An investigation of peanut storage pests in Queensland 2. Insecticidal treatment of bulk nut-inshell peanuts in silos

Abstract

The effectiveness of sprays of malathion or synergised pyrethrins in protecting groundnuts stored in the shell in silos in Queensland against insect attack was investigated in 1961-62 and 1962-63. The sprays were applied to cleaned groundnuts passing on a conveyor belt prior to storage at rates of 1 gal. 3.6% malathion emulsion spray to 2 1/3 tons groundnuts, giving a concentration of 66 parts malathion per million, and 1 gal. synergised pyrethrins (0.2% pyrethrins with 3-2% piperonyl butoxide) to 3 tons groundnuts, giving concentrations of 3 and 48 p.p.m., respectively. In addition, the surface of the bulk and the roofs and exposed walls of the bins were sprayed every two months with 0.5%. malathion combined with synergised pyrethrins (0.04% pyrethrins and 0.64% piperonyl butonide). After six months, the malathion spray gave good control of all species except Cadra cautella; adults of this species were virtually absent from the treated bins, however, though numerous in the untreated controls. The pyrethrins spray was in general less effective, but gave slightly superior control of C. cautella. In a trial in 1962-63 to determine the most effective concentration of malathion, the minimum needed to control a heavy infestation in which the principal species were T. castaneum, O. mercator, C. cautella and Carpophilus dimidiatus was 41 p.p.m. ; this left residues of up to I p.p.m. in the kernels and 9 p.p.m. in the shells. The cost of treatment compared favourably with that of fumigation, and a further advantage of the spray was its persistence. In 1963, the whole of the crop stored in the silos was treated with malathion at 44 p.p.m. and remained virtually free from infestation until January 1964, when light infestations of Cadra cautella occurred in some bins on the roofs of which groundnuts heavily infested with this species were stored. The practice of storing groundnuts in this manner was subsequently discontinued and a higher level of control was obtained in the 1964 crop. Localised accumulations of moisture and residues on the walls of many bins permitted infestations of Carpophilus dimidiatus to build up, but numbers were low and it is thought that aerating the bins should prevent this type of infestation and also greatly improve the persistence of malathion. Observations on the abundance and distribution of the more important insects in bins treated with malathion are reported in an appendix.