Straw Bale Building and Sawtoothed Grain Beetles

Written by Andrew Morrison

Sawtoothed Grain BeetlesI received an unusual question from a builder today and I do not have an answer to his question. Hopefully you can help me. Here is his question:

“We were involved in the construction of a commercial facility using straw bales. We have since run into a variety of issues with Saw Tooth Grain Beetles that have evidently arrived with the bales and are now setting off smoke detectors. Do you have experience with this ???”

Here is all the information I have been able to find. If you have anything you think would be helpful, please let me know.

The following information was taken from a website provided by Stuart Bennett and is dated 2003. Thank you Stuart for this informative information. Unfortunately, the sawtoothed and merchant grain beetles are fairly prevalent and can contaminate food supplies. The information below is descriptive; however, it speaks to food storage issues, not straw bale houses. I hope it will be helpful nonetheless in dealing with the infestation. If all else fails, Stuart says, there is always fumigation. Of course, that is something we all want to avoid I imagine.

It sounds as if moisture control, as with bale buildings in general, is paramount with population control. I hope that the food source in the bales is very small and that there is not enough to sustain any new generations. My concern is that if the beetles are in the home, tripping smoke detectors, they also must have access to your stored food. This would lengthen their life cycle and ability to breed. I think the sooner you can take action against them, the better.

Sawtoothed grain beetles are common stored-food product pests that infest cereals, cornmeal, cornstarch, popcorn, rice, dried fruits, breakfast foods, flour, rolled oats, bran, macaroni, sugar, drugs, spices, herbs, candy, dried meats, chocolate, bread, nuts, crackers, raisins, dried dog and cat food, and other foodstuffs, making them unsalable and unpalatable.

These beetles are capable of chewing into unopened paper or cardboard boxes, through cellophane, plastic, and foil wrapped packages. Once inside, populations build up rapidly often spreading to other stored foods and into food debris accumulated in the cupboard corners, cracks, and crevices. Sometimes all life stages (egg, larva, pupa, and adult) may be found.

These insects contaminate more food than they consume, and usually are discovered leaving the infested food to crawl about the house. Adults and larvae are external feeders, feeding on finely divided food particles and not whole grains. The insects have running legs (ambulatory) much like cockroaches and penetrate “tightly sealed” packaging.

Life cycle:

Sawtoothed grain beetles lay eggs singly or in small batches in the food material where the life cycle is completed. The sawtoothed grain beetle cannot fly. Adults usually live about 6 to 10 months, with some living as long as 3 years and 3 months. Female sawtoothed grain beetles usually emerge in April and lay an average of 300 eggs. Egg laying begins about 5 days after emergence and continues up to 3 to 4 weeks. Eggs hatch in about 8 days, larvae mature in 37 days, and pupa about 67 days.

The life cycle of sawtoothed grain beetles can be completed in 51 days or as early as 27 to 35 days depending on temperature. Merchant grain beetles, which are almost impossible to tell apart from sawtoothed grain beetles by the untrained eye, lay an average of 200 eggs over 28 to 42 days, requiring about 35 days to complete the life cycle. There may be as many as 6 to 7 generations under warm conditions of 85°F to 95°F and 70% relative humidity, with fewer generations throughout the winter months. Adults remain active and feed. The sawtoothed grain beetle prefers cereal-based products, whereas the merchant grain beetle prefers nuts, seeds, and dried fruit.


Although broken kernels are the preferred food of both species, sound kernels will sometimes be penetrated and fed on. The dry weight of grain may be reduced, but total weight may increase because of water absorption caused by the metabolic processes of insect populations. Molds may begin to grow on the grain, further reducing grain quality and value.


Prevention is the best strategy to avoid insect problems in stored grains. Proper bin sanitation before introduction of new grain minimizes the need for pesticides. Good sanitation involves the removal of old grain and dust in and around the grain bin/silo. This includes removal of old grain from corners, floors, and walls. Any grain remaining when a bin is emptied can harbor insect infestations which will move into the new grain. Grain that is to be stored for longer than six months may need a protective application of an approved insecticide.

Grain placed in a clean bin should be checked at two week intervals during warm months and at one month intervals during cooler months for the presence of hot spots, moldy areas, and live insects. If any of these conditions exist, the grain should be aerated to lower the moisture level and temperature.


If you have anything to add to this post please post a comment by using the comment box below.

Happy baling,


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27 Responses

  1. Straw bales can be made from rice, wheat, rye, flax, barley, oats and maybe a few other crops too. Does anyone know whether or not saw tooth grain beetles find all equally palatable or can the crop choice make a difference?

  2. John,
    That is a great question. From the research I have done, it seems they like all kinds of grains. There is another beetle, very similar in shape and size, that is often mistaken for the saw tooth grain beetle. Those beetles prefer more oily foods like nuts and seeds. Unfortunately, I do not think the type of grain makes a difference for the beetles in question. I have not heard of this problem before, so I think the best thing is to inspect the bales before you put them in the walls. Paying special attention to any bugs may save you from the headaches later.

  3. John,
    I would not abandon your plans for a bale building because of this post. I have helped hundreds of people build their straw bale home and this is the first time I have heard about any beetle problem. Further more, these are “pantry pests,” not a danger to the house or the inhabitants. The beetles came in with the bales and will die out after the “food” source is depleted. For some perspective, I have heard of many people, myself included, who have had issues with pantry moths. They love to get into all of my pasta and other dry goods. Over time, they die out and no one is the worse for wear, except the pasta! So don’t give up your plans for the bale house because of this small bug. After all, you could build a conventional house and get these bugs anyway. They are most well known for living in grain towers, not houses.

  4. John,
    Straw bale walls, once cement rendered are sealed. Any bugs and vermin will eventually die due to lack of oxygen. Most of the pest will disappear during construction time. Also, if bales are well compressed, the little monsters would be external only. There would be only a small amount of grain seeds left after the baling process.
    So, don’t panic and keep your dream home real!

    Rudy Stoffel, Bale-Up Strawbale construction

  5. Hi,
    I’m a trained agricultural engineer and although I have no direct experience with Saw Tooth Grain Beetles, I have worked in entomological research in Central America and possess basic knowledge in the field. Here is the result of some Internet research on the genus Oryzaephilus (Latin name of this beetle’s genus):
    They feed on grain are are not that good at it, as they have trouble attacking whole, healthy seeds. They are actually described as “secondary pests” because they rather follow on other bugs who started the munching. They are mainly going for the germ of the seeds and damage grain more than they eat it because they are quite choosy. This is because grains are awfully dry and it actually is quite a challenge to live on dry stuff only. Ever seen bugs in a pot of dry sugar? And sugar (glucose) is _the_ universal food for all living organisms! (Grain starch itself is just long chains of sugar molecules.)
    Now straw is principally made of hard cellulose (75%) and wood lignin (17%) and very few animals are able to consume it. Cows need four stomachs to get the job done more or less efficiently. Grain Beetles cannot do it, it is no food for them.
    Conclusion: those bugs in your bales are in trouble. They will be moving out of them to find food in your kitchen or elsewhere. So don’t fight them in the bales, because they only want to get out of there anyway. Rather protect your food in the kitchen until the bugs all die out (a few months at most). Just use sealed containers in glass or tin for the most attractive stuff (starch, cereals, chocolate, …) and wait for a few weeks before taking any further step.
    Bugs are everywhere, most are essential for pollinating crops, recycling dead matter and keeping a mutual balance, a few are a nuisance because we created incredibly favorable conditions for them (monocropping with all their predators dead)…
    So those bugs are probably much more in trouble than you are!

    Happy baling,


  6. Actually, I would rather see those beetles as shell-shocked survivors of a brutal attack by a 10 ton harvester that processed their whole habitat in a few (I suppose long…) seconds and left them with a cut “forest” of straw and all the good food gone. They may be eating the few grains left and will have to move on to find some living grass elsewhere on whose grain they will be able to feed. But things look really bad for them on the eave of winter and I don’t think many of them will manage to complete their cycle… The beetles who stayed with the grain in the harvester are the lucky ones!
    Now there are organisms that are able to feed on dry straw, but they are few and all have very specific needs of humidity, etc, because eating straw is no easy job. You’ll notice that no special precautions are taken when storing straw bales in barns for several years.
    Packed, dry bales plastered on both sides are very unattractive to any living being, and certainly less than, say, exposed wooden beams.

  7. Diatomaceous Earth. Is currently used for grain storage pest control and is in almost every food you eat. Please look up it’s uses and you will be pleasantly surprised. Buy the kind from the local feed store or organic store. DO NOT buy the processed version used in Pool filters. Made from the crushed shells of fresh water diatoms is extremely safe, does not ever expire, is not toxic(don’t breathe the dust, like any other dust it’s bad for your lungs) embedded in a wall sealed by stucco I cannot think of a better pest control.
    do not put it anywhere you want bug to be. in the walls it’s perfect, back in your pantry is great. in your garden it’s devastating. You’ll never use anything else again.

  8. D.E Diatomaceous Earth kills bugs by scratching the waterproof shell (Chitin) and allowing the bugs (or ants) to dehydrate. Although it’s considered “safe” and is non toxic, it could still be dangerous to consume it (or breath it) as it is capable of damage to the lining of the stomach/lungs in the same way it scratches bug skin.

  9. My bale house has beetles and pseudoscorpions.
    At one time there were moths as well, but that has been taken care of, thankfully.
    I know that neither bug is considered a pest but they tend to “freak” people out and any bug problem casts a dark shadow and could limit the real possibility that more people around my community will build with bales.
    They have to go, but I am at a loss for how to accomplish this.
    How could I use Diatomaceous Earth to help?

  10. How long have the beetles and fake scorpions been around? Have you been able to locate any high concentration areas? If you have specific areas you think are problems, I would start there with the Diatomaceous Earth and see if you can stop it at the source.

    Let me know where the “source” is and how long the bugs have been hanging around. This may affect the final strategy.

  11. Thanks for answering my questions. They have been around for a few years now.
    I noticed that they appear most in a couple places.
    The black beetle is the biggest problem as they are easily seen and bothered by the tennants of the house. I am many hundreds of kilometers from the house and my dad is helping with the house. The tennant said the problem is worse in the summer months.
    My dad thinks there must be a population in the house that is thriving for us to have so many bugs for so long.
    Do you know the life expectancy of these beetles? Do you think they are coming in from outdoors each fall or might i have an ongoing population?
    Thanks again for your help.

  12. Humidity is a factor in bale structures for sure. Whether it affects the insect population, I do not know; however, I think the bugs are more the result of the conditions described above than of humidity.

  13. I think the bugs may be getting in from outside. It is possible for them to be in an established colony within the building, but they would need a food source, water, and the ability to thrive in order to maintain or grow that population. There may be some areas of the house that are not properly sealed to the outside and are letting the bugs in. I suggest checking the perimeter of the house along the bottom and see if you can find any points of entry along the base of the plaster. There should be separation from the ground and plaster, thus allowing you to see any bug tunnels or place of travel. If there is not separation, start by creating it. I suggest at least 6-8 inches of space between the ground and the bottom of the plaster. Some builders finish the plaster right to grade and that is a mistake. Start there and see what you find. If you can find a point of entry, you can close it off and/or kill the bugs or discourage them from entering.

  14. Hi there….
    we have 2 problems with bugs in our strawbale house…. first of all we have a lot of those grain bugs… they evidently live in our roof that is filled with mostly canola straw and a little of grass that has had seeds (no clue what that is called in English – just know that it’s NOT hay). They come when it’s cold outside (we’re in Denmark and it tends to cool down here in the fall) and warm inside. You can see that they really like our chimney. We had some last year – but this year they seem to have increased in numbers – and not like the pest control guy said: decreased. So what are our options? Open up the roof and spray? hmmm…. It drizzles with them – and it’s a bit gross 🙁

    The other problem we have is during summer – here we have a problem with book/bark mites/lice. We have linen in parts of the construction around the windows – could they be coming from there? We have a fair amount of them – especially on the cool side of the house.

    I’m getting a bit weary of vacuuming – but it never ceases. Both insects have been in the house since we moved in 2 years ago – so: HELP!

  15. I’m sorry to hear about these problems. Is the material in the attic loose fill material or is it baled? I’m not a fan of using straw as attic insulation anyway, so my first thought would be to remove it fully and reinsulate with regular insulation. That would also allow you to investigate the attic space and look for other potential areas where the bugs are getting in. The grain bugs usually don’t last long if they are in the bale walls because they need a food source which the straw does not supply. If there is still seed available to them in the attic, then they can survive there longer. If they leave the house in the summer and come back when things cool down, then there must be a way for them to get in.

    The lice are harder to deal with because the use of pesticides does not work very well. In general, it’s the use of dry heat that kills them off. If the house is damp, then they will survive. If the house is hot and dry, then they will eventually perish. The problem is that the bales are so think and insulative that it’s impossible to heat the center of the bales up enough to kill off the book lice. That said, you may be able to drive them into another part of the house and then starve/dry them out there. As long as you bales are not damp, the bales themselves should not be a source for the bugs or even a happy home. They need to moisture to survive.

    I don’t know if this is a big help for you or not, but this is my “first step advice.” Remove the material from the attic. Fully clean the space (vacuum it up entirely). Inspect the attic space and fix any gaps that would allow access to the bugs. Consider smaller screening for attic vents. Reinsulate the attic with standard materials. Inspect the walls for moisture intrusion. Check the moisture content of the walls with a bale probe in areas with high concentrations of the lice. Once you are sure there is no source for additional moisture in the walls, heat up the house and dry it out. Continually vacuum up whatever bugs are driven out of the walls (this could be thousands). Keep the house warm and dry for several weeks (warmer than you are used to). You may need to wear shorts and a t-shirt for a while inside. I hope this helps and good luck.

  16. Hi Andrew!
    Thank you so much for your reply. The beetles have actually dissappeared by themselves. And the lice also. But it seems that every time we get rid of one insect another one shows up.
    We now have a problem with meal Worms. We have no clue where they are coming from – since we have so many different materials in the house. They apear from the window seal and strange Places like in our lamps…. and of course we have the Black beetle that comes a long when the larvae hatches. Now we’re considering spraying. The good news is that they are mainly Upstairs and away from all our food.

    Around the Windows we have linen as insulation and then paper wool.

    The straw in our roof is not in the attic, so it is not as simple as removing it. The straw in our roof should be closed all the way in with 2 layers of drywall and plaster. I defintely would not do it Again. The canola straw has been “blown” into the roof pocket with a special machine.

    I don’t think these beetles are coming from the roof. I think they are coming from the linen. But I don’t know.

  17. What a bummer. I’m sorry to hear this continues. I have never heard of such a thing. I wish you the best of success with spraying. I think it may have come to that as things seem to be continually developing for you. Please let me know how it goes.

  18. Hi guys……..just seeing these posts. Sprinkling boric acid on the floors and all around before bed and vacuuming in the morning works. As does a spray made with vodka (as the alcohol carrier) neem oil and cedar oil. Both of these things work on all beetles.

  19. Hi Andrew,
    good to have someone to go to , to help. We just recently build a straw bale house in Mexico and its beautiful and cool. But we have cigarette beetle infestation. we have been fumigating for 2 months now, we took off all the elec outlets and spray inside them…. so are you saying to seal everything so to cut off the oxygen? We moved out of the house and took all dry goods out ect and vacuum daily.
    Any advise is appreciated,
    thank you

  20. Hi Ellen. That is a tough one as the bugs can retreat into the center of the bale and they won’t be impacted by the treatments there. I don’t have enough experience with this to know of the right solution. Have you found a solution since your original post?

  21. Hi Andrew, I’ve just stumbled across this thread, I have a newly built house from straw bale panels in Australia with a fairly major weevil infestation. The producers of the panels advised they may be eating the remnants of the grain in the straw which wouldn’t be much but they seem to be getting worse not better. I have heard of DE as a fix however without breaking the walls open, do you know how this could be applied? We had our panels installed over an unseasonably wet period and whilst we rendered straight away, it’s possible there’s some moisture in the walls…could this be adding to the problem? We’re hoping that after a summer season these problems will settle but any advice would be appreciated. Thank you!

  22. Hi Clair, I’m sorry to hear that you are having a weevil infestation to contend with. Typically, bugs need moisture to survive. Since your panels and plaster were applied during an unseasonably wet time, there may still be some lingering moisture within your walls. I would imagine the summer season will help the drying process, and as the wall dries out, the weevils will die or move to a more hospitable home that is not your straw bale walls.

    Diatomaceous Earth can help with bugs in certain situations if you can apply it to the sources. DM is usually applied around or on the exterior of the wall. I’m unsure how you could get it inside the wall without damaging the render and wall.

    I hope the weevils leave as your walls continues to dry out. Please let us know if you see a significant improvement. Your experience will be helpful to others.

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