Efficacies of low- to high-volume (960-10 700 litre ha-1) citrus sprayers for applying petroleum spray oil to control Chinese wax scale

Petroleum spray oil (2, 4 and 6% in water) was applied to Valencia orange, Citrus sinensis (L.) Osbeck, for the control of Chinese wax scale, Ceroplastes sinensis del Guercio, using a low-volume (< 2000 litre ha-1) air-blast (LV AB) sprayer, a low- to high-volume (L-HV) (up to 7000 litre ha-1) sprayer with four fan-assisted rotary atomiser (FARA) spray heads mounted on a vertical tower, and a high-volume (> 7000 litre ha-1) oscillating boom (HV OB) sprayer. The most effective sprayer was the L-HV FARA sprayer. The most cost-effective treatment was a 20 ml litre-1 (60 litre oil ha-1) spray applied at 3000 litre ha-1 by the L-HV FARA sprayer. It gave mortality equivalent to a standard 20 ml litre-1, 10 700 litre ha-1 spray (214 litre oil ha-1) applied by the HV OB sprayer but with 72% less spray and significantly less oil deposited per cm2 of leaf area. Equivalent or significantly (P = 0.05) higher mortality, than that given by the 10 700 litre ha-1 HV OB spray, was given by the 40 ml litre-1, 3000 (120 litre oil ha-1) and 60 ml litre-1, 2180 and 3000 litre ha-1 (130.8 and 180 litre oil ha-1) L-HV FARA sprays, but the 60 ml litre-sprays deposited more oil per cm2 than the 20 ml litre-1 HV OB spray and were considered to be potentially phytotoxic. The least effective sprayer was the LV AB sprayer, which applied a 60 ml litre-1 spray (57.6 litre oil ha-1) at 960 litre ha-1. Linear relationships were established for Chinese wax scale mortality, transformed using an angular transformation (aresin proportion), versus log10 spray volume for the 20, 40 and 60 ml litre-1 sprays applied by L-HV FARA at 1260, 2180 and 3000 litre ha-1, mortality versus log10 micrograms oil cm2 and log10 micrograms oil versus log10 volume of oil sprayed.