Fumigation to control insects in stored grain

The major advantage of fumigation with phosphine (often referred to as ´bombing´ or ´gasing´ using Fostoxin, Fumitoxin etc) is that insects can be controlled without moving the grain. See below for dose rates and other critical recommendations.

  • Diagram of sealed silos
    Diagram of sealed silos

General information on phosphine fumigation

Phosphine gas moves readily through grain from the point of application. Phosphine leaks quickly through holes in silos or sheeting. Wind and large temperature changes accelerate phosphine loss. Most phosphine is lost within four days from fumigations in ordinary, unsealed storages.

Insects are killed slowly by phosphine gas. The fumigant must be kept in contact with the insects for at least 7 days to kill all stages of the insect´s life cycle that usually exist in stored grains . Fumigation in ordinary, unsealed storages will kill some adults but most eggs, larvae and pupae will survive to continue the breeding cycle. A silo that is built to be sealed gas-tight is needed to contain sufficient phosphine concentration for long enough to kill all stages of the insects.

Fumigation gives no residual protection to stored grain. In other words, insects will begin breeding, after the phosphine gas concentration has dropped to low levels.

Phosphine fumigant itself leaves minimal residues, and is acceptable to most markets. However, the solid powdery residues left by phosphine generating tablets when they are mixed with the grain are a concern to some markets. It is against label recommendations to mix solid phosphine generating tablets directly into the grain.

Phosphine is effective against insects in most types of grain. But some commodities (for example, oilseeds - linseed, cottonseed) soak up phosphine very quickly, leaving little to kill insects. 

Applying phosphine

Most grain growers use phosphine-generating tablets when fumigating. These tablets react with moisture in the air to release phosphine gas. Up to three days is needed to release all the gas, longer if the grain is cool or very dry. The gas then moves through the grain, usually within a day in volumes up to a hundred tonnes.

The problem of tablet residues in the grain can be avoided by putting tablets on trays, suspending trays in the head space or placing trays on the grain surface. An alternative to using tablets is to use phosphine products which are sold as bag chains, belts or blanket formulations. Do not place tablets in heaps on trays - as the tablets on the top smother the tablets underneath preventing full gas release.

Warning : Do not add water to tablets. This causes a dangerous, quick release of gas which is less effective against insect pests.

Do not enter a fumigated storage to retrieve the spent fumigant formulation - phosphine is toxic to humans. Once the full exposure period plus airing period has passed, the spent formulation can be removed from outside the silo and buried. Some phosphine will be given off by the spent formulation dust, so do not carry it in a confined space.

The SIROFLO® system for applying phosphine as a gas

The SIROFLO® application system continually introduces a low concentration of phosphorine into a silo from a cylinder for 21 days plus. The gas flows through the silo and out, usually at the top of the silo. Originally developed for very large silos, SIROFLO® is now available for small-scale storages. The advantage of this method is that the silo does not have to be sealed to the high standard required for conventional fumigation.

Although much more expensive than using tablets, it has application in specific bulk storage systems. The system using the product named Eco2Fume® is available through Cytec Australia Holdings, Baulkham Hills, NSW, Ph: 02 9846 6200.

Dose rate and fumigation times for solid formulations of phosphine

Effective fumigation of grain needs 1.5 tablets per cubic metre of total storage capacity (i.e. not the tonnage of grain actually in the storage).

A tablet releases 1 gram of phosphine, so 1.5 tablets per cubic metre is equivalent to 1.5 grams per cubic metre.

Examples of application rates of tablets for storages of various sizes are shown in Table 1 below. Phosphine moves through the whole air space, so apply the same number of tablets whether the storage is full or partly full with grain.

Table 1. Phosphine tablets - application rates for phosphine tablets in storages of various sizes
Storage capacity Number of tablets required
Tonnes Bushels Cubic metres
20 730 27 40
50 1830 65 100
100 3660 130 200
300 11000 400 600

The first knock (K1) was applied at the 2 tiller stage (early), and second knock (K2) was applied 7 days later when plants had >6 tillers (late). Seed production data are an average of 2009 and 2010 results. fb = followed by.

Notes on Table 1 :

A storage needs the same amount of fumigant regardless of whether it is full, partly full, or empty. For example, a storage with enough space to hold 100 tonnes of wheat always needs 200 tablets no matter how much grain it contains.

Other solid formulations release various quantities of phosphine ranging from 0.2 grams to one kilogram. Follow label recommendations for those formulations.

Minimum fumigation times following application of phosphine are:

  • 7 days at grain temperatures above 25oC
  • 10 days at 15 to 25oC.

Grain below 15oC should not be fumigated with phosphine - insects are very hard to kill at low temperatures.

Fumigant takes longer to distribute in storages with more than a few hundred tonnes capacity, unless forced circulation is used. Longer exposure periods are required in larger storages. See product label for details.

Airing (ventilating) and withholding periods for phosphine

Fumigated grain must be aired before handling to remove toxic gas and to minimise phosphine gas residues. Flow-through ventilation aided by a fan usually allows completion of airing in 24 hours. Without a fan this could take five days. A withholding period of two days after the airing period applies before the grain is used for human food or stockfeed. Grain can be legally transported during this period.

Total fumigation time for phosphine

The minimum period allowable between application of phosphine and use of the treated grain is 7 days exposure + 1 day airing with fans + 2 days withholding period, or 10 days total . If grain temperature is less than 25oC so that a longer exposure period is needed, or airing is done without fans, the minimum allowable period between application and use of the grain can be as long as 17 days. In storages larger than 300 tonnes, an exposure period of 20 days is required, so total fumigation time is up to 27 days. Always refer to the label.

Safety with phosphine

Phosphine is very toxic to people and farm animals. Health workers specify an air concentration of 0.3 ppm as acceptable in work areas. Gas detector tubes are one method of monitoring workplace concentrations. See the full report on insect management in stored grains for:

  • information on phosphine detectors
  • guidelines for staying safe when using phosphine
  • improving effectiveness with phosphine
  • shortcuts for quick treatment of grains

Slowing resistance to phosphine

Australia´s grain industry relies heavily on phosphine fumigation to meet market requirements for insect-free grain. The development of resistance in insects to phosphine is a threat to the future effective use of phosphine. The frequency of strong resistance to phosphine is increasing in four of the five major insect pest species.

The grain industry can help slow this build up of resistance by adopting an Integrated Pest Management approach to grain insect control by close management of moisture content and temperature of grain in storage - the cooler and drier the grain, the less insects like it. Maintaining strict hygiene standards in grain handling equipment and storage facilities is also a vital component of the pest management programme. These non-chemical approaches greatly reduce the threat from insects and take pressure off both fumigants and protectant insects. See Resistance of grain storage insects to phosphine fumigant and protectant insecticides.

Fumigating stored grain with Vapormate Fumigant® (ethyl formate)

Vapormate Fumigant® (166.7 g/kg ethyl formate) has recently been approved by the Australian Pesticides and Veterinary Medicines Authority and released on to the Australian market by BOC, as a post-harvest fumigant for the control adult stages of certain insect pests in stored cereal grains, oilseeds, grain storage premises and equipment (and certain horticultural produce). Vapormate Fumigant® is supplied as a liquified gas under pressure and is to be applied in sealed, gas-tight chambers. Visit BOC Gases for full information.

This fumigant is to be used only by licensed fumigators who hold appropriate State/Territory license, and are trained in the proper use of required detection devices. It is not to be applied to farm grain silos as a general use recommendation, by unauthorised, untrained operators.

Vapormate Fumigant® is approved for control of adult stages only of Rice weevil, Lesser Grain Borer, Rust Red Flour Beetle and Psocids. Application rate in stored grains is 420 g/m³. Required exposure period is 24 hours. For treating grain storage premises and equipment, application rate is 420 g/m³ and exposure time is 6 hours.

Other grain treatment methods to consider

Controlled atmospheres - Insect control in stored grains using controlled atmospheres

Controlled or modified atmosphere (CA) refers to the process of altering the proportion of atmospheric gases oxygen, nitrogen and carbon dioxide (CO2) to produce a gas mixture toxic to insects. The advantage of the CA technique is that it provides a disinfestation method that is chemical-free and suitable for "organic" grain, but it is expensive. See the full report for more information.

Aeration for cooling to control insects

Controlled aeration cooling can greatly reduce insect and mould activity in stored grain, as well as preserving grain quality. Reducing grain temperature slows insect development, for example, flour beetles can complete their development in three weeks at 35oC and 70%relative humidity (RH), but take ten weeks at 22.5oC and 70% RH. Although adult grain insects live a long time at cool temperatures, their young stages stop developing at temperatures below 15 oC for weevils or 20oC for most other species.

Aeration may need to be used in conjunction with other pest control methods. It may fail to control insects because of heating of the surface layer, particularly in summer, or because of insects flying into the surface layer. Both these problems can be overcome by mixing amorphous silica (see full report). Aeration cooling is strongly recommended for seed or malting barley to maintain germination and malting quality.

Last updated 17 November 2010