1. Introduction Postharvest losses due to storage pests have been recognized as an increasingly important constraint to maize production. In spite of the great value of maize, its availability and utilization have been impaired due to grain damage caused by notorious postharvest insect pests in developing countries. Maize weevil, S. zeamais (Coleoptera: Curculionidae), is a small agricultural insect pest in the beetle family Curculionidae (snout beetles). It is the single most important primary pest that attacks stored maize grains, among others insect pests such as the larger grain borer Prostephanus truncatus Horn (Coleoptera: Bostrychidae) [1, 2]. Weevil infestation causes an estimated annual loss of 30–50% of stored maize grains in tropical Africa [3]. Farmers normally control insect pests using synthetic insecticides. However, these chemicals are expensive and are Hindawi International Journal of Zoology Volume 2021, Article ID 8814504, 14 pages https://doi.org/10.1155/2021/8814504