The fumigation of grain

What does the fumigation of grain involve?

Fumigation is the safe and effective combating of grain storage insects in stored grain in such a manner that the grain will be safe for the end user and free of live insects.

How does it work?

Fumigation involves the application of formulations which release phosphine and which fill the area around the grain kernel with a toxic gas.  It is important to understand that the grain is not fumigated, but the space around the grain.  Due to this fact there will be no phosphine residue on the grain.  When grain is sprayed physically, the substance used will break down to an acceptable residue level, which is safe for the end consumer, after 21 days.

Integrated pest management

It is a program in terms of which you do everything in your power to eliminate insects and fumigation is the last way out.  In the industry we talk about store hygiene.  It goes hand-in-hand with cleaning actions in and around silo complexes, the preparation of storage structures / silo bins, sealing of structures and regular monitoring.  It does not help to apply poison but to neglect the cleaning process.  It is like a kitchen.  You do not put food on a dirty plate - the plate has to be clean.  The same applies in respect of store hygiene.

The poison breaks down after approximately 21 days, after which the grain will be exposed to insects once again.  Store hygiene is the most important point of departure in the elimination of re-infestations.

Substances used

Phosphine, methyl bromide and certain other products are used to combat insects.  At this stage we use phosphine.  Phosphine reacts with the atmosphere (moisture, heat and oxygen) to release a gas.  Spraying substances also work well, but due to the residue levels, many buyers prefer not to buy sprayed grain.  Methyl bromide is very expensive and is in the process of phasing out because it is an ozone depleting substance.

Types of fumigation

Mass fumigation - silo bins, silo bags and storage dams where grain is stored in large quantities. Tablets, pills or circulation fumigation are used.  Tablets and pills are worked into the grain and a gas is released.

Circulation - a formula is added to the bin and the air (gas) circulates inside the bin, which kills the insects.

Bag fumigation – When bagged products are fumigated.  It is a labour-intensive way of fumigation.  Safety is always important, but even more so when this type of fumigation is done.

How long is the fumigation process?

You require a minimum exposure period of 120 hours.  For this reason, irrespective of the method used, the structure has to be airtight.  The KT-value has to be reached, which is the exposure measured against the gas concentration over time.  A low concentration will require a longer exposure time.

Kills all stages in the life cycle of an insect

Fumigation must kill all stages in the life cycle of an insect - insects, larvae, pupae and eggs.  Insects and larvae are the most active - they inhale the poison and die first - within 24 hours!  Pupae and eggs take longer to absorb the poison.  Pupae and eggs are sometimes exposed, but not for long enough to kill them.  The result is that the next generation will build up a resistance to the poison. 


Regular inspections are carried out at silos.  We look at store hygiene and the right fumigation and preparatory procedures.  Legislation relating to safety and the use of registered substances at the correct dosages is complied with at all times.


A full-face mask with a filter can must be worn when fumigation is done.  Every type of poison has its own filter can, inorganic gasses and organic vapour.  Act 85 of 1993, Occupational Safety and Health, regulates a safe work environment.  Regular monitoring must be done in order to prevent exposure and acute poisoning. All silos are limited space environments and have to be employee-friendly.

Health risks have to be considered when working with toxic substances.  Good training is essential.  Labels have to be read and understood.  The types of toxic substances and the effect thereof on the applicator must be understood. 


There are approximately 2 million species. The most important storage insects in the Senwes area are maize-weevils, flat grain beetles, rust red grain beetles, grain moths and grain lice, each with its own subspecies. During the winter, at temperatures below 15°C, the insects are less active and they do not reproduce.  However, at warmer temperatures and in moist conditions, they flourish.

The impact of insects

With every non-effective fumigation action, the next generation of insects builds up a measure of resistance.  Life cycles are between 20 and 40 days.  One surviving insect can lay up to 300 eggs.  The larva consumes everything in its reach in order to generate a strong insect and the main purpose of the insect is to reproduce.

What do the insects target?

Insects target the germ - exactly the part which germinates and which contains the highest nutritional value. Insect damage on grain therefore affects the grading of the grain.


Some years ago the khapra beetle occurred from time to time, but fortunately not any longer.  The occurrence of such a beetle would result in a total silo complex or store being placed under quarantine.  This is one of the reasons why we have laws regulating the importing and exporting of grain.

When is it necessary to fumigate?

Sifting tests and quality control tests on bins are done on a weekly basis.  As soon as insect infestation occurs, fumigation has to be done.  The Fumigation division is very busy during the summer months.

Problems being experienced?

Untrained persons sometimes use incorrect dosages and follow the wrong procedures when they fumigate.  Fumigation is then ineffective and the insects build up a resistance.  The grain ends up at some or other silo, with detrimental results. The current warm winter temperatures result in insect presence throughout the year and it is becoming more difficult to combat the insects.

Advice for farmers

Use a knowledgeable person, registered as a grain fumigator at the Department of Agriculture in terms of Act 36 of 1947, to fumigate your grain.  Follow the directions on the label - take note of health warnings and preventative measures, instructions for use and the symptoms of poisoning.

How big is the area covered by the Fumigation division?

There are approximately 60 business units within Senwes, which have to meet certain standards at all times.  Every business unit has a registered person with a registration number who takes responsibility at all times.  In addition, Renier and his staff render assistance at any business unit where they are needed. Advice is rendered on dosages, new substances, new practices and any other fumigation matters.

Cost of fumigation

The cost of fumigating one bin amounts to between R5 000 and R8 000, depending on the size of the bin, the type of substance and the method used. 

Exposure period

Fumigated grain may only be marketed 14 days after fumigation.  It is a very important point to remember, since responsibility is accepted for the grain until it is offloaded at the end user.

One learns something new every day and you have to be innovative.  Successful fumigation is of the utmost importance.  The decision to fumigate is taken jointly by the Silo Manager and the Head of Fumigation and in many instances it is based on experience gained in practice and not on what you can learn from a book.

Fumigation responsibility

When fumigation is done, no mistakes may be made.  The end results are the most important.  "You work with people, and you have one chance only".