Residue analysis of DDT, malathion and kelthane on pears.

Malthion, DDT, kelthane and their mixture (3 : 10 : 5) were applied to pear fruits at the rate recommended by the Ministry of Agriculture. The rate of disappearance of their residues on pear fruits was investigated under refrigerated conditions (2 +/- 1 degree and 47% relative humidity (RH)). The same insecticides were tested in higher concentrations (1.5 and 2 times) to determine the effect of insecticides on the black spots usually found on pears after storage. No relation was found between the tested insecticides and the appearance of the black spots. Residue-disappearance curves of the three tested concentrations of these insecticides and their mixture were drawn on semi-log paper. The residues of each material found on pear fruits after one hour, 1, 8, 15, 30 and 45 days' storage in a refrigerator, estimated biologically by C. pipiens larvae and D. magna were recorded. The highest concentrations of the mixture and of the individual insecticides left higher residues on pears than the lower ones of all the insecticides either as a mixture or alone. Stored pears, refrigerated under the above conditions immediately after treatment with the mixture of insecticides recommended for the pest control of pears, cannot be consumed fresh after 45 days.