Development of an improved postharvest cold treatment for Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae)

Abstract

Some of South Africa’s fresh produce export markets require postharvest cold treatment of fruit as a phytosanitary risk mitigation treatment for Thaumatotibia leucotreta. Currently, the most widely used treatment entails a 22-d exposure to sub-zero temperatures. This treatment has been primarily based on studies conducted in the 1950s and 1960s. Given the age of the data and subsequent indications that a reduced duration treatment may be effective, a new set of cold treatment efficacy data was generated in this study. The treatments were tested at the probit 9 level, using fourth and fifth instar T. leucotreta, these instars having previously been shown to be the most cold-tolerant. Three time-temperature combinations using sub-zero temperatures were initially evaluated: 20 d, 18 d and 16 d at average temperatures in the range of −0.16°C to −0.44°C; and thereafter a time-temperature combination using an above-zero temperature was evaluated: 19 d at an average temperature of 1.05°C. All treatments were shown to cause mortality at or in excess of the probit 9 level (99.9968% efficacy at the 95% confidence level). These results demonstrate the efficacy of the following potential postharvest cold treatments for T. leucotreta for phytosanitary treatment purposes: 16 d at −0.2°C, 18d at −0.4°C, 20d at −0.4°C and 19d at 1.0°C, which are recommended for conversion into the following treatment protocols: 16d at or below −0.1°C, 18d at or below −0.3°C, 20d at or below −0.3°C and 19d at or below 1.2°C, commencing once all probe readings are at or below: −0.2°C, −0.4°C, −0.4°C and 1.0°C respectively. Relative to the cold treatments currently in use for T. leucotreta, the shorter duration and in some cases higher temperatures will be less costly and logistically easier to apply with reduced adverse effects in the form of fruit chilling injury and hence trade restriction.