Aircraft applications of insecticides in East Africa. XV.--Very-low-volume treatment of a seed-bean crop with DDT in oil solution.

Abstract

Experiments are described on the aerial application of very low volumes (0·5 gal./acre) of oil solutions of DDT to control two pests, Heliothis armigera (Hb.) (bollworm) and Acanthomia horrida (Germ.) (brown bug), of the seed-bean crop in northern Tanzania. The method was compared with an improved variant of the standard commercial method of aerial control, in which DDT emulsion is applied at 1·85 gal./acre. This improved variant was also compared with a standard commercial treatment. Very-low-volume (solution) treatments were made with rotary atomisers, normal volume (emulsion) treatments with boom-and-nozzle equipment. The results were assessed, by measuring the amount of DDT deposited on filter papers and on the leaves of the crop and by estimating pest mortality from counts made immediately before and 48 hours after spraying.A direct comparison of very-low- and normal-volume treatments under identical conditions was not possible, but it was established that under similar conditions the proportion of DDT emitted that was deposited at crop level (percentage recovery) was much the same for the two methods, varying between 41 and 62 per cent, according to conditions. The lower leaves of a fairly young crop of an open variety received about 60 per cent, of the deposit on the upper leaves, but in a mature crop of a tall dense variety this percentage fell to 35.Standard commercial practice, with the aircraft flying very close to the crop, gave a recovery of about 55 per cent, in moderately good conditions but with a variation in deposit density across the swath of over 7:1. An identical application from 10 ft. above crop, as in our method gave in rather better conditions a recovery of 62 per cent., with the deposit-density variation reduced to 2:1. This performance was much the same as that achieved by the type of emulsion application used in the other experiments described.In similar conditions, both very-low and normal-volume treatments gave very similar mortalities of Heliothis. Acanthomia was present in only one experiment and was less readily controlled; it required deposits at crop level of about 0·4 lb./acre to give satisfactory mortality, whereas in the same experiment 0·25 lb./acre gave 96 per cent, mortality in a relatively young Heliothis population containing only 10 per cent, of sixth-instar larvae.Older Heliothis larvae required higher dosages, 0·4 lb./acre at crop level killing only 53 per cent, of a population containing 62 per cent, of sixth-instar larvae. It was shown that there is a regular decrease in the kill inflicted by a given dosage on the successively later instars (from 92 per cent, of the third to 66 per cent, of the sixth) with a recorded deposit of 0·63 lb. DDT per acre.Except in the first experiment, mortalities were much below those claimed for commercial treatments, even at lower rates of DDT per acre. The reasons for this are discussed, together with the advantages to be expected from a change to the very-low-volume technique with solutions.