Phosphine Fumigation Systems
for Insect Control in Stored Grain
|GTS LowPH3low||For poorly sealed structures|
|GTS CircPH3low||For better sealed structures|
|GTS PH3ReCirc||For well sealed structures|
Declining Options: There are very few chemicals that are suitable for fumigating stored grain to control insect pests, Methyl-Bromide and Phosphine being the two most commonly used in the past. Methyl bromide is currently being phased out because of its ozone-depleting effects, leaving phosphine as the front-line fumigant to control insect pests in the world's grain stocks.
Phosphine: has traditionally been supplied in tablet form, consisting of either aluminium or magnesium phosphide that reacts with moisture from the air to produce phosphine gas. Tablets are commonly mixed with the grain or placed on the grain surface, relying on convection currents and natural diffusion to distribute the phosphine gas throughout the grain mass.
Whilst phosphine is very toxic to mammals, its toxicity to insects is more variable, and depends not only on the concentration of the gas, but also on the grain temperature, and the time that insects are exposed to it. Its toxicity differs from one insect species to another, and between each life stage of the insects, the eggs and pupae usually being least susceptible to the effects phosphine. Furthermore its toxicity is affected by the development of insect "resistance" that results from selective breeding of survivors from poorly fumigated structures.
See http://www.phosphine.com for more detailed information on Phosphine.
Temperature: The stored grain temperature for fumigation should normally be above 15oC. Below this temperature, insects become too dormant to be fully affected by the toxin. Phosphine is usually most effective at temperatures between 20 and 35oC.
Time and Concentration: Minimum fumigation times and concentrations are inter-related, and depend on the type of insect to be killed and the grain temperature. In the past it was commonly recommended that insects be exposed to concentrations of phosphine >35 ppm for a period of not less than 15 days, or >20 ppm for not less than 28 days. Much higher concentrations of around 700 ppm were recommended for 7 day fumigations. The gradual (and sometimes rapid) development of insect resistance has forced an increase in minimum recommended dosages which in some cases may be as high as 100 ppm for 28 days or 200 ppm for 15 days. It is no longer possible to make generalized recommendations since insect resistance can vary so much between countries, regions and even individual storages.
Gas Loss: Unless the grain storage structure is gas-tight, then phosphine generated from tablets will leak out through the perforations in the structure through which air will also leak in. This causes localized dilution of the phosphine concentration in the grain resulting in insect survival. Loss of gas and ingress of air can be very pronounced in tall silos which are subject to the "Chimney Effect" caused by temperature differences between the air inside and outside the silo. In cases of very leak silos subjected to high temperature differentials, its entire fumigant content can be lost in a matter of hours. (For further information, see Pressure Tests).
Resistance: Very few countries routinely use good fumigation practices to ensure that an adequate concentration of fumigant is maintained for a long enough period of time to guarantee the mortality of all insect populations.
The application of phosphine to leaky storages may allow some insects survive, the survivors having the greatest tolerance to phosphine. This is the "selection mechanism" through which the most tolerant insects pass on their "phosphine resistant" genes to their progeny, and through which the manifestation of "insect resistance" is developed.
Repeated tablet fumigations in poorly sealed storages have resulted in the development of serious insect resistance in many countries which has dramatically reduced the effectiveness of phosphine as a fumigant. This may represent a serious threat to the world's food-stocks of the future if nothing is done to abate the trend.
Sealed Storage: The Stored Grain Research Laboratory within CSIRO (Australia's Commonwealth Scientific and Industrial Research Organization) was the first to establish a properly defined sealing standard for fumigation of grain storages, requiring that storages be capable of sustaining an air-pressure differential such that the period of time that it takes for the pressure to drop to half its initial value should be no less than 8 minutes for a full storage and 15 minutes for an empty one. This standard has been adopted in most storages built in Australia since 1975, however very few other countries have so far followed this lead.
As a result most of the rest of the world stores and fumigates its grain in poorly sealed structures rendering fumigations only partially effective. Only two effective solutions to this problem has so far been devised - LowPH3low and CircPH3low fumigation technologies.
LowPH3low and CircPH3low technologies have been developed specifically to overcome the problem of ineffective fumigations in leaky storages. They offer additional benefits including:
- lower environmental impact through lower gas emissions
- lower phosphine residues in the grain
- greater safety for operators
- reduced risk of fire and explosion
In many cases, particularly with older storages, it is impractical to achieve a satisfactory level of seal to sustain effective fumigation practices using phosphide tablets. It was with this in mind that CSIRO developed the LowPH3low and CircPH3low technologies which differ from traditional fumigations in that they inject a continuous flow of phosphine by means of a small fan at the base of the storage that maintains a small positive air pressure within the storage structure. This ensures that air does not leak back into the structure to dilute the phosphine concentration; such leakage that occurs being phosphine and air exiting the structure.
By this means:
- Phosphine is continuously bled into the storage during the entire period of the fumigation, replenishing gas that is lost through leaks in the storage structure and maintaining a preset toxic concentration;
- The fumigation period can be extended to as long as may be required to ensure complete 100% mortality of all insect stages;
- Much lower phosphine concentrations are required than in "one-shot" fumigations in which a high initial concentration is needed in order to allow for losses during the fumigation period.
This results in several benefits:
- The fumigation time and concentration can be controlled and adjusted to ensure that all insect stages and species are controlled;
- Resistant insects can be controlled adjusting the concentration and/or exposure time;
- Because phosphine concentrations are lower, there are lower residues of phosphine in the grain;
- Because low concentrations are used, workspace concentrations of phosphine are very low;
- Because low concentrations are used, emissions of phosphine into the environment are very low.