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In combating infestation in the storage environment the choice of control method and appropriate setting of dose or exposure level needs to take into account how long it is likely to take for the applied measure to reach the site of infestation, and then the local environmental conditions of temperature and humidity before deciding how long is needed for insects to be controlled. Other factors such as the physical properties of the stored product, the nature of the packaging, the time of year and the lighting conditions, may also be of critical importance. This review examines some of the defence mechanisms insects employ and environmental niches they utilize when confronted by control measures. The responses of insects to natural or induced environmental stimuli may be divided into behavioural and metabolic responses. Behavioural responses include orientation towards more favourable conditions or attractants, and avoidance measures such as retreating from treated surfaces into a refuge or descending a concentration gradient of a repellent gas or fumigant. Metabolic responses include aspects of increased metabolism such as when a toxicant is actively excluded from entering the body or the activation of enhanced detoxification pathways following uptake, and aspects of reduced metabolism such as the shutting down of activity, an induced delay in development prolonging a tolerant stage, or a switch to less active biochemical pathways such as anaerobiosis. The response of insects to physical and chemical gradients, treated surfaces, their temperature-related activity responses, their survival at temperature extremes and survival thresholds in toxic atmospheres are discussed in the context of pest survival and the development of resistance in the storage environment. |
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