Fumigant toxicity of monoterpenoid compounds against the confused flour beetle, Tribolium confusum Jacquelin du Val. (Coleoptera: Tenebrionidae)

Abstract

This study was carried out to determine the fumigant toxicity of the monoterpenoid compounds; a-pinene, p-cymene, eugenol, cuminaldehyde, linalyl acetate, linalool, alpha-terpinene, gamma-terpinene, limonene, beta-pinene, allyl isothiocyanate and diallyl disulphide against all life stages of Tribolium confusum Jacquelin du Val. at 25 degrees C and 65% RH. Preliminary biological tests were performed to test the fumigant activity of monoterpenoid compounds against all life stages of T. confusum exposed to a dose of 100 mu LL-1 for 24-h. Preliminary biological tests indicated that allyl isothiocyanate and diallyl disulphide had high fumigant activity on all life stages of the test insect with mortality rates ranging from 92 to 100%, while cuminaldehyde was highly toxic to only its egg stage. Other monoterpenoid compounds, apart from cuminaldehyde, allyl isothiocyanate and diallyl disulphide, showed low fumigant toxicity to all life stages of the test insect with mortality rates ranging from 0 to 27.3%. Allyl isothiocyanate and diallyl disulfide were the only compounds to kill all life stages of T. confusum. Lethal concentration tests showed that allyl isothiocyanate was more toxic to T. confusum larvae, pupa and adults with 5.99, 2.69 and 3.50 mu LL-1 LC90 values, respectively, than diallyl disulphide by 98.06, 42.26 and 47.57 mu LL-1 LC90 values, respectively. The complete mortality of all life stages of T. confusum was achieved at a Ct product (Concentration x time) of 254 mg h L-1 of allyl isothiocyanate. Based on the toxicity data, allyl isothiocyanate has potential as a fumigant for controlling stored-grain insects.