Guideline No. 15
A fumigant is a chemical, which, at a normal ambient temperature and pressure, exists in the gaseous state and is lethal to the target pest.
Fumigation is the process of releasing and dispersing a chemical so that it reaches the organism as a vapour. The fumigation of agricultural commodities such as bulk grain and fresh fruit is a very important pest control method and is often required by quarantine or export/import regulations. In agriculture, two types of fumigation are used:
- Space fumigation to control pests in enclosed buildings or commodity storage facilities. Here the gas is strictly confined by the structure surrounding the commodity.
- Soil fumigation to control weeds, insects, fungi and nematodes in soil prior to crop establishment.
The application of fumigant pesticides is a specialised operation and certain uses are controlled by regulations that restrict the operation to licensed operators. Strict observance of safety measures is required.
OBJECTIVES OF RESIDUE STUDIES FOR FUMIGANTS
The aim of space fumigation is to determine the maximum amount of residue that will likely result in or on a raw agricultural commodity or in or on the food or feed commodity as a result of post-harvest fumigation.
It is desirable to know how much is taken up by or reacts with the foodstuff during treatment and how quickly it is lost during aeration/ventilation.
Because of the volatile nature of fumigants the objective of soil fumigation is to confirm that a crop grown in soil, treated with a fumigant, contains no residues.
CONDUCT OF RESIDUE TRIALS FOR SPACE FUMIGANTS
The residue studies should adequately reflect the intended use pattern and be representative of those commodities and packaging types that might be fumigated.
It is important that the maximum residues likely be determined after applying the maximum concentration for the longest time specified to airtight containers and followed by the minimum ventilation period specified. Residues resulting from an elevated concentration should also be determined.
The effect of aeration/ventilation (procedure and duration as recommended) on the decline of residues should be demonstrated.
Details of the container dimensions and material of construction and the method to create gas tightness to avoid gas leakage or entry of air from the outside should be recorded. Care should be taken to ensure even and thorough dispersal of the fumigant within the container; details should be provided (e.g. use of fans).
The temperature and relative humidity within the storage and fumigation space, and moisture content of the commodity can influence residues (as well as efficacy) and should be reported.
Supervised trials should be carried out over a wide range of storage facilities and using valid sampling techniques.
After fumigation, the remaining fumigant has to be removed safely over a period of time which must be practical. The fumigation space is opened to the air by various means and forced ventilation (using flashproof fans) can be utilised or may be required in some circumstances.
For some fumigants that are absorbed by the commodity, the rate of desorption can be the determining factor in ventilation requirements.
Sampling treated commodities
Sampling should take place after the specified ventilation process has elapsed and at several times thereafter to determine the rate of loss of residues.
It is useful to know the rate of decline of a residue under normal conditions to below the level of the maximum residue limit (MRL) and then to the limit of analytical quantitation.
Obtaining a representative sample from a large bulk container (eg. of cereal grain) is difficult. Ideally, samples should be taken from the stream during transfer to another container. Probe samples may be acceptable if it is possible to reach every part of the storage container and many samples should be taken and mixed to produce a representative sample.
Residues may be higher in dust fractions. These need to be checked.
Stacks of bags may be difficult to sample evenly and a number should be sampled. The position of the bag in the stack should be noted.
For disinfested fresh commodities, a number of samples must be taken and analysed separately to determine the range of residues that are present.
Because fumigants are volatile, precautions must be taken to prevent loss from samples during the time they are taken, handled, packaged, frozen, transported and finally analysed.
It may be necessary to determine storage stability (see Residue Guideline No. 8, Stability of residues during storage).
Some fumigants absorb into or react with certain commodities. This should be investigated where appropriate.
Where a commodity is normally processed after fumigation, it may be necessary, depending on residues detected in the commodity, to conduct processing studies (see Residue Guideline No. 7, Processing studies).
Animal transfer studies
Where the commodity is fed to animals and contains a finite residue >0.1 mg/kg an animal feeding study might be required depending on metabolism studies (see Residue Guideline No. 1, Animal transfer studies).
CONDUCT OF RESIDUE TRIALS ON SOIL FUMIGANTS
Fumigants applied to the soil disperse through the air spaces to contact the target organisms. For them to be efficacious, the soil needs to be prepared to a fine tilth with no clods and contain no undecomposed crop residues. The fumigant must be delivered to the optimum depth and the soil surface sealed as appropriate by watering, rolling or the application of sheeting. After a specified time the soil is aerated (cultivated) and a certain time should elapse before the crop is planted.
Temperature and soil moisture should be recorded.
If there are no residues in the soil following aeration prior to planting, no residue studies on the crop at maturity are required. However, if there are residues in the soil then residues in a few representative crops should be analysed. Where the label recommends use in both leafy vegetables and root crops, data demonstrating the residues in the root crop would be expected to reflect the maximum residue situation. If no residues are found in these commodities, there is no need to test further crops.
PROPOSALS FOR MRLS
residues are present, an appropriate MRL should be proposed.