The differences between methods of grain fumigation are related primarily to the type of fumigant initially applied to the grain mass. Gas-tight structures that will retain a sufficient concentration of fumigant for the required period of time are essential for effective treatments. For the fumigant phosphine, gas tightness is particularly important because this gas can penetrate to escape much more easily and rapidly than other fumigants.
In the case of phosphine it is essential to maintain uniform concentrations for sufficient periods of time to kill all insects present; high dosage rates do not compensate for inadequate standards of gas tightness with this fumigant. The objective of the fumigation must be complete control of all stages of all species of pest so that resistance to the fumigant does not develop.
In flat storage units, in which the depth of the grain does not exceed 10 m, tablets, pellets or sachets containing aluminium phosphide may be used. To simplify application, it is advisable to level the surface of the grain as much as possible before the work is begun.
Phosphine or hydrogen phosphide (PH3) is a low molecular weight, low boiling point compound that diffuses rapidly and penetrates deeply into materials, such as large bulks of grain or tightly packed materials. The gas is produced from formulations of metallic phosphides (usually aluminium or magnesium phosphide) that contain additional materials for regulating release of the gas.
AIP + 3H2O à PH3^ + Al(OH)3
Mg3P2 + 6H2O à 2PH3^ + 3Mg(OH)2
Aluminium phosphide is formulated 8S tablets, pellets or small sachets of powder with additional materials such as ammonium carbamate, ammonium bicarbonate, urea and paraffin to regulate release of fumigant and suppress flammability. The magnesium phosphide is similarly manufactured in tablets or pellets. It is also prepared in flat plates; here the formulation is embedded in a plastic matrix that regulates access of moisture and hence controls release of the gas. After the phosphine has evolved from a formulation, the residue that remains consists mainly of aluminium or magnesium hydroxide. Small amounts of undecomposed aluminium phosphide may also remain in the greywrlite dust from tablets, pellets or sachets.
Phosphine ranks as one of the most toxic fumigants of stored product insects. It is a slow acting poison that is effective at very low concentrations if the exposure time is long enough. Usually, exposure times of four or more days are required to control insects, depending on temperature. The toxicity of phosphine to insects declines as the temperature falls to 5°C, so that longer exposure times are required for it to exert its effect. It is not recommended for use below 5°C. The exposure time cannot be shortened by increasing the dosage; in fact, high concentrations can have a narcotic effect on insects thereby reducing mortality