Biological Activity of Humulus lupulus (L.) Essential Oil and Its Main Components against Sitophilus granarius (L.)

Besides its use in the brewing industry, hop cones appear as a powerful source of biologically active compounds, already checked for their putative anticancer, antimicrobial, and other bioactivities. Conversely, hop use in pest control remains to date under-investigated. Therefore, the biological activity of hop essential oil (EO) and its main constituents was investigated here against Sitophilus granarius. Adult contact toxicity was found 24 h after treatment with hop EO (LD(50)/LD(90) 13.30/40.23 µg/adult), and its three most abundant components, α-humulene, β-myrcene, and β-caryophyllene (LD(50)/LD(90) 41.87/73.51, 75.91/126.05, and 138.51/241.27 µg/adult, respectively); negligible variations at 48 h, except for α-humulene (LD(50)/LD(90) 26.83/49.49 µg/adult), were found. The fumigant toxicity of the EO and terpenes was also checked: in the absence of wheat grains, β-myrcene showed the highest inhalation toxicity (LC(50)/LC(90) 72.78/116.92 mg/L air), whereas α-humulene, β-caryophyllene, and the EO induced similar values (LC(50)/LC(90) about 130/200 mg/L air); with the exception for EO, the wheat presence increased (30–50%) LC(50)/LC(90) values. Moreover, EO and terpenes were perceived by insect antennae and elicited repellent activity. Only β-caryophyllene showed an anticholinesterase effect, this suggesting that different mechanisms of action should be responsible for hop EO toxicity. Therefore, hop EO appears suitable for developing control means against this pest.