Crop Pest Relations and Global Climate Change

A major potential problem confronting farmers would be where global climate change results in the occurrence of much larger populations of troublesome insect pests in specific regions. In temperate and subtropical climates, the extent to which insect pest populations build up during the growing season can depend on how well the insect pests survive the winter and how fast their populations increase during the growing season. Relatively small increases in minimum temperatures during the winter may result in substantial increases in the survival of specific insect pests that could cause catastrophic increases in their populations during the growing season. Higher temperatures during the growing season would also aggravate this effect by increasing the development rates of insect pests. Plant breeding can contribute to solving this problem by developing cultivars with resistance to various insect pests, but this is not an easy task, in many cases, and may become more complicated due to effects on plants of elevated [CO2].

Elevated [CO2] has often caused increases in the C/N ratio of plant tissue (Conroy, 1992) and this reduces the nutritive value of leaves for insects. Also, elevated [CO2] may cause changes in the concentrations of proteins that are toxic to insects and other allelochemicals (Lincoln et al., 1993). These changes in tissue composition can influence the feeding behaviour and performance of insect pest herbivores. For example, in different experiments, insect herbivores ate 20-80% faster when fed foliage from plants grown under elevated compared with normal [CO2], presumably due to lower and limiting nitrogen content of foliage from plants grown under elevated [CO2] (Lincoln et al., 1993). For optimal crop production, higher soil N levels will be needed for environments where potential productivity is enhanced by increases in atmospheric [CO2] and this will act to decrease the C/N ratio of plant tissue. However, the critical nitrogen concentration in leaves that is required for optimal grain production may be lower under elevated atmospheric [CO2] than under present-day conditions (Conroy, 1992), so even with higher soil N levels the C/N ratios in plant tissues still may be lower than current levels. Consequently, it is possible that under the elevated [CO2] of the next century the extent of crop damage by insect pest herbivores may change and the extent of varietal resistance to these pests also may change.

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