Maria Suely Siqueira Ferraz

Essential oils and their major compounds have been studied to protect stored grains, especially for the control of insects. In this context, this research aimed to investigate the fumigation and contact toxicities of the essential oil of Piper hispidinervum C. DC. (Piperaceae) (sin. Piper hispidum Sw.) to Callosobruchus maculatus adult individuals and the effect on insect progeny. We also assessed the essential oil’s effect on stored-cowpea quality. The fumigation bioassay used essential oil at 14.3, 57.1, 100.0, 142.9, and 185.7 µL/L of air, whereas the contact bioassay tested concentrations of 60, 80, 100, 120, and 140 µL/kg. Insect mortality was appraised after four days (fumigation) or one day (contact). In turn, oviposition and emergence rates were evaluated after seven (fumigation) or fifty (contact) days of storage. Grain quality was also analyzed after 50 days of storage. Safrole was confirmed as the primary compound of the essential oil. P. hispidinervum essential oil proved its fumigant and contact toxicities to C. maculatus adult individuals. The concentrations lethal to 50 and 95% of the population were, respectively, 91.23 and 242.59 µL/L of air (fumigation) and 101.51 and 208.52 µL/kg of cowpeas (contact). In both application forms, C. maculatus oviposition and progeny rates declined with the increase in the essential oil concentration. Furthermore, cowpea bean quality was preserved even at sublethal doses.

This study aimed to evaluate the toxicity of Piper hispidinervum essential oil (PHEO) against 11 Brazilian populations of Sitophilus zeamais (Coleoptera: Curculionidae). The effects of sublethal doses of PHEO on the behavior (walking and flying), respiration, and population growth (r_i) of the insect populations were investigated. PHEO toxicity was determined through concentration–mortality bioassays, with mortality curves established using increasing PHEO concentrations ranging from 140.00 to 1000.00 μL kg⁻¹. Behavior was evaluated based on walking distance, walking time, walking speed, walking time proportion, flight height, and flight takeoff success. Respiration was measured via the respiratory rate, while population growth (r_i) was assessed through the instantaneous growth rate. All 11 populations of S. zeamais were susceptible to PHEO, showing no signs of resistance. The populations exhibited varying behavioral and physiological responses to sublethal exposure to PHEO, indicating different mitigation strategies. The results confirm that PHEO possesses insecticidal potential for controlling S. zeamais populations. However, the observed behavioral and physiological responses should be considered when establishing control measures in pest management programs for stored products.

The objective of this study was to investigate the influence of leaf drying techniques (bench and oven at 35 and 45 °C) on the essential oil (EO) yield of Piper aduncum L., Piper anonifolium Kunth, Piper crassinervium Kunth and Piper hispidinervum C. DC., and to analyze the chemical profile of EOs and the insecticidal potential of these oils against Ascia monuste orseis (Godart), Atta sexdens L., Zabrotes subfasciatus (Boheman), Cryptolestes ferrugineus (Stephens) and Sitophilus zeamais Motschulsky. EO yield was evaluated using four replicates of 100g of dry leaves. The EOs were obtained by hydrodistillation and subjected to GC-MS analysis to assess the chemical composition. Concentrations of 2.60 and 157.25 nL/cm² were used in the oil toxicity bioassays. EO yield was higher in the species P. aduncum and P. hispidinervum using leaves dried in oven at 45 °C, with average yields of 4.72±0.04% and 2.61±0.26%, respectively. The major constituents present in the EOs of P. hispidinervum and P. aduncum were Safrole (98.80%) and Apiole (90.00%). For P. anonifolium, the major constituents were α-Muurolene (23.11%), y-Muurolene (16.60%) and Cadina-1(10), while for P. crassinervium, they were Viridiflorol (27.70%) and Sabinene (15.50%). It was found that the EOs of P. aduncum, P. anonifolium, P. crassinervium and P. hispidinervum had a toxic effect on insects, except for P. anonifolium and P. crassinervium for S. zeamais. EO yield was higher in the species P. aduncum and P. hispidinervum, and these oils caused a higher mortality rate for the investigated insects.

Bioinsecticides are regarded as important alternatives for controlling agricultural pests. However, few studies have determined the persistence of these compounds in stored grains. This study aimed at optimizing and validating a fast and effective method for extraction and quantification of residues of safrole (the main component of Piper hispidinervum essential oil) in cowpea beans. It also sought to assess the persistence of this substance in the grains treated by contact and fumigation. The proposed method used headspace solid-phase microextraction (HS-SPME) and gas chromatography with a flame ionization detector (GC/FID). Factors such as temperature, extraction time and type of fiber were assessed to maximize the performance of the extraction technique. The performance of the method was appraised via the parameters selectivity, linearity, limit of detection (LOD), limit of quantification (LOQ), precision, and accuracy. The LOD and LOQ of safrole were 0.0057 and 0.019 μg kg⁻¹, respectively and the determination coefficient (R²) was >0.99. The relative recovery ranged from 99.26 to 104.85, with a coefficient of variation −1 (contact) and from 2.16 to 0.12 µg kg ⁻¹ (fumigation). The levels measured up from the fifth day represented less than 1% of the initial concentration, proving that safrole have low persistence in cowpea beans, thus being safe for bioinsecticide use. Thus, this work is relevant not only for the extraction method developed, but also for the possible use of a natural insecticide in pest management in stored grains.