Atividade inseticida dos óleos essenciais de Alecrim, Cravo-da-Índia, Cedroda-Virgínia e Toranja sobre Sitophilus zeamais em milho armazenado.

Abstract

The maize weevil (Sitophilus zeamais Motschulsky) is a major primary pest in maize production and storage, causing significant damage to grains and facilitating infestation by secondary pests. Its reproductive capacity, long life cycle, and ability to survive at great depths among the grain explain its destructive potential. Given the damage caused by S. zeamais, the use of post-harvest technologies to control this pest is necessary and essential oils are a promising alternative because they do not pose a threat to human health or the environment. The aim of this study was to evaluate the toxicity of rosemary (Rosmarinus officinalis), clove (Eugenia caryophyllus), eastern red cedar (Juniperus virginiana), and grapefruit (Citrus paradisi) essential oils against S. zeamais. Contact and ingestion tests were conducted to determine the lethal concentrations (LC50 and LC90) of each essential oil, and evaluate the fumigant effect of C. paradisi and E. caryophyllus essential oils. The contact and ingestion experiments were conducted in Petri dishes containing 20g of maize, in which different concentrations of the evaluated essential oils were applied using an automatic pipettor. The plates were manually shaken to ensure that the oil spread between the grains and then, 10 adults of S. zeamais were confined in each plate for 48 hours. After this period, the percentage of mortality was analyzed, the results were subjected to PROBIT analysis, and lethal concentrations (LC50 and LC90) and the toxicity ratios (TR) were calculated. Each concentration was tested with 5 replicates. To evaluate the fumigant effect of C. paradisi and E. caryophyllus essential oils on S. zeamais, different concentrations of the oils were applied to filter paper strips, which were positioned on the bottom surface of the lid of glass containers with a volume equivalent to one liter.. In each fumigation chamber, 10 adults of S. zeamais were confined, using maize as substrate. An organza clothc barrier was placed to prevent direct contact of the insects with the oils. The experiment was carried out in an entirely randomized design with five repetitions. After 48 hours, mortality was analyzed and the results were submitted to Probit analysis. The results obtained demonstrated the insecticidal effect of C. paradisi, E. caryophyllus, R. officinalis, and J. virginiana essential oils on S. zeamais by contact and ingestion. The estimated lethal concentrations (LC50 and LC90) for the essential oils of C. paradisi (40.30 and 81.56μL/20g maize, respectively), R. officinalis (8.00 and 12.92μL/20g maize, respectively), and J. virginiana (39.42 and 102.20μL/20g maize, respectively), in the contact test corroborate with the literature. However, it was not possible to determine the LCs of E. caryophyllus, since already at the lowest concentration (0.5 μL/20g maize) it caused 100% mortality of insects. As fumigants, however, the essential oils of C. paradisi and E. caryophyllus did not show insecticidal activity, since they did not cause significant mortality even at the highest concentration tested (280μL/L of air). Among the oils evaluated in this study, R. officinalis showed the most promise for the management of S. zeamais, since it presented the lowest lethal concentrations (LC50 and LC90) and the highest toxicity ratio (RT).