The climate change may indirectly affects the forest ecosystems through the activity of phytophagous insects. The climate change has been claimed to be responsible of the range expansion northward and upward of several insect species of northern temperate forests, as well as of changes in the seasonal phenology. Several papers have dealt with the prediction of the most likely consequences of the climate change on the phytophagous insects, including some of the most important forest pests. Increased levels of CO2 in the atmosphere involve an increase of the C/N balance of the plant tissues, which in turn results in a lower food quality for many defoliating insects. Some insects respond by increasing the level of leaf consumption and consequently the damage to the tree, whereas others show higher mortality and lower performance. The level of plant chemical defences may also be affected by a change of CO2. The temperature is affecting either the survival of the insects which are active during the cold period, such as the pine processionary moth, or the synchronization mechanism between the host and the herbivores, as in the case of the larch bud moth. An increase of temperature may alter the mechanism by which the insects adjust their cycles to the local climate (diapause), resulting in faster development and higher feeding rate, as in the case of the spruce web-spinning sawfly outbreaks in the Southern Alps.
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