Efficacies of low‐ to high‐volume (960‐10 700 litre ha^{− 1}) citrus sprayers for applying petroleum spray oil to control chinese wax scale

Abstract

Abstract 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 cm 2 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 −1 sprays deposited more oil per cm 2 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 (arcsin proportion), versus log 10 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 log 10 μg oil cm −2 and log 10 μg oil versus log 10 volume of oil sprayed.