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June / July 2020


SA Fruit Journal: June / July 2020

A promising postharvest treatment against grain chinch bug and banded fruit weevil

The presence of phytosanitary insect pests is a limiting factor influencing the international trade of fresh horticultural products.

Postharvest control of these phytosanitary insects requires the application of cost-effective postharvest mitigation treatments that disinfest the commodity while maintain-ing market quality. Postharvest fumigation with methyl bromide, which has been used worldwide as a broad-spec-trum, fast-acting fumigant, was phased out of use in 2015 and ultimately banned due to its ozone-depleting properties. But exemptions are granted for phytosanitary purposes until alterna-tives are available. Therefore, safe alternative postharvest fumi-gation options are needed, and so far ethyl formate has proven to be a promising alternative. Aestivating grain chinch bugs (Macchidemus diplopterus) and the banded fruit weevil (Phlyctinus callosus) are key phytosan-itary pests of SA export table grapes, and strict phytosanitary regulations are enforced against them. Interceptions result in rejection of consignments, which is not only a direct financial loss, but it also jeopardises access to existing and potential new markets. Postharvest mitigation treatments to control these insects on export fruit are needed.

In work conducted by Dr Shelley Johnson from Stellenbosch University, research has shown that ethyl formate is highly effective against the grain chinch bug, at concen-trations that do not cause phytotoxic damage to the commodities tested so far. However, application of ethyl formate as a fumigant is challenging. Although a formulation of ethyl formate exists, known as Vapormate®, the dispensing technology for Vapormate® from individual gas cylinders limits the scale of the application. The volume of export fruit that requires post-harvest control for grain chinch bug and banded fruit weevil requires application at a large scale.

With the above in mind, Dr Johnson extended the work to table grapes, in order to address the following three objectives:
• Establish the efficacy of ethyl formate fumigation of grain chinch bug and banded fruit weevil on table grapes, and establish what concentrations would not be phytotoxic. This was done in small laboratory trials.
• Learn from the experience of other researchers who are already experimenting with ethyl formate fumigation of phytosanitary pests in other countries. This was done by way of overseas visits.
• Develop a system to dispense liquid ethyl formate into shipping containers.

Dr Johnson was successfully able to prove that both banded fruit weevil and grain chinch bug can be effectively controlled by ethyl formate fumigation. Banded fruit weevil is more tolerant of the treatment, but effective concentrations do not cause phytotoxic damage to table grape cultivars that were tested in the project. The work demonstrates that ethyl formate fumigation is a promising phytosanitary treatment for table grapes and other export commodities. This work was co-funded through a number of industry funding mechanisms, together with Hortgro Science and the Postharvest Innovation Fund (DST and FPEF) as part of a larger research project. A system for dispensing liquid ethyl formate into a shipping container under safe conditions was developed. This places Dr Johnson and her research team in a position to use the system to obtain efficacy data for various insect pests, conduct more fruit quality assessments and to upscale the system for larger scale application in a cold room facility. ✤

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