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Resistance can be explained by natural instability of insect genetics, and by the great reproductive capacity of most species. Any given population of insects is in state of developing resistance against a chemical if they are repeatedly exposed to sub-lethal concentrations of it. As soon as one individual in a colony has become resistant, the high reproductive rate will extend this resistance to the offspring (there is no competitive pressure anymore, as susceptible individuals will eventually die in the presence of the pesticide).
It is necessary to distinguish between real resistance (an irreversible condition that leaves the insects undisturbed when exposed to a given pesticide) and tolerance (where the insect shows a diminished susceptibility to the pesticide, but will eventually die if the appropriate exposure time and concentration are maintained).
Resistance can be achieved by three main routes (roughly):
- Elusion: Through behavioural changes, insects avoid contact with the pesticide
- Active exclusion: Low intake rate of pesticide (through change in target of metabolic processes or “rejection” of active compound)
- Break down: Active chemical degradation of pesticide.
Although resistance to phosphine has been reported, in most cases it would be more correct to speak about tolerant strains of insects to phosphine. This can be regarded as positive, as best fumigation practices will eventually achieve a correct control of the problematic strains.
These best practices have four key words:
- Sealing
- Time
- Concentration
- Temperature
The fumigation must provide the necessary C*T (Concentration * Time) for a given species under given conditions (Temperature is fundamental).
It is obvious that a good fumigation starts with good sealing. If the fumigant has the least chance to escape, there will never be a build up of gas that allows an effective control throughout the fumigated object.
Time and concentration go hand in hand in phosphine fumigation, but time is in fact the decisive factor. As opposed to other gases, phosphine does not perfectly follow a C*T curve, and time is far more important than concentration. Best practices indicate at least 300 ppm for three consecutive days in order to achieve a correct control. However, if we take into account the time needed for the chemical reaction to produce phosphine, the time required for penetration and the presence of less susceptible stadia in the insect population (eggs and pupae), anything below 7 days is beyond best practices.
It is also vital to ensure that during the whole exposure time, deadly concentrations of phosphine are maintained.
Finally, temperature has also an important role to play. Different exposure times can be coupled to different temperatures. In general, the higher the temperature, the more active the insects populations and the swifter the production of phosphine. As a rule of thumb, phosphine fumigations below 10°C are to be avoided.
This general approach must be substantially reviewed when dealing with tolerant strains, which may need higher concentrations and will undoubtedly and with no exception require longer exposure times.
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