By Tim Motis

Elevated CO2 levels have been shown to kill insect pests in sealed containers (Navarro 2006). Most insects in stored seeds are killed when the atmosphere in the room or container is comprised of 60% or more CO2. Elevated CO2 levels have little or no adverse effect on the germination (Gupta et al. 2014; Navarro et al. 2012) of stored seeds. Most farmers would not have access to CO2 cylinders/tanks, but perhaps they could generate their own CO2 with yeast, sugar and water. Is CO2 generated in this way effective against insects that damage seeds?

About the image

An airtight jar of weevil-infested seeds (A) is flushed with CO2 generated in a bottle (C) containing water, yeast, and sugar. An optional check valve (B) ensures one-way transfer of CO2 into the jar of seeds. As CO2 builds up in the seed jar, gas is exhausted through a tiny hole (A) in the lid.
Source: Tim Motis

Ken Thompson made a simple CO2 generator and flushing device for storing seed in CO2. In AN 128, he wrote:

"For resource-poor communities, carbon dioxide (CO2) or biogas can be easily generated at a low cost. In our experiments, we collected pure CO2 from a yeast-fermented sugar solution (Figure 4). The CO2 was gently injected into the bottom of the seed-filled vessel through a straw, and because CO2 is heavier than oxygen, it pushed the oxygen out the top of the bottle. Seeds stored for six months in this atmosphere at 24°C have shown no loss of viability compared to the pre-storage germination rate and compared to vacuum-packed controls (packed in laminated foil bags with a commercial vacuum packer). We have tried this method with several species so far with good results, but there is still some doubt as to whether it is safe for all species normally capable of long term storage (also known as orthodox seeds). Also, with this method, seed must be very dry before flushing with CO2 gas. Use caution and do your own experiments before sharing this technique with farmers."

Thompson’s method of replacing oxygen with CO2 is also shown in a video called CO2 Flushing.

Tim Motis has also experimented briefly with a CO2 generator to control bruchids in cowpea seeds. See the image in this post for a photo of the components used.

  • Select a drill bit slightly smaller than the diameter of polyethylene airline tubing. Drill a hole into the lid of a plastic bottle and push several centimeters of tubing through the hole.
  • Fill a 2-L bottle container about two-thirds of the way full with warm water. The end of the airline tubing, inside the bottle, should be well above the level of the water.
  • Pour 1 to 2 cups (237 to 473 ml) of sugar into the water and then shake it up to completely dissolve the sugar; NOTE- the sugar will dissolve faster in warm than cold water. There should not be any sugar settling to the bottom of the container.
  • Add 1/4 of a teaspoon (1.2 ml) of yeast.
  • Screw the lid (with tubing in it) tightly onto the bottle of yeast/water. Connect the other end of the airline tubing to the lid of a mason jar; some creativity is needed to make a leak-proof connection.
  • Check for leaks. After about 30 minutes, press the the sides of the water/yeast container. They should feel firm, a sign of pressure build-up as CO2 is being generated. You will also see some foam produced at the top of the container. Place a few drops of water around the area where the tubing connects to lid of each container. Any bubbles indicate leaks. If necessary, use glue to ensure leak-proof connections.
  • Make a very tiny hole for gas to exit as CO2 is generated. To do this, Motis made a hole in the jar lid with a hammer and small nail, covered the nail-hole with tape, and then punctured a portion of tape over the nail hole with a needle. As CO2 is generated, CO2 and/or oxygen escapes through the needle-hole in the tape. Since CO2 is constantly being generated, it builds up in the seed container.

Following the above steps, using 0.5 L (2 cups) of sugar, CO2 was generated over a period of about two weeks, with all cowpea bruchids being killed. How can you tell if CO2 is being produced? Place a drop or two of water over the tiny hole in the mason jar lid. If you see bubbles, CO2 is still being generated.

Areas for future research:

  • What ratios of sugar, water, and yeast to achieve optimum results? Adding more yeast speeds up the production of CO2. Doing so also uses up the sugar faster, reducing the duration of CO2 production. Using less yeast extends the duration, but reduces the rate/speed, of CO2 production. Adding more sugar extends the duration of CO2 generation, but there is only so much sugar that can dissolve in a given volume of water (e.g., 0.5 L [2 cups] of sugar is at or near the maximum for 2 L of water).
  • How much time is needed, with any given method or recipe used, to replace oxygen with CO2 in a container? Is there a simple way to determine when the oxygen has been replaced by CO2?
  • Once the CO2 replaces all or most of the oxygen in a container, can the container be sealed, reducing the time over which CO2 needs to be generated? If that works, it would reduce the amount of sugar needed.
  • How long do various species of insects need to be exposed to CO2 to control larvae, adults and eggs? A literature review will show what insects have been studied already.

Further Reading

Don't miss Dr. Motis' recent post on using Biogas to eliminate pests in seeds.