The lethal effects of Detech®, lemon peel, and their mixture were tested against the granary weevil (Sitophilus granarius L.) on wheat grains. For the single-dose test, 2000 ppm of Detech®, lemon peel, and a 50% (w/w) mixture of lemon peel and Detech® were applied to 10 g of wheat grains. Ten granary weevils were added to each sample under controlled conditions of 27±2°C and 60% relative humidity. The samples were then placed in complete darkness for a period of seven days, allowing the granary weevils to feed and breed without any external influences. At the end of the incubation period, the lemon peel + Detech® mixture demonstrated almost 100% mortality rate, which was comparable to that of the Detech® alone treatment. Additionally, a dose–response study was conducted using the lemon peel + Detech® mixture to determine lethal concentration values (LC₁₀, LC₃₀, LC₅₀, and LC₉₀). The calculated LC₁₀, LC₃₀, LC₅₀, and LC₉₀ were 200 ppm, 600 ppm, 1400 ppm, and 9100 ppm, respectively. These results revealed a very high mortality rate in both single-dose and dose–response tests, highlighting the notable toxicity of the lemon peel + Detech® mixture against Sitophilus granarius adults. Consequently, this study supports the development of environmentally sustainable pest control strategies in grain storage systems, providing alternative solutions for sustainable agriculture.