Fumigants

Fumigants have been used for centuries as a method of controlling stored insect pests. Because some insects deposit their eggs inside the grain kernel, fumigants must possess certain properties so that they exist in a gaseous state to penetrate the kernel to effectively control the infestation. In recent years the number of fumigants used worldwide has decreased to only two, phosphide and methyl bromide (1). The industry must take greater care in the use of the available products and exercise greater caution so that misuse cannot occur. Improper sealing of an area to be fumigated, poor application techniques coupled with wrong dosages, and inadequate fumigation time always result in failure. One of the principal concerns with poor fumigations is the development of resistant insects.

Newer government regulations require that applicators become more knowledgeable in regard to fumigant chemicals and their application. Safety of the applicator is a prime consideration for any fumigation, both during application of the chemical and the subsequent aeration of the fumigated commodity. The total procedure will only be successful if the space to be fumigated has been properly sealed so that the gas concentration can be maintained for the required time period to ensure total control. Improper sealing will result in loss of gas during the fumigation, so it is recommended that the applicator take gas readings during the fumigation to determine gas release and concentration. It is always surprising to hear how much time was taken for a fumigation that failed only to hear that some area was not sealed and that gas readings were not taken. Applications of this nature can only result in the development of resistance because generations of insects become exposed to nonlethal concentrations. The industry still has sufficient means to control stored product pests, but these methods must be used effectively and with proper techniques.

Each fumigation is different and, therefore, the selection of the appropriate fumigant to be used is crucial. It is especially significant that the properties of each chemical is considered for both success and to determine how best to apply the product so that all areas of the space to be fumigated have sufficient gas concentration during the treatment period. Fumigant products vary in mode of action depending on many factors including temperature, relative humidity, sorption, diffusion, and penetration.

A good fumigation will require prefumigation planning. All aspects of the fumigation must be decided so that success will be achieved. Among the factors to be considered are safety of the applicator, proper sealing of the structure, properties of the fumigant to be used, application equipment or techniques, gas readings, placement of warning signs, posting of guards, and notification of emergency personnel in the area in the event of an episode. The proper method of aeration of the treated commodity must also be analyzed and planned so that the release of the fumigant will not result in exposure or risk to anyone. Once all aspects of the fumigation have been established, it then is necessary to choose which fumigant will be used from among those available.

AVAILABLE FUMIGANTS Metal Phosphides

There are two metal phosphide products that are presently used as a source of hydrogen phosphide: aluminum phosphide and magnesium phosphide.

Aluminum Phosphide. This product is manufactured as a tablet or pellet and is packaged in a formulation in a bag or sachet or as a tablet or pellet prepac. The paraffin-coated, 3-g tablets will produce 1 g of hydrogen phosphide gas when exposed to certain heat and humidity conditions. The pellets are one-fifth the size of the tablets and produce 200 mg of hydrogen phosphide gas. This gas has a density of 1.18 as compared to air and, as a result, has excellent diffusing and penetrating properties. The tableted product usually contains ammonium carbamate, which will break down into ammonia and carbon dioxide when exposed to heat. The gases escape from the tablet or pellet by causing ruptures in the paraffin coating. The ammonia also acts as a warning gas to the applicator. Atmospheric moisture then penetrates the ruptured coating to react with the aluminum phosphide resulting in the release of hydrogen phosphide gas, which is the resultant fumigant. Tablet and pellet prepacs release hydrogen phosphide in an identical manner. These formulations cause a delayed release until appreciable gas concentrations are reached and act as a safeguard to the applicator. Bag or sachet formulations also have a delayed release; atmospheric moisture must penetrate the paper or cloth packaging material before any toxic gas is released. It is important to follow the manufacturer's applicator's manual for use of aluminum phosphide because low temperature and low relative humidity can result in only partial reaction and a minimum of gas release. The result will be low concentration of gas, a good chance at fumigation failure, and possible problems on deactivation of the unreacted metal phosphide. When temperatures are lower or relative humidity is lower, then it is wise to consider the use of a magnesium phosphide product.

Magnesium Phosphide. These products are produced and marketed as either tablets or pellets or as magnesium phosphide impregnated in a plastic matrix. No bag or sachet formulation of this chemical is available. There is a magnesium phosphide pellet prepac product that is used only as a spot fumigant for plant processing equipment. The reactivity of magnesium phosphide with atmospheric moisture is so much faster than aluminum phosphide, it can be used in conditions of high temperatures, low relative humidity or low temperatures and low to medium relative humidity.

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