Fumigating stored grain
Grain should not require fumigation during the first year of storage in Minnesota, especially if sanitation and residual insecticides are used properly (see Preventing Stored Grain Insect Infestation). Fumigation, however, may be justified if insect prevention efforts fail.
Fumigants act on all insect life stages. They reach these pests by diffusing through the spaces between grain kernels as well as into the kernels themselves. So fumigants are able to penetrate into places that are inaccessible to insecticide sprays or dusts.
All fumigants are poisons–highly toxic to humans and other warm-blooded animals, as well as to insects. Consequently, they are classified as Restricted Use Pesticides and in accordance with Minnesota State Pesticide Law can be applied only by certified and licensed fumigators.
Currently, only three fumigants are labeled for use in stored grain: phosphine, chloropicrin, and methyl bromide. Each of these products has special limitations and restrictions governing its use.
Chloropicrin is still registered for grain fumigation as well as empty bin fumigation to combat insect infestations. It is especially useful for the control of insects in the sub-floor aeration area of empty bins.
Methyl bromide is an effective grain fumigant for the control of stored grain insects at all stages of development. It evolves into a gas at temperatures above 39° F and has virtually no odor or irritating qualities to indicate its presence.
Phosphine-producing materials have become the predominant fumigants used for the treatment of bulk-stored grain throughout the world. It is available in solid formulations of aluminum phosphide or magnesium phosphide. When exposed to heat and moisture the formulations release phosphine, a highly toxic gas. The time required for release of phosphine will vary with temperature, grain moisture, and formulation.
Before applying phosphine, level the grain surface and break up any encrusted areas. These areas are sometimes hidden from view just below the grain surface. Failure to locate and break up encrusted grain will result in uneven penetration of fumigants.
Seal the bin as well as possible before fumigation. A high degree of gas tightness is essential to achieve the required combination of gas concentration and exposure time necessary to kill insect grain pests. Roof ventilators and aeration ducts should be covered with plastic bags that are gathered at the base and taped in place. Doors should be sealed with a foam seal or tape. Also make sure that the roof-wall junction is sealed. All cracks and holes in the bin wall or ceiling must also be sealed.
Determine the amount of phosphine required to treat the volume of grain in the bin and the head space. If you use the tablet formulation, apply 90 tablets per 1,000 bushels. Allow extra tablets for the head space unless the grain is going to be covered with a plastic sheet. Increase this rate up to as many as 180 tablets per 1,000 bushels if the bin cannot be adequately sealed or if the temperature of the grain is below 68° F. Decrease the dosage to as few as 60 tablets per 1,000 bushels if the bin is exceptionally gastight, contains clean, dry grain, or if the grain temperature is above 80° F. Divide the bin into quarters before application and place 25 percent of the required number of tablets in each pie-shaped section. Divide the total number of tablets/quarter by five to determine the number of probe applications needed per quarter. For example, if the total number of tablets needed for fumigation is 280, the number for each quarter is 70. Thus each quarter of the bin would be probed 14 times with 5 tablets each. When inserting the tablets with a probe, place the first one in the probe when the probe is 5 feet into the grain, then raise it 1 foot and place the next tablet in the probe. Continue until all tablets are used. The last tablet should be about 6 inches below the grain surface. After all sections of the bin have been probed, close the bin and seal the access point as described before. Gas loss can be reduced by placing a polyethylene sheet cut to size over the grain before sealing the door. Leave the bin sealed for at least 5 days after application of the tablets. Never leave plastic on the grain surface more than 7 days. Following fumigation, the grain should be ventilated so that phosphine gas concentrations are below 0.3 parts per million (ppm) before re-entry into the bin. Accurate monitoring of gas concentration is required.
|Application Pattern for Phosphine|
Number of tablets needed for fumigation = 280
280 tablets ÷ 4 quarters = 70 tablets per quarter
70 tablets ÷ 5 tablets per probe site = 14 probe sites per quarter
Monitoring and respiratory protection–Respiratory equipment required for application and re-entry into the fumigation area is based on phosphine gas concentrations. Monitoring of these gas concentrations can be achieved with three different monitoring devices: personal breathing zone pumps, electronic direct readout devices, and colormetric tube detectors. During the application of phosphine producing products, gas concentrations are determined over an 8-hour time-weighted average (8-hour TWA). No respiratory equipment is required to be worn if the gas concentrations do not exceed 0.3 ppm per 8-hour TWA. If gas concentrations are between 0.3 ppm and 15 ppm during the 8-hour TWA, then a full-face canister gas mask approved for phosphine must be worn. If the gas concentrations exceed 15 ppm 8-hour TWA, a self-contained breathing apparatus (SCBA) must be worn. For farm application of phosphine to outdoor bins, a full-face canister gas mask approved for phosphine protection must be worn if monitoring is not done. After fumigation the grain must be aerated so that phosphine gas concentrations are below 0.3 ppm. (This is not an 8-hour TWA.) If gas concentrations are higher than 0.3 ppm, proper respiratory protection must be worn to re-enter the bin (re-read concentration levels this paragraph). The above detailed fumigant concentration monitoring and respiratory protection requirements should be provided by a professional fumigator.
Effective fumigations result from following several recommended guidelines including the following:
- Level the grain below the vertical wall of the bins.
- Remove or break up any crust on the grain surface.
- Seal all cracks, making the bin as airtight as possible.
- Fumigate when the grain temperature is between 70 and 90° F.
- Keep the bin closed and post warning signs until the gas concentration is below 0.3 ppm.
- DO NOT ENTER the bin during or after fumigation until gases have been reduced to safe concentrations. (0.3 ppm).
Chloropicrina (tear gas) (*Chlor-O-Pic, Quasar, Larvacide, etc.)
- EPA exposure limit of 0.1 ppm.
- Gas reading required on each fumigation.
- Gas mask or SCBA above 0.1 ppm.
- Empty bin (plenum or sub-floor) fumigation recommended.
- Fumigate on warm, calm day.
- Seal fan and low openings.
- Post warning signs.
- Apply chloropicrin.
- Wait 2 - 3 days.
- Unseal, gas reading, remove signs.
Methyl bromide (Meth-O-Gas, etc.)
- Primarily applied by professional fumigators (fast, economical).
- Good sealing is critical.
- Complete kill beyond 20 feet unlikely without recirculation.
- Leveling of grain or probing peaks recommended.
- EPA exposure limit of 5.0 ppm.
- Gas reading required on each fumigation.
- SCBA above 5.0 ppm.
- 100% methyl bromide can be used on stored grain (and food).
Phosphine (hydrogen phosphide) (Phostoxin, Detia, Fumitoxin, Phostek, Gastoxin, etc.)
- Widely used on stored grain.
- Good sealing is critical.
- Time is critical.
- Probing greatly improves results.
- Recirculation or modified technique for grain depths greater than 30 feet.
- EPA exposure limit of 0.3 ppm.
- Gas reading(s) required on each fumigation except in some situations.
- SCBA above 0.3 ppm (gas mask okay to 15 ppm or escape).
College of Agricultural, Food, and Environmental Sciences
Phil Harein and Bh. Subramanyam