In¯uence of spray volume and oil concentration on the ef®cacy of petro- leum spray oil against Myzus persicae (Sulzer) (Hemiptera: Aphididae) GA Herron, 1 GAC Beattie, 2 A Kallianpur 2 and I Barchia 1 1 Elizabeth Macarthur Agricultural Institute, PMB 8, Camden, NSW 2570, Australia. 2 University of Western Sydney, Hawkesbury, PO Box 436, Richmond, NSW 2753, Australia. Abstract A Potter spray tower was used to assess the ecacy of petroleum spray oils against the green peach aphid, Myzus persicae. Increasing spray volume (1.00±5.65 mL) of C23 D-C-Tron NR resulted in a signi®cant (P < 0.001) increase in aphid mortality when oil deposits were held constant (40 g/cm 2 ). The practical implications of this result are discussed. LC95 (lg oil/cm 2 ) values of 107, 147 and 87 against C20 Total Citrole, C21 Caltex Lovis and C23 Ampol D-C-Tron NR, respectively, were calculated using a 35 mm Petri dish. These values are higher than those previously recorded for M. persicae using a large 70 mm Petri dish which restricted air ¯ow and reduced deposit. Key words bioassay method, concentration mortality, spray volume. INTRODUCTION Since the 1970s, aqueous volumes of pesticide sprayed in horticultural crops have been minimised to reduce ap- plication costs and reduce wastage. Such emphasis has led to recommendations for reduced droplet size and in- creased concentrations of pesticide in tank mixes (Mat- thews 1979). However, recent research indicates that plant and petroleum oil spray ecacies are signi®cantly enhanced by increasing spray volume and reducing con- ventionally used oil concentrations (Beattie et al. 1991; Herron & Rophail 1994). It is an accepted technique to use Petri dishes with a Potter spray tower (Holland & Chapman 1995; Kabir et al. 1993) to both contain insects and provide a post-spray treated surface. However, Petri dishes can obstruct the end of the spray tube and in¯uence spray deposit (Busvine 1971). Herron et al. (1995) showed that the 70 mm di- ameter Petri dishes used to contain the green peach aphid, Myzus persicae (Sulzer), were suciently large to in¯uence the amount of spray deposit, as those in the 70 mm Petri dish were half that obtained on a ¯at surface and required a recalibration of the Potter spray tower. In this paper, we use a 35 mm Petri dish with the Potter spray tower which did not require recalibration and use this method to test M. persicae against three petroleum spray oils. The technique was then used to test for a mortality relationship between spray volume and oil concentration sprayed. MATERIALS AND METHODS Aphid strain The strain of M. persicae tested was maintained on pot- ted `Bollhead' cabbage at the Biological and Chemical Research Institute, under insecticide-free conditions, in an isolated rearing unit under constant illumination at 23  3°C (Herron et al. 1995). Petroleum spray oil speci®cation Commercial formulations of C20 Total Citrole (Total, Putecaux France), C21 Caltex Lovis (Caltex Oil Aust., Sydney, Australia) and C23 Ampol D-C-Tron NR (Ampol Petroleum Ltd, Sydney, Australia) were tested because they were considered typical of petroleum spray oils available in Australia. The speci®cations of C21 Caltex Lovis and C23 Ampol D-C-Tron NR are tabu- lated in Herron et al. (1995). Speci®c properties of C20 Total Citrole, as analysed by SocieÂte GeÂneÂrale de Sur- veillance (SGS), Redwood (Belgium) are: relative density at 15/4°C (test method: D 1298, American Society for Testing Materials 1969a) 0.8254; kinematic viscosity at 40°C (cSt; test method: D 445, American Society for Testing and Materials 1969b) 6.449; and acid number (mg KOH/g; test method: D 979, American Society for Testing and Materials 1969a) 0.08. General characteris- tics are given in Boutourlinsky et al. (1996). Bioassay method Petri dishes were prepared using methods described in detail by Herron et al. (1996) for the bioassay of the western ¯ower thrips, Frankliniella occidentalis Pergande. Brie¯y, the method used small 35 ´ 10 mm Petri dishes which were ®lled with 3 mL of hot, melted agar. While the agar was still liquid, a single snug-®tting (34.5 mm) cabbage leaf disc was ¯oated onto the agar surface and then the agar was allowed to set. To standardise the life stage tested, only apterous adult female aphids were collected from the culture with Australian Journal of Entomology (1998) 37, 70±73