Environmental Fate of Acephate
Breakdown of Chemical in Soil and Groundwater: Acephate dissipates rapidly with half-lives of <3 and 6 days in aerobic and anaerobic soils, respectively. The major metabolite was CO2 in both soil types. TLC and soil column studies indicate acephate is mobile in most soils but that aged residues (excluding acephate and its degradate methamidophos) are immobile in sandy loam soil. Most of the applied acephate and degradate methamidophos degrade to immobile compounds in 20 days. Methamidophos and carbon dioxide were identified as the major soil metabolites .
Breakdown of Chemical in Surface Water: No information was currently available.
Breakdown of Chemical in Vegetation: Acephate is quickly absorbed, translocated, and transformed in pine seedlings and cotton plants. The chemical was metabolized via cleavage of the amide bond to form methamidophos and an unknown, but insecticidally active compound, which were identified in the roots, stems, and leaves. Methamidophos was also found in cotton leaves following a single application of acephate. Four additional degradation products were formed - two of which were tentatively identified as O,S-dimethyphosphorothioate and S-methyl acetylphosphoramidothioate. The amount of methamidophos and the four products represented about 9% and 5% of the applied amount, respectively. In studies on tobacco leaves, citrus fruit, greenhouse tomatoes, and celery and lettuce, half-life disappearance of residues ranged from 1 to 15 days, depending on the crop species and the part of the plant analyzed. This same study showed residues of both acephate and methamidophos on carrots and potatoes even though no direct spraying of the underground portion of these crops occurred. Carrots contained much higher residues (up to 5.2 mg/kg) than potatoes (up to 3.6 mg/kg). In contrast to carrots, potatoes and the fruit bearing vegetables studied, the amount of rainfall occurring was directly proportional to the disappearance of both acephate and methamidophos residues on lettuce and celery. The level of residues on the eight crops studied on day 3 after application generally reflected the weight to volume ratios of the crops except where translocation appeared to give higher residues than would be expected. In carrots, potatoes, peppers and tomatoes, residues on day 7 were higher than on day 3 and day 14, thus suggesting absorption and translocation from foliage to root, tuber or fruit. Methamidophos was identified in the eight crops studied, at high levels on peppers, but at very low levels on lettuce and celery. Acephate is rapidly absorbed into the leaf tissue of cotton plants when applied foliarly, with nearly 40% of the applied acephate present in the internal extract and 25% remaining on the leaf surface 24 hours after application. The unrecovered acephate probably was translocated from the leaves or bound in unextractable form in the leaf tissue. The low vapor pressure of acephate indicates that loss due to volatilization would be negligible. Translocation of acephate into the fruiting body of the cotton plant following foliar application is not sufficient to be toxic to cotton insect pests. Little to no degradation of acephate to methamidophos occurred on the leaf surface. Methamidophos was more persistent in plant tissue than acephate (i.e. acephate was degrading to methamidophos faster than methamidophos was degrading to another compound)
