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The grain store calendar

Wednesday 3rd July 2024

It will come as little surprise that your grain store is a very attractive place for certain types of insects.

These are called the SPIs- the stored product insects. In this article, we ask our MD and resident Pest Geek, what they are and when to do battle.


Q.Martin, what types of insects make their home in a grain store:

They can be broadly broken down into two groups:

1. The animal cache insects (also called primary infesters). These insects are particularly widespread. Studies have shown that 90% of squirrel caches contain SPIs. If you think about it, your grain store is no different to an animal cache. It’s a place where animals (humans in this case) keep their food when it’s not growing or being eaten.
2. The wood infesting insects (also called secondary infesters). These are also extremely common but they tend to live on the stuff that grows on grain rather than the grain itself. Where their natural habitat might be under the bark of a tree, they are equally happy to survive in a grain store where they can seek out mould that grows on grain.


Q. OK, now we know what we’re dealing with, what actions should I take through the year?

These are the actions you can take at each stage of the year cycle of the grain store to discourage SPIs from getting into the grain.


Emptying the store

Once the grain has gone from the store it is time to start the stopwatch. This is your opportunity to limit the ability of grain-infesting insects to get a toehold, and time is money.

I am well aware cleaning the grain store is the worst job of the year, so most people leave it to the students and labourers for a rainy day just before harvest.

Very few people are aware that leaving the cleaning of grain store so late can be extremely damaging.

Most of the animal cache insects, that will be interested in a mostly empty grain store, have quite a low breeding rate. Think of grain weevils for instance that only lay 40-60 eggs per female.

Leaving the remnants of your stored grain is leaving them in peace to start building populations. Let’s not forget, with the store mostly empty, it will be impossible to do any cooling and your grain will be at ambient temperature. This situation is more or less perfectfor our primary SPIs. They have all the things they need.

1. Infestable material
2. Uncontrolled temperatures
3. Peace and quiet to build populations slowly.

Thinking in these terms, you can see how beneficial it might be to get the store cleaned as soon as it is empty.

If you can’t face the task or if you simply aren’t able to, a quick call to Dealey will soon get one of our cleaning team out to your site to give you a quote. I promise you will be pleasantly surprised and it will be money well spent.


Pre Harvest

Just before harvest, temperatures have started to really amp up. This starts to encourage insects to breed quicker, including both primary and secondary SPIs.

This increased breeding rate is only limited by the availability of food for the insects. Hence the importance of proper cleaning early on.

You can check for insects at this stage using an insect trap available from Dealey.

Many grain stores will opt to have the store treated with an insecticide before harvest. This is a treatment that is largely reliant on timing. Insecticide efficacy data shows an effective period of around six weeks, which is also a rough life cycle for many SPIs – the time it takes for an insect to get from egg to breeding adult. Insecticides have a very limited impact on eggs, meaning the optimum time to treat a grain store is 5-6 weeks before harvest.

If you wait right up until harvest to have a treatment it may be that you will be controlling the adults and larvae but not any of the eggs. These eggs will then hatch out into a grain store filled with new grain and begin infesting your products with no upper limit on the population size.



After the grain is in store, it is a critical time to control conditions for breeding insects.

Studies have shown it takes very little encouragement for an insect to make its way into your grain store.

For the primary SPIs, it can be something as simple as a single degree in temperature difference which makes them decide whether to remain outside the store or make the journey into a new environment inside the store. With grain temperatures from the field being sometimes in the upper 30 degrees it can be a very attractive proposition to enter a filled grain store on a cooling evening.

For secondary SPIs the slightest whiff of either some insect pheromones (from previous years’ infestations) or mVOCs (the smell of mould that grows on grain) to encourage them into the store.

This means it is very important to have a dry, cool grain which is much less attractive to a grain-infesting insect.

It is very difficult at this point to get the temperature in the grain bulk to drop quickly. Sometimes ambient temperature remains stubbornly high after harvest and no amount of blowing seems to drop the grain temperature below the magic number of 15 degrees.

Many storekeepers will opt for a post-harvest treatment at this stage. We call it an envelope treatment.

This is a surface treatment of insecticide on the top of the grain bulk to either discourage or kill any insects which might be making the attractive journey into the grain store.

The treatments are below the accepted maximum residue levels for insecticides in stored grain and will give any storekeeper peace of mind that if temperatures do not go down quickly enough, they will remain protected from infestation.


During the storage season

From filling to outloading can be a long time. Grain must be kept cool and dry.

This can be harder than it sounds, with hot spots developing, condensation leading to grown grain on the surface, or fines in the grain leading to uneven cooling.

At this point, many customers will request we carry out monthly grain checks to monitor the moisture, temperature and storage conditions to be sure an insect-friendly environment isn’t being developed.

During every check of the grain, a 2mm fine sample is taken from the entire surface of the grain to check for insects.

Surface treatments of diatomaceous earth can be very useful at this point to stop infestations in their tracks if one or two insects are found.

It is all part of the GrainCare service we provide, which is a monthly check of all stored grain to prevent the nasty surprise of rejection at the weighbridge when the grain is delivered.


Q. That all sounds great, but what happens if I’m too late and the insects have made a lovely home in my grain?

Fumigation- it’s the only remedial option once an infestation has taken hold.

There are many old wives’ treatments for insects in grain. Many that are advertised on social media are completely illegal and utterly ineffective!

The only insecticidal treatment for infested grain which controls all life stages (including eggs) and will penetrate right through the grain bulk is fumigation – the use of toxic gases.

This is a very specialised skill in pest control and should not be attempted by anyone other than a trained fumigation specialist. They all carry Fumigation Specialist cards which you should ask to see.

At Dealey, we have the largest fumigation team in the UK where our technicians see grain infestations every day. This is one of the reasons we are experts at every stage of the grain store calendar up until the fumigation stage, we know how to stop things from getting to this point.

Every fumigation treatment is guaranteed to work and is certified with a clearance certificate once the fumigation is complete. This reassures buyers that infestations have been completely controlled.

This is a very detailed treatment which should only be performed by trained trusted experts.


Q. Once it’s been fumigated, will that stop the little blighters coming back next year?

Sadly not. This may bring us to the end of the grain store calendar where the grain is emptied, but the cycle needs to begin again.


Q. What help and support can Dealey offer?

We offer help and support at every stage of the grain store year.

We can help you at every stage of the grain store year to keep your grain insect-free and provide proper reassurance backed by scientific study that you will not receive rejections for insect-infested grain.

For any information on any of the stages above, please contact us through our webpage at

Contact us for a free consultation
01359 269713