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Cowpea (Vigna unguiculata) is a crop of major socioeconomic importance in Northern and Northeastern Brazil, but its productivity is strongly constrained by infestations of the black aphid Aphis craccivora, which causes direct damage through sap feeding and indirect losses by transmitting plant viruses. Although insecticides are widely used, their intensive application raises concerns related to resistance development and environmental impacts. Silicon (Si) has been investigated as a sustainable alternative for enhancing plant resistance to biotic and abiotic stresses by strengthening structural barriers and activating biochemical defense pathways. This study evaluated the influence of silicon fertilization on the plant–insect interaction between cowpea genotypes and A. craccivora. Three bioassays were conducted under greenhouse and laboratory conditions to assess aphid preference among genotypes, the effect of silicon on insect attractiveness, and plant susceptibility with and without silicon supplementation. Rice husk biochar and a commercial silicon source were used as Si fertilizers. Although the landrace genotype accumulated higher silicon levels, this did not result in reduced aphid populations or mitigation of plant damage. A negative correlation was observed between silicon accumulation and chlorophyll content, suggesting silicon-associated physiological adjustments under herbivory stress. Overall, under the evaluated conditions, silicon application did not confer effective resistance against A. craccivora, indicating that its role in integrated pest management strategies for cowpea requires further investigation.