Petroleum spray oils (PSOs) are now an essential part of many integrated pest management (IPM) programs in agricultural crops (Beattie et al. 1995; Mensah et al. 1995). Despite these benefits, the use of PSOs has been limited in cotton due to a perceived risk of PSO-induced phytotoxicity and the fact that PSOs do not have a quick kill effect. Recent research on citrus and a range of other horticultural crops had led to the development of PSO additives such as UV light absorbers that can reduce the risk of phytotoxicity. The additives also enhance the persistence and activity of UV sensitive products such as biological insecticides to improve their efficacy and also can be mixed with synthetic insecticides to improve efficacy against a wide range of pests (see Mensah at al this proceedings; Beattie et al. 1995; Mensah, et al 1995; Jeyakumar and Gupta, 1999). The mode of action of PSOs appears to be multifaceted. Recent studies by Mensah et al. (2001, 2000) have shown that application of PSOs can affect Helicoverpa spp. egg lays on a range of host plants. Deterrence of oviposition by any compound or product against any pest should have a significant effect on the pests&#39 population by reducing the number of eggs deposited by pests on the plant leading to a reduced pest population (Hagen et al 1971). For cotton growers to utilize the oviposition deterrent activity of PSOs in their pest management program, they need to understand the mechanisms underlying the oviposition deterrent activity of PSOs. This will give growers a detail understanding of PSO use pattern effective against cotton pests particularly Helicoverpa spp. The aim of this study was to determine the mechanism that may be involved in the oviposition deterrence activity of PSO against Helicoverpa spp