Sublethal co-exposure of zinc and iron mitigates paraquat herbicide toxicity affecting the physiological responses of Egyptian cotton leafworm Spodoptera littoralis

Abstract Metals and herbicides present in the environment affect the vital functions of insect pests, including their development, tolerance, resistance, and survival. Such metal-herbicide interactions will have inevitable, but complicated effects on insects, which remains poorly understood. Here, the interaction of zinc (Zn), iron (Fe), and paraquat (PQ) at subleathal dose on the physiological responses of the Egyptian cotton leafworm Spodoptera littoralis was examined. Co-exposure of Zn/ Fe enhanced the survival of leafworms (10 to 20 mg 100%; 40 mg 85%) compared to separate exposure. Low exposure to Zn/Fe/PQ had the lowest toxicity, possibly because metal complexes have efficient chelating activity, allowing the leafworm to survive longer. Low exposure to Zn, Fe, and Zn/Fe increased food intake and larval weight significantly, and affected frass production. PQ alone hindered feeding, enhanced frass production, and reduced larval weight. In contrast, Zn/Fe and PQ/Zn alleviated toxic effects. Interestingly, Zn/Fe/PQ combined increased larval and pupal weight in surviving individuals. Larval mortality was highest with high doses of PQ (40 mg) and in metal co-exposure diets, with no larvae reaching the pupal stage. Zn/Fe appeared to be essential for the ecdysis of larvae to pupae, with lower larval mortality and extended pupal ecdysis (% days). This study demonstrated the realistic conditions of Zn and Fe interactions with PQ, providing important information regarding physiological toxicity and pest management. Tests on haemolymph-bacteria of S. littoralis using disc diffusion method revealed significant differences with Zn, Zn/Fe, Zn/Fe/PQ, stating their interaction might play a crucial immunomodulatory role in S. littoralis .