Fumigation to combat phytosanitary pests

Basics, benefits, and the rise of ethyl formate. By Grethe Bestbier.

Worldwide, fumigation is recognized as an effective way of controlling phytosanitary pests like chinch-bugs, weevils, mites, and moths. These insects all meet their match in powerful fumigants that safeguard our fruit exports. However, recent environmental regulations have led to the withdrawal of widely relied-upon chemicals, and research is paving the way for introduction of an alternative.

Fumigation 101

Fumigation is a method of phytosanitary pest control in which an area is completely filled with a gaseous chemical, called a fumigant, to smother or kill storage insects and pests. The technique creates a modified atmosphere that is toxic to its target, and works particularly well for the control of external pests. Pests, like grain chinch-bugs, weevils, and mites, that are found on the surface of fruit can easily be reached by fumigants. On the other hand, pests like larvae of fruit flies or false codling moths that burrow into fruit are mostly treated with other methods, such as cold sterilisation.

“There is no one-size-fits-all treatment. You need to understand the insect and also look at the fruit, to decide which approach you are going to use,” says Dr Renate Smit, postdoctoral research fellow in the Department of Conservation Ecology and Entomology at Stellenbosch University, who conducted her doctoral research on commodity and insect tolerances in relation to phytosanitary post-harvest treatments.

The fumigation process generally involves a few standard phases. For the fumigant ethyl formate, the treatment area must be gas-tight. Because certain fumigation chemicals are explosive, the room or container’s oxygen levels must be lowered, often by increasing the amount of nitrogen in the air. At the correct oxygen level, the fumigators dispense a specific dose at the required concentration for the target pest.

As Smit explains, each pest is different and requires a unique treatment type and concentration of fumigant, for a specific duration.

After applying the fumigant for the correct amount of time, the area is opened and ventilated until it is safe to enter and handle the fruit. Sometimes, to speed up the ventilation process, cold air is blown through the room or container. Post-operation ventilation of the area is a critical safety aspect of fumigation.

The old and the new

Several fumigants are available for pest eradication. Each fumigant relies on a different mechanism of action, depending on its chemical composition. So, how does one choose the best fumigant for your pest problem? According to Smit, it is not always a straightforward decision. Many fumigants have a broad spectrum, so there are other aspects to consider.

“To choose the correct fumigant, you need to look at the history of the fumigant: what pests can it deal with, how successful has it been, how toxic is it, are there issues with pest resistance?” explains Smit. “From there, you can narrow it down to the most appropriate treatment.”

Methyl bromide has long been widely used as a broad-spectrum, highly penetrating, and fast-acting fumigant. However, due to its ozone-depleting characteristics, methyl bromide’s production and use was restricted by the Montreal Protocol, and it is in the process of being phased out globally. While some exemption treatments of quarantine pests with methyl bromide are still allowed, the chemical will eventually be completely banned.

“When methyl bromide was discovered, it took over in some ways, and many of the other fumigants took a back seat. Methyl bromide is very penetrating, which simply means it reaches all the insects, regardless of where they are on the fruit, making it extremely popular,” explains Dr Shelley Johnson, Hortgro research associate in the Department of Conservation Ecology and Entomology at Stellenbosch University. “Now that we can’t use methyl bromide anymore, we are looking into other options, and one of them is ethyl formate.”

Ethyl formate was first used in the 1920s to fumigate grains. In some ways, it is not the ideal replacement for methyl bromide – it is less penetrating, less toxic to the target pest, and highly explosive.

“It is not just as simple as replacing methyl bromide with ethyl formate, because of the differences in application. Because of its explosive characteristics, ethyl formate is not as easy to apply,” says Johnson.

However, it is still a viable option, especially with methyl bromide’s rising cost due to reduced production and low availability of stock. Smit and Johnson are currently researching the application of ethyl formate fumigation on a larger scale than has been done thus far.

First do no harm

Post-harvest phytosanitary treatments can compromise the taste or appearance of the fruit, and affect fruit quality. Killing the pest while still preserving fruit quality is a fine balance. Cold sterilisation, for example, is effective against a variety of phytosanitary pests, but some fruit cultivars are chill-sensitive and will suffer quality issues when exposed to such low temperatures.

“At the fumigation concentrations we use, the fruit is not damaged. However, if you overdose, there can be changes to the consistency or quality of the product. That is why we conduct trials where we look at the different concentrations and treatment durations, to determine where and when there is a phytotoxic response on a specific product,” explains Johnson.

Furthermore, if you’ve done your homework, fumigation can be a fairly quick treatment option in comparison to other types of treatments. Smit and Johnson are also working on making the fumigation process more affordable. Currently, application of the commercial formulation of ethyl formate called Vapormate can be quite costly, but Smit jokes that even this is “still cheaper than export rejection.”

Still, fumigation is a hazardous operation, and working with fumigants requires strict safety protocols. Sensors are used inside the sealed area to monitor fumigant levels.

“Sensors ensure that operations are within the fumigant’s explosive limit, and when it is safe to enter the fumigated area. It is the same procedure you would have in a controlled-atmosphere room. You have your safety checklist, and you need to make sure you follow all the correct steps. Then you should be fine,” says Smit.

Johnson is optimistic that they have found an alternative to methyl bromide. “Hopefully, once we have this set up, we can start getting it out to industry. We will need to create some awareness for ethyl formate fumigation, and explain how it should be applied.”