For thousands of years grain has been stored in sealed containers to keep it safe from the elements and protected from damage by rodents, birds, and insects. In ancient times, agricultural societies stored their grain reserve in underground pits covered with mud or bricks. Storing grain in sealable clay pots and metal containers continues in many places of the world today.
The last half of the 20th century saw a huge growth in the use of chemical pesticides and fumigants to protect stored grain. Most fumigants have lost their U.S. EPA registration status primarily because of their negative environmental impact, or because of toxicity and safety concerns. One fumigant still in use is phosphine (PH3) gas. Phosphine is relatively inexpensive, easy to obtain and easy to use, making it the most widely utilized grain fumigant worldwide. Phosphine may be applied to stored grain via solid pellets or tablets, via pressurized cylinders, or via on-site generators.
For a fumigation to be successful, the gas concentration needs to be held for a sufficient amount of exposure time to kill all insects at all life stages. When the fumigation is complete, fresh air is forced through the grain to remove the fumigant. Care must be taken that the storage structure is sealed well before fumigation. Leaks present in the storage structure, whether a silo or warehouse, will invariably result in low fumigant concentrations that fail to kill the more resilient insect pests. In the event of an incomplete kill, PH3-resistant insects continue to produce offspring, thus creating a population of insects that are resistant to the fumigant.
Resistance to insecticides, and in particular PH3, is a global problem. Research by several scientists has documented that the number of resistant insect populations has increased significantly in the last decade in North America, South America, Australia, Africa and Asia. In some cases of extreme resistance, dosages and treatment times have to be substantially increased to kill stored product insects with PH3. Because PH3 is relied upon so heavily, insect resistance is a major challenge that grain producers, handlers, processors, and exporters need to begin paying more attention to. If PH3 loses its ability to kill insect pests, the global grain industry will have a much more difficult time maintaining stored grain quantity and quality – and it will be much more expensive to do so.
The reality is that around the world little research is being sponsored to develop practical and cost-effective alternative stored grain insect pest treatment technologies. Why? In large part industry and government are not providing funding to invest in the needed research, development and technology transfer, and at least as important there are fewer scientists in industry, government and universities with the expertise and experience to conduct the necessary long-term research.
The importance of sealing
Fumigating in unsealed silos has been cited as a main cause for fumigation failure and the emergence of insect resistance. A sealed structure keeps the fumigant within the grain mass long enough and at sufficient concentrations to achieve a complete kill of all insects at all life stages. Commonly used bolted steel (“open top”) silos are usually not manufactured as sealed nor can they be easily sealed well after construction to hold gas effectively. The vast majority of on- and off-farm grain storage structures around the world are not engineered to be sealed for adequate levels of gas-tightness. Instead, they have to be sealed temporarily before fumigation which, especially in larger silos, may add substantial labor and material costs, and results in greater risk of fumigation failures from inadequate sealing than with silos sealed by design.
Sealed grain storage was the subject of much research and development in Australia in the 1970s and 1980s, largely in response to Australia’s commitment to zero live insects in export wheat and concerns over PH3-resistant insect populations found in grain stores. Grain producers, handlers, processors and exporters needed a means to effectively kill stored grain insect pests without leaving pesticide residues on the grain.
Recognizing the threat to PH3 as an effective fumigant due to insect resistance and human safety risks, the Australian government published standard AS 2628 for sealed grain silos in 2010 (AS 2628-2010). According to this standard, a grain silo may be considered “sealed” only when an applied pressure on the inside of the structure depletes by 50% in no less than 3 (for older silos) to 5 minutes (for new silos). For example, if a new silo is pressurized to an internal pressure of 500 pascals (Pa), it should be sufficiently airtight to lose no more than 250 Pa in 5 minutes. Today, silos as small as 10 tonnes and silos more than 250,000 tonnes are sold and constructed to conform to the Australian sealing standard. The 5-minute pressure decay time indicates a level of sealing that minimizes the amount of fumigant loss due to wind and chimney effects. This level of sealing allows the structure to hold the fumigant long enough to kill all insects at all life stages without having to add more fumigant during the fumigation or extend treatment time.
Sealed storage should not be confused with hermetic (i.e., airtight) storage. Hermetic storage is designed for zero air exchange between the inside and outside of the structure, whereas sealed storage allows for some amount of leakage. The goal is that leakage does not cause fumigant concentrations to fall below levels needed to kill all insects at all life stages.
Given the rising concern over PH3 resistance development in stored grain insects and the success of sealed silos in Australia, a research project was undertaken at Kansas State University (KSU) funded in part by the Australian Plant Biosecurity CRC to evaluate the feasibility of utilizing sealed silos to ensure long-term efficacy of PH3 fumigation. Results from that completed study are being utilized at Iowa State University to refine and apply computer models to further study fumigant dispersion in large-scale silos, warehouses and bunkers as well as levels of gas tightness needed to assure PH3 fumigation efficacy.
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Two hopper-bottom steel silos holding 55 to 65 tonnes of maize were constructed at the KSU Grain Science Complex, a SCAFCO Grain Systems Co. (Spokane, Washington, U.S.) 1503HBT, and a Bird’s Silos and Shelters (Popanyinning, Western Australia) 2250 sealed silo, which was used as a benchmark for gas tightness. Assessments included gas tightness, fumigant application and dispersion, mortality of stored-grain insects, and stored grain quality maintenance. Efforts focused on sealing the U.S. bolted corrugated steel hopper silo during construction in order to minimize gas loss during fumigation. Results were compared with those of the Australian steel hopper silo that was engineered to be sealed.
Gas tightness is easier to achieve the fewer holes and seams a silo has. The Australian silo was designed to reduce the number of these openings. The sidewall rings were constructed out of one sheet so there was only one vertical seam per ring. Rivets were used to close the ring and connect stacked rings. Sealant was applied between the overlap sections of each ring. Likewise, the roof and hopper sections were made from one piece of sheet metal each that were formed into shape so only one seam resulted, eliminating potential leak sites. Sealant was applied to the seams connecting the wall rings, hopper and roof sections.
The U.S. silo had more than 1,800 bolt holes and consisted of 57 individual sheets, including the hopper, wall and roof sheets. With so many potential leakage sites, a liberal amount of sealant was applied between sheets, around bolt holes, and along all seams inside the silo. In practice, the biggest problem areas for sealing “open top” silos are the roof-wall and the wall-hopper (or wall-foundation) junctions. A sealing kit was provided for the roof-wall junction which included foam blocks to seal most of the gaps created by the roof ribs. Sealant was applied around the blocks to close off the remaining gaps.
Because of the difficulty in sealing the slide gate discharge plate, the bottom of the U.S. silo was modified to use a butterfly valve instead based on the design of the Australian silo. On both silos a fumigant reaction chamber was installed beneath the butterfly valve housing in which the PH3 tablets and pellets were placed for gas release.
A thermosiphon recirculation pipe was installed on the outside of each silo to connect the gas release chamber to the headspace of the silo, approximately 0.5 meters from the peak. The Australian-designed thermosiphon recirculation technology effectively distributes fumigant throughout the grain mass of a silo as a result of solar radiation and differential temperature effects. It increases overall fumigant movement and dispersion within the silo. A U-type manometer was installed in line with the thermosiphon for seal testing, measuring gas tightness, and functioning as a pressure relief valve.
To determine the distribution of PH3 inside the silos, monitoring lines were installed. The lines were placed in the center, north, south, east, and west directions of the grain mass, with 24 sampling points that terminated at various depths in the grain mass and silo. As the fumigant dispersed throughout each silo and grain mass, gas samples were drawn through the lines and read using a PH3 monitoring device at a common point outside the silos.
The gas tightness of each silo was assessed with the Australian sealing standard. The U.S. silo ranged in pressure half-life times from 20 to 50 seconds, and the Australian silo ranged from 31 seconds to 2 minutes, 43 seconds. Neither silo reached the 5-minute minimum time prescribed by the Australian sealing standard for a new silo.
During a fumigation, the gas concentration should reach sufficiently high levels to kill all insects at all life stages in every area of the structure. Insects may migrate to areas of the structure that do not reach lethal fumigant concentrations and survive. Analyzing fumigant distribution within the silo may help to understand why certain areas may not reach lethal concentrations.
The figure on page 78 shows the average concentrations in the silos when the thermosiphon in the U.S. silo was turned off, preventing gas recirculation in the silo and through the grain mass. That forced the PH3 to diffuse upward through the grain mass through the gap around the butterfly valve. That is a slower process than allowing the gas to move through the thermosiphon and reach the headspace. The average concentration did not get above 250 ppm compared to the Australian silo where the average concentration reached 430 ppm. It took about 24 hours for the average concentration to peak with the thermosiphon on compared to 48 hours with the thermosiphon off. Phosphine distributed throughout the silo without recirculation from the thermosiphon took place at a slower rate. During dispersion of the fumigant, the south and west sides of the silo, which received the most sunlight (as is characteristic in the northern hemisphere), had consistently higher concentrations (average 187 ppm) than the north and east sides which did not receive as much sunlight (average 77 ppm).
Even though the average concentration, or the concentration at one point may be sufficient to kill all insects at all lifestages, areas of sub-lethal concentration may persist in the grain mass. The top 1 meter of grain is where insect infestations are most commonly found. Thus, maintaining lethal concentrations in this area and the headspace is critical to fumigation success.
Throughout the fumigations, no PH3 above the detectable 0.1 ppm level was measured outside of the silos at any time. This indicated that PH3 leaked to the outside at low levels and was immediately diluted in the air to undetectable levels.
Bioassays using live stored grain insects were used to demonstrate the efficacy of the fumigations. These were 100% effective in killing PH3-resistant and PH3-susceptible R. dominica (lesser grain borer) and T. castaneum (red flour beetle) adults. In the PH3-resistant strains of R. dominica and T. castaneum, five adults emerged from immature stages after six weeks of incubation, compared to 586 adults that emerged from the untreated control. In the bioassays containing PH3-susceptible S. zeamais (maize weevil) and T. castaneum, all adult insects were killed, and there was no emergence after six weeks of incubation.
After 6-7 months, maize quality was affected by being stored in non-aerated and non-vented silos. Mold was found in the topmost layer (0.3 meters) in both silos. Condensation collected on the underside of the roof and dripped onto the grain surface as evidenced by the observed ring of mold directly underneath the support ring attached to the underside of the roof of the Australian silo. In the top layer, maize moisture increased from 11.5% to 17% in both silos due to moisture equilibration with humid headspace air and moisture condensation over the duration of the storage period. Test weight of maize reduced from approximately 77 to 65 kg/hL.
This research project evaluated the viability of using sealed grain silo technology to achieve successful fumigations, maintain grain quality, and control insects without building up PH3 resistance. Effective fumigations are critical for grain producers, handlers, processors and exporters to assure PH3 remains a viable fumigant in the future. Sealed grain storage is a proven technology and a viable tool for the global grain industry to protect the quality, quantity and value of stored grain. With properly sealed storage silos of any size, fumigation treatments work more effectively and with substantially lower costs. They may prevent resistant strains of insects from developing as a result of fumigation failures. They also may prevent insects from re-infesting the stored grain, which is a typical problem with “open top” silos. However, moisture condensation in the headspace of sealed silos has to be managed particularly in temperate climates where ambient temperatures fluctuate seasonally between cold winters (-10 to -20 degrees C) and hot summers (25 to 35 degrees C). More research is needed to further assess and demonstrate the effectiveness of sealing large-scale silos and storage structures in order to assure fumigation efficacy and overcome PH3-resistant insects.