We determined the efficacy of direct applications of a range of concentrations between 1% and 10% of an nC24 petroleum spray oil (PSO) (Sacoa, Biopest) for use against the cotton aphid, Aphis gossypii. We focused on the effects of the oils on aphid behaviour and mortality. We also determined the indirect effects of the deposits of the PSO. We focused on the effects of the oils on acceptance of cotton as host plant by the aphids, as well as on aphid mortality rates and their success in establishing colonies.