The persistence, as assessed by biological and chemical methods, of field deposits on apples from two spray programmes, each of DDT, carbaryl or azinphos-methyl, was compared throughout eight weeks following the first application in an orchard in south-eastern England. Newly emerged larvae of the codling moth, Cydia pomonella (L.), were used to assess, in the laboratory, the biological activity of the deposits. The spray programmes studied were (1) two applications of conventional concentrations (i.e., 0·1 per cent, for DDT and for carbaryl, 0·04 per cent, for azinphos-methyl) three weeks apart and (2) three applications of half these concentrations at two-weekly intervals. DDT wettable powder, carbaryl and azinphos-methyl were compared in 1961, DDT emulsion, DDT wettable powder and carbaryl in 1963. Results of biological and chemical assessment of the deposits are presented graphically. Over-all, the performance of the three-application programmes was as satisfactory as that of the two-application programmes, although a smaller amount of chemical was used. The serious reduction in deposit caused by heavy rain shortly after the first application of DDT wettable powder and of carbaryl in 1961 is noted. In the absence of heavy rain, carbaryl was biologically the most persistent, but the best performances of the other chemicals were almost as good as that of carbaryl.
LD50 values of fresh deposits obtained by field spraying were shown to be similar to those obtained by laboratory dipping.
Comparison of the toxicity of fresh and weathered field deposits of similar magnitude indicated that the biologically available proportion of a DDT deposit from either formulation decreases as a result of weathering. This phenomenon was not found to occur with carbaryl and either not at all or to a much lesser extent with azinphos-methyl.
The deposits obtained in the comparison of the spray programmes were separated into their fresh and aged components, so that differences in the contribution to the final deposit provided by each application could be seen. With the very persistent DDT emulsion, only about half of the six-week deposits was composed of chemical from the final application, but, with the much less persistent carbaryl and azinphos methyl, the six-week deposits were almost entirely derived from the final application. By substitution in an equation to obtain the rates of decay of the deposits from the three-application programmes of all chemicals, it is shown that deposits from the second and third applications were, in all cases, more persistent than those from the first. Possible reasons for this difference are discussed.
The significance, in terms of field control, of the results for the larvicidal activity of the deposits from the different spray programmes is indicated. It is concluded that data on the LD90 of insecticidal deposits on apples, together with measurement of their persistence in the field, can be used to assess the probable field performance of codling-moth insecticides and to determine the most efficient means of employing them, although the deposit level that ensures 90 per cent, larval mortality in the field is higher than the LD90 determined in the laboratory.