THE PERSISTENCE AND FATE OF MALATHION RESIDUES IN STORED BEANS (PHASEOLUS-VULGARIS) AND MAIZE (ZEA-MAYS)

Two experimental models simulating the traditional storage conditions prevalent in Kenya, i.e. the open basket model and the modern wooden b ox model, were used to study the rate of dissipation and fate of malat hion residues in maize grains and beans stored for periods of up to on e year at ambient temperatures averaging 23 degrees C. The grain sampl es were initially treated with 10.36 mg kg(-1) of radiolabelled malath ion dust prior to storage and portions analysed at regular intervals f or malathion, malaoxon and the transformation products isomalathion, m alathion alpha-monocarboxylic acid and malathion beta-monocarboxylic a cid using a combination of chromatographic, radioisotopic and mass-spe ctrometric techniques. The findings showed a gradual penetration of ma lathion into the grains in amounts which were slightly higher in maize than in beans irrespective of the method of storage. After 51 weeks o f storage, 34-60% of the initial residues persisted in all the grains. The total residual levels were slightly higher in beans than in maize irrespective of the storage methods though the persistence was a litt le higher in the wooden box than in the open basket. The rates of diss ipation of the pesticide from the grains decreased with storage time a nd followed a biphasic pattern. Applying first-order reaction kinetics , the following half-lives were obtained: maize grains stored in open basket: 194 days; maize grains stored in closed wooden box: 261 days; beans stored in open basket: 259 days; beans stored in closed wooden b ox: 405 days. Beans stored in the wooden box had higher levels of boun d residues than those sampled from the open basket. This trend was sim ilar in maize grains although the concentrations were lower. The analy sis of malathion metabolites confirmed the degradation trend of the re sidues.