Nuclear polyhedrosis virus (NPV) and Bacillus thuringiensis (Bt) are the most commonly used biopesticides for the control of Helicoverpa spp. larvae on cotton crops in Australia. These naturally occurring entomopathogens can regulate populations in agricultural and forestry ecosystems. However, in many instances, entomopathogens have not provided consistent control of pests to an acceptable level (Benz 1987) and in some cases yield loss has occurred (Mensah 2002). In Australia, NPV and foliar Bt are used to control Helicoverpa spp. on conventional cotton crops particularly early in the cotton season. The efficacy of NPV and Bt against Helicoverpa spp. larvae is often found to be inconsistent and can be inadequate when population pressure is high (Mensah 2002). This may be due to the narrow host range, retarded response and/or poor residual activity of biopesticides after application (McGuire 2000). Ultra-violet light (UV) is known to cause pathogens such as NPV and Bt to lose at least half their original activity within days of being applied in the field (Bull et al. 1976; Krieg et al 1980; Jeyakumar and Gupta, 1999). For these biopesticides to fulfil their role as effective, selective Iarvicides in cotton, it is essential that their persistence and efficacy be improved. Most studies aimed at overcoming the constraints of short persistence and low efficacy of entomopathogens, have focussed on the formulation of the pathogens, viz. fungi virus and Bt in oils within a biologically based framework (Inglis et al 2000). Oil based formulations has been reported to increase the adhesion of propagules to the insect integument, enhance spread of inoculum over the insect body, enhance penetration of the insect cuticle, protect propagules from ultra-violet radiation and enhance infection under low humidity (Ingris et al.2000). Recent research on citrus and a range of other horticultural crops led to the development of a PSO formulation which incorporated heavy base oil for maximum efficacy (Beattie et al. 1995, Beattie and Smith 1997) and UV light absorbing compounds to reduce the detrimental effects of UV light on unstable oil molecules. Minimising UV induced breakdown of the petroleum base oil can in turn reduce the potential of the PSO to damage plants (Hodgkinson et al 2002a; Hodgkinson et al 2002b). Further PSOs were developed based on this premise, to improve the effectiveness of UV labile biopesticides against cotton pests. The aim of this study was to determine the effect on persistence and efficacy of NPV and Bt of a UV protected PSO. The effect was measured by the control of Helicoverpa spp larvae, in relation to days after treatment application.