Fumigant toxicity and Cytotoxicity evaluation of monoterpenes against four stored products pests

In the present study, eight pure monoterpenes: citronellol, geraniol, linalool, eugenol, thymol, cinnamaldehyde, p-cymene and α-pinene were tested for their fumigant toxicity against four stored product pests, Callosobruchus analis, Sitophilus oryzae, Stegobium paniceum and Tribolium castaneum. Monoterpenes tested showed varying degrees of fumigant toxicity against different species of stored product pests but were highly dependent upon dosage and exposure duration. Cinnamaldehyde, p-cymene and α-pinene at a concentration of 5 -10 µl/mL air caused 90 - 100% mortality among all the four pests after 24 hrs of the treatment. α- pinene was highly toxic with LC50 values of 0.03, 0.12, 1.21 and 1.43 µl/mL air after 12 hrs of treatment for C. analis, S. paniceum, S. oryzae and T. castaneum respectively. Citronellol was least toxic having LC 50 of 1.0, 1.6, 2.3 and 2.8 µl/mL air after 12 hrs of treatment for the same sequence of the four insects, respectively.  Eugenol and thymol also showed potent fumigant toxicity causing high mortality for all the insect species. LC50 values of all these monoterpenoids tended to decrease at longer exposure times. In general, monoterpene hydrocarbons exhibited high toxicity as compared to oxygenated monoterpenes. Among the four insect species tested, C. analis and S. paniceum were the most susceptible, having highest mortality of the species tested for each compound, while T. castaneum and S. oryzae were the most tolerant of the four insect species tested. These monoterpenes have also been examined for cytotoxicity towards BV2 (microglia) cell line by employing MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and found that monoterpenes exhibit low cytotoxicity.