A fumigant is a chemical that, at 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. Fumigation of agricultural commodities such as bulk grain and fresh fruit is a very important pest-control method and is often required by quarantine and import-export regulations. In agriculture, 2 types of fumigation are used:
- Space fumigation to control pests in enclosed buildings or commodity storage facilities
- Soil fumigation to control weeds, insects, fungi and nematodes in soil prior to establishing crops
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.
1. Objectives of residue studies for fumigants
The aim of a space fumigation residue study 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.
The study should provide information about how much of a fumigant is taken up by or reacts with the foodstuff during treatment and how quickly it is lost during aeration and ventilation.
The objective of a soil fumigation residue study is to confirm that a crop grown in soil that has been treated with a fumigant contains no residues. This type of study is necessary because of the volatile nature of fumigants.
2. Conducting residue trials on space fumigants
The residue studies should adequately reflect the intended use pattern and be representative of the commodities and packaging types that might be fumigated.
It is important to determine the likely maximum residues after applying the maximum concentration to airtight containers for the longest time specified, followed by the minimum ventilation period specified. You should also determine the residues that result from an elevated concentration.
The effects of aeration and 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. You should take care to ensure even and thorough dispersal of the fumigant within the container. Details of the means of dispersal (such as use of fans) should be provided.
The temperature and relative humidity in the storage and fumigation space and the 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 using valid sampling techniques.
Aeration and ventilation
After fumigation, the remaining fumigant has to be removed safely over a period of time that must be practicable. Some fumigation spaces can be opened to the air by various means; forced ventilation (using flash-proof fans) can also be used or may be required.
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 period 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 and then to the limit of analytical quantitation.
Obtaining a representative sample from a large bulk container (of cereal grain, for example) can be 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. With probe sampling, 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, so a number should be sampled. The positions of the sampled bags 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.
You may need to determine storage stability; for further information, refer to the stability of residues during storage guideline.
Some fumigants absorb into or react with certain commodities; this should be investigated where appropriate.
For a commodity that is normally processed after fumigation, you may need to conduct processing studies depending on the residues detected in the commodity. For more information, refer to the processing studies guideline.
Animal transfer studies
If the commodity is fed to animals and contains a finite residue greater than 0.1 milligram per kilogram, depending on the results of metabolism studies, you may need to conduct an animal feeding study. For further guidance, refer to the animal transfer studies guideline.
3. Conducting residue trials on soil fumigants
Fumigants applied to the soil disperse through the air spaces in the soil to contact the target organisms. For these fumigants to be effective, the soil needs to be prepared to a fine tilth with no clods and no undecomposed crop residues. The fumigant must be delivered to the optimum depth and the soil surface sealed as appropriate by watering or rolling or through the application of sheeting. After a specified time the soil should be aerated (cultivated) and a certain time should elapse before the crop is planted. Soil temperature and moisture should be recorded.
If there are no residues in the soil following aeration prior to planting, you will not need to conduct residue studies on the crop at maturity. However, if there are residues in the soil, residues in a few representative crops should be analysed. If 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.
4. Proposals for maximum residue limits
If residues are present, you should propose an appropriate maximum residue limit.