Mode of action of the rediscovered fumigant - ethyl formate

Ethyl formate is a potential replacement for the ozone-depleting fumigant, methyl bromide. We have investigated the mode of toxic action of ethyl formate as part of an evaluation of its efficacy against insects. The acute toxicity of a range of alkyl esters, ethanol and formic acid were tested in the adult rice weevil, Sitophilus oryzae (L.) and adult grain borer, Rhyzopertha dominica (F.). All alkyl formates and formic acid were similarly toxic to S. oryzae and at least twice as potent as ethyl propionate, methyl acetate or ethanol. The order of potency was similar in R. dominica for the tested substances. Alkyl formates were rapidly metabolised in vitro to formic acid, when incubated with insect homogenates, presumably through the action of esterases. S. oryzae and R. dominica fumigated with a lethal dose of ethyl formate had 8 and 17-fold higher concentrations of formic acid, respectively, in their bodies than untreated controls. Cytochrome c oxidase activity from isolated insect mitochondria was unaffected by acetate and propionate esters or salts, ethanol or methanol but sodium formate was inhibitory. Toxicity of volatile formate esters to insects is much higher than related alkyl esters, due to hydrolysis of the formate esters to form formic acid and its inhibition of cytochrome c oxidase. From our knowledge of the mechanism of action, the development of insect resistance to ethyl formate is predicted to be slow.