Stopping grain chinch bugs
Grain chinch bugs — Macchiademus diplopterus — are indigenous to the wheat-growing areas of the Western Cape. They are pests of cereals but also cause problems for fruit exporters. By Engela Duvenage.
“Ironically, they do not damage fruit at all, but just seek shelter there during a certain life stage. This unfortunately coincides with fruit harvesting times. Bugs land up on fruit and in packaging, and so become pests of phytosanitary concern to certain markets,” says Dr Shelley Johnson of the Department of Conservation Ecology and Entomology at Stellenbosch University.
Johnson has been studying grain chinch bugs since 2006. A survey she conducted in 2006 and 2007 showed that they were especially common around Ceres, Porterville and Piketberg — all towns surrounded by wheat fields.
A rum business?
Imagine if controlling grain chinch bugs could be as easy as adding a thin sheet or lining to packaging to ensure that the pesky bugs are not hiding among fruit when these reach overseas markets?
That’s what researchers in the Department of Conservation Ecology and Entomology at Stellenbosch University hope to achieve by using ethyl formate. They want to develop an ethyl-formate formulation that can be incorporated into fruit packaging, and that will slowly release ethyl formate to kill grain chinch bugs — similar to how sulphur-dioxide sheets kill decay-causing organisms.
Rum-smelling ethyl formate is used as a food flavouring and contributes to the flavour of raspberries. The United States Food and Drug Administration considers it generally safe and suitable for use in human and animal food.
Ethyl formate is a natural product and therefore considered an alternative to fumigants that are toxic or harmful to the environment.
“Ethyl formate is highly reactive and volatile, making it ideal for our purposes,” says doctoral student Khaya Gould. “One of its modes of action is to bind to cytochrome, a protein responsible for oxygen uptake in cells, thus inhibiting their ability to function and inhibiting oxygen uptake in cells.”
Once ethyl formate has done its job it readily breaks down, being either metabolised by cells or hydrolysed by atmospheric water, leaving behind non-toxic environmentally neutral products.
“In essence, once used it should have no effect on the environment or the fruits being treated, unlike commonly used sulphur-dioxide sheets,” says Gould. “The concentrations at which it is dangerous to humans are much higher than those needed for it to act as a pesticide.”
Scientists versus chinch bugs
“Ethyl formate was the buzzword in phytosanitary-treatment research when I started my master’s in 2014, as it is considered a safe product,” says Dr Renate Smit, recalling the early years of studying the compound as an alternative fumigant. Smit is the manager of the new Hortgro Phytosanitary Laboratory.
Her PhD in Entomology, completed in 2019, investigated whether ethyl formate or Controlled Atmosphere Temperature Treatment Systems — CATTS for short — could work against grain chinch bugs and two other pests of concern.
CATTS is a technology that combines heat and atmospheric stress. Unfortunately, Smit found that grain chinch bugs have a high thermal tolerance. She soon realised that CATTS was not suitable as a phytosanitary weapon against the bugs because the heat required to kill grain chinch bugs damages fruit.
Ethyl formate, on the other hand, was a solid option to treat stone fruit, but more care is needed with pome fruit. Laboratory-based studies showed a threshold at which phytotoxic damage develops in pome fruit. There are, therefore, limits to how much can be used to do the job, Smit reported in Scientia Horticulturae.
Her next question was simple. “It is great that it works, but how do we apply it?”
Smit started playing with the idea of infusing material with ethyl formate after reading a paper by Canadian researchers that described the development of encapsulated ethyl formate for activated release application. Subsequently, they released it as a vapour into a blueberry-storage facility and successfully killed all life stages of spotted-winged drosophila. Japanese researchers have since used it for insect management in export-quality mushrooms.
Packaging the solution
Preliminary work on packaging that incorporates ethyl formate started last year. The project is led by Johnson and Smit, who are both seconded to Stellenbosch University by Hortgro. They are working with Gould, and polymer expert Prof. Albert van Reenen of the Department of Chemistry and Polymer Science at Stellenbosch University.
“We want to see if it is possible to include a precursor into packaging, such as the wrappers around apples and pears. It could be something similar to the sulphur-dioxide sheets used by the table-grape and blueberry industries to curb antimicrobial activity and decay,” explains Smit.
“If you pack it along with fruit, it could release ethyl formate and break down over time while being shipped,” she adds. “If you could get the concentration of ethyl formate being released from the material high enough, without damaging the fruit, you would theoretically be able to kill all insects.”
Gould has already successfully synthesised an ethyl-formate precursor in powder form. “It releases gas when exposed to moisture or weak acid,” he notes. “Using it as a powder allows us to avoid working with its liquid form, which is highly flammable.”
Van Reenen is currently developing a suitable polymer material to infuse with the precursor.
Smit hopes that a prototype material could already be trialled by mid-year in a small-scale study. “I want to be sure it works before applying for further funding,” she says. Her ethyl-formate trials will be among the first conducted at the new Hortgro Phytosanitary Laboratory.
Smit also plans to test whether ethyl formate can work against known South African pests such as banded fruit weevils and red spider mites, and post-harvest plant pathogens causing stem-and calyx-end mould.