Effects of coniferous essential oils on the survival and reproduction of the granary weevil (Sitophilus granarius L.)

The present study evaluated the insecticidal efficacy of six coniferous essential oils (EO) – common juniper (Juniperus communis L.), Scotch pine (Pinus sylvestris L.), Mediterranean cypress (Cupressus sempervirens L.), cedar (Cedrus spp.), European silver fir (Abies alba Mill.) and the Norway spruce (Picea abies (L.) H. Karst.) – against granary weevil (Sitophilus granarius L., 1758) (Coleoptera: Curculionidae) under different storage conditions. Fumigant bioassays were conducted at five temperatures (15, 20, 25, 30, 35 °C), two relative humidity (RH) levels (55, 75 %) and four concentrations (250, 500, 750 and 1000 μL per 226.2 mL Petri dish). Both initial (immediate) adult mortality and progeny production of S. granarius were determined as functions of EO concentration, temperature and relative humidity. Mortality was strongly affected by the oil type and storage conditions. Common juniper EO showed the highest and most consistent activity, achieving almost complete suppression of adult survival at 15–20 °C and moderate RH (55 %), even at lower concentrations. Mediterranean cypress EO exhibited a similar trend, with high efficacy under colder conditions but reduced activity at 25–30 °C. Scotch pine EO was moderately effective, particularly at intermediate temperatures (20–25 °C) and higher concentrations, while cedar, European silver fir and Norway spruce EOs showed variable responses depending on storage conditions and concentration. The efficacy of EOs decreased at higher temperatures (25–35 °C) and high humidity (75 %), while controls showed the highest survival, confirming insecticidal effect of the EOs. Common juniper and Mediterranean cypress EOs are effective for cold storage, but warm and humid conditions may need additional strategies. Coniferous EOs represent an environmentally friendly alternative to synthetic insecticides in stored products pest management.