RESIDUAL EFFECTIVENESS OF INSECTICIDES AGAINST SOIL-INHABITING INSECTS

Abstract

Results of tests with just-hatched grubs of Diaprepes abbreviatus L. as biological indicators of the residual effect of insecticides mixed with soil were much more consistent and showed little loss of toxicity in 3 years as compared with the tests using field-collected white grubs. Thus tests with third- or last-instar grubs of Phyllophaga (or Lachnosterna) portoricensis Smyth and P. vandinei Smyth, as well as with first-instar white grubs of these species, although not entirely conclusive, appear to indicate that, after being mixed with soil for 3 years, Chlordan is not nearly so toxic to white grubs as when originally applied. But the latest results with weevil grubs are almost identical with those of 3 years ago. Even when freshly applied at the rate of 10 pounds per acre, the gamma isomer of benzene hexachloride did not kill just-hatched grubs of Diaprepes abbreviatus L., and cannot be recommended for field application to soils infested with them. This insecticide continued to be almost as effective against white grubs 3 years later as when originally applied to soil infested principally with them. Aldrin appeared to be fully as effective against the weevil grubs of Diaprepes abbreviatus L., as when first applied 3 years earlier, and was found to be very toxic to the grubs of Ligyrus (which are of very minor economic importance). Even if it is less toxic to third-instar Lachnosterna white grubs at the concentrations recommended for original field application (2 pounds per acre) it will kill these grubs in the first-instar. DDT appeared to be only slightly toxic to Ligyrus grubs as compared with Aldrin. For cucumbers and for the grubs of Diaprepes abbreviatus L., and the endemic species of Lachnosterna, DDT experienced no appreciable loss in toxicity in the soil with which it had been mixed 3 years earlier.