Toxicity and repellency of the monoterpenoids carvone and eucalyptol on Sitophilus zeamais: contact and fumigation-based interactions

Abstract

Corn is a globally important crop; however, pest infestations during storage represent a major challenge. The maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae), is a primary pest of stored maize, commonly controlled through phosphine fumigation. Nevertheless, extensive reliance on this insecticide has led to the emergence of resistant populations. Thus, the objective of this study was to evaluate the contact and fumigant toxicity of the monoterpenoids carvone and eucalyptol against S. zeamais , addressing the previously unexplored contact toxicity and repellency of these compounds following impregnation into maize grains, as well as their effects on population growth, insect fresh weight, and grain mass consumption. Experiments were conducted under laboratory conditions (28 ± 2 °C, 60 ± 10% relative humidity and a 12 h photoperiod) using a completely randomized design. Lethal concentrations (LC₅₀ and LC₉₅) were determined for contact and fumigation bioassays, and the LC₅₀ values obtained in the contact tests were used in repellency assays. Carvone exhibited higher toxicity than eucalyptol in both bioassays. In the contact toxicity test, LC₅₀ values were 3.48 µL/20 g for carvone and 14.35 µL/20 g for eucalyptol, while in fumigation assays the LC₅₀ values were 7.68 µL/L and 59.56 µL/L, respectively. Both compounds showed strong repellent activity against S. zeamais , with LC₅₀ treatments attracting only 0.2 and 0.8 insects to eucalyptol- and carvone-treated grains, respectively, corresponding to repellency levels above 90%. The treatments significantly reduced adult emergence and resulted in negative instantaneous population growth rates (r i ranging from − 0.0255 to − 0.0512), indicating population decline. Grain weight loss was reduced to 1.6% for carvone and 0.9% for eucalyptol when LC₅₀ concentrations were applied. These results demonstrate that carvone and eucalyptol exhibit insecticidal and repellent properties and may represent promising alternative agents for the management of S. zeamais in stored maize.

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