Insecticidal activities of monoterpenes and phenylpropenes against Sitophilus oryzae and their inhibitory effects on acetylcholinesterase and adenosine triphosphatases

In the present study, six monoterpenes [(−)-citronellal, p-cymene, (−)-menthone, α-pinene, α-terpinene, and (−)-terpinen-4-ol] and two phenylpropenes [trans-cinnamaldehyde and eugenol] were evaluated for their contact and fumigant toxicities against Sitophilus oryzae adults. The effects of these compounds on the mortality of S. oryzae adults in stored wheat and their inhibitory effects on acetylcholinesterase (AChE) and adenosine triphosphatases (ATPases) were examined. The tested compounds showed varying degrees of contact toxicity, with trans-cinnamaldehyde (LC50 = 0.01 mg/cm2) being the most potent compound, followed by (−)-menthone (LC50 = 0.013 mg/cm2) and eugenol (LC50 = 0.015 mg/cm2). In a fumigant toxicity assay, the monoterpenes α-terpinene, p-cymene, and (−)-menthone showed the highest toxicities (LC50 = 50.79, 52.37, and 54.08 μl/L air, respectively). Trans-cinnamaldehyde, (−)-citronellal, and eugenol were the least toxic (LC50 > 100 μl/L air). In general, the oxygenated compounds exhibited high contact toxicities while the hydrocarbon compounds exhibited high fumigant toxicities. When tested for their insecticidal activities against S. oryzae in stored wheat, trans-cinnamaldehyde was found to be the most potent compound, with 73.9% mortality at an application rate of 0.5 g/kg and complete mortality (100%) at 1 and 5 g/kg after 1 week of treatment. All of the tested compounds showed AChE inhibition, although (−)-citronellal and trans-cinnamaldehyde presented the strongest enzyme inhibition, with IC50 values of 18.40 and 18.93 mM, respectively. On the other hand, (−)-terpinene-4-ol exhibited the highest inhibition of ATPases, followed by α-pinene and α-terpinene.