malaoxon in the blood and malathion mono- and di-acids in the urine 
as well as monomethyl phosphate, dimethyl phosphate, and dimethyl 
thiophosphate (Knaak and O'Brien, 1960; Mattson and Sedlak, 1960; 
Seume and O'Brien, 1960). 
Although both phosphatase and carboxyesterase activity are present 
in insects, it is the lower activity of the carboxyesterases and the 
higher production of malaoxon that apparently accounts for the 
higher insect toxicity (Cook et al., 1957b; 1958a,b; O'Brien, 1957b; 
Krueger and O'Brien, 1959b; Lewallen and Nicholson, 1959; Kojima et al., 
1960, 1963a; Perry 1960a; Weidhaas, 1959; Matsumura and Brown, 1961, 
1963; Bigley and Plapp, 1962; Mengle and Lewallen, 1963). 
Dimethyldithiophosphate was found in plants treated with 
malathion (Tomizawa and Sato, 1960b). When malathion treated maize 
and wheat was stored in sealed jars for six months and analyzed 
monthly, dimethyl phosphorothiolate, malathion mono-acid, and 
malathion di-acid were identified by thin-layer chromatography. Other 
studies showed that wheat grain also converted malathion to dimethyl 
phosphate and malaoxon. Malathion breakdown appeared to be principally 
hydrolytic rather than oxidative, (Rowlands, 1964, 1965). In rice 
bran, the rate of disappearance of malathion was greater in those 
samples having the higher acid content. Hydrolysis proceeded, 
apparently, by both enzymatic and chemical routes. Dimethyl 
phosphorothionate and dimethyl phosphorothiolothionate were found 
(Rowlands and Clements, 1965). 
122 
