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    Factors determining the insecticidal efficacy of inerting.

    Factors determining the insecticidal efficacy of inerting.

    The main factors influencing the effectiveness of inerting with carbon dioxide (CO₂), or possibly with dinitrogen (N₂), are (in decreasing order of importance):

    • grain temperature
    • gas concentration (CO₂) on injection into the charged container
    • inerting maintenance time
    • the species of insect to be eradicated.

    The influence of each of these factors on inert gas efficiency has been studied by numerous R&D teams in all developed countries producing cereals, oilseeds and pulses (Australia, USA, Canada, Israel, Germany, UK, Italy, Spain, Portugal and France). Today, we have robust scales guaranteeing the efficacy of disinsectisation using the two most commonly used inert gases (CO₂ and N₂) for the disinsectisation of 'organic' seeds, legumes and field cereals under AB or quality label schemes.

    To help you understand the best practices for effective disinsectisation using CO₂, the effect of each of the four factors determining disinsectisation efficiency is shown in the following graphs.

    ‣
    Effect of temperature on insect mortality rate.
    Figure: Effect of temperature on the mortality rate of adult rice weevils kept in an inert atmosphere of pure nitrogen (data from Fleurat-Lessard and Le Torc'h, 1991).
    Figure: Effect of temperature on the mortality rate of adult rice weevils kept in an inert atmosphere of pure nitrogen (data from Fleurat-Lessard and Le Torc'h, 1991).
    ‣
    Influence of CO₂ content on the lethal exposure time (LD99) of the weevil and a Chinese bruchid.
    Figure: Effect of CO
    Figure: Effect of CO₂ content on grain weevil mortality rate (DEL = lethal exposure time) (data from Annis, 1987).
    Figure: Effect of CO
    Figure: Effect of CO₂ content on the mortality rate of the Chinese bruchid (DEL = lethal exposure time) (data from Annis, 1987).
    ‣
    Influence of gas holding time on the mortality rate achieved.
    Figure 15: Influence of duration of exposure to a CO
    Figure 15: Influence of duration of exposure to a CO₂-enriched atmosphere on the mortality rate of cereal weevils (data from Annis & Morton, 1997).
    ‣
    Difference in sensitivity of different weevil development stages.
    Figure 16: Relative sensitivity of different weevil development stages to exposure to CO
    Figure 16: Relative sensitivity of different weevil development stages to exposure to CO₂-enriched atmospheres (data from Annis & Morton, 1997).

    An abacus is available to help users determine the minimum duration for maintaining a certain CO₂ concentration, resulting in insect mortality at all stages of development. The nymph is the most resistant stage, particularly when the CO₂ concentration is around 50% (around 10% oxygen remains). This tolerance to anoxia is linked to the insect's very low respiration rate at this stage of metamorphosis.

    Figure: minimum duration of exposure to a carbon dioxide atmosphere at different levels for complete disinsectisation (for maximum safety, these durations can be increased by 20% for leguminous plants).
    Figure: minimum duration of exposure to a carbon dioxide atmosphere at different levels for complete disinsectisation (for maximum safety, these durations can be increased by 20% for leguminous plants).

    Conditions for optimum CO₂ inerting efficiency

    The inherent insecticidal effect of CO₂ only manifests itself in the presence of a certain level of residual oxygen: exposing wheat grains infested by the hidden forms of the weevil, Sitophilus oryzae, to an atmosphere of pure CO₂ (100% CO₂) gives poorer disinsectisation results than an atmosphere of 50% CO₂, combined with 4, 10 or 20% oxygen (diazote excipient).

    Figure: Reduction in emergence as a function of duration of exposure of wheat infested with hidden forms of the rice weevil, S. oryzae, to CO
    Figure: Reduction in emergence as a function of duration of exposure of wheat infested with hidden forms of the rice weevil, S. oryzae, to CO₂-modified atmospheres (from Fleurat-Lessard and Le Torc'h, 1991).