Due to the enormous uncertainties surrounding global climate change, estimates of cropland reductions vary widely from 10 to 50 per cent. But this much is clear: global warming is likely to alter production of rice, wheat, corn, soybeans, and potatoes--staples for billions of people and major food crops in both North America, Asia and Africa.By some estimates, agricultural productivity in Africa, Asia and Latin America is expected to decrease by as much as 20%, with less developed countries suffering the greatest negative effects (IPCC, 2007). 

Global change will affect the pattern of population dynamics of insects in different ways. Global warming will not only lead to greater overwinter survival, earlier appearance in spring, an increase in the number of generations in a year, lengthening of the reproductive season, etc., but also affect their biotic associations as a result of changes in inter-specific interaction. Changes in the density of insects in response to unusually hot summers provide us with useful indications of the potential effects of global warming. Different insect guilds respond differently to hot summers, which sometimes result in an increase in density and sometimes a decrease. These effects may occur immediately or be delayed by 1 or 2 years. As long as the regime remains unchanged, the affected population can recover sooner or later. Even a single year change in climate, however, it allows predators to outbreak, may be strong enough to cause a regime shift. Most insects are susceptible to heat stress between 28 and 38°C, global warming could have a more profound impact on the population dynamics and biodiversity of agricultural insects than has previously been predicted at global level. Incidence of insects would be most severe in tropical region due to climate change. Agricultural insects of plain agroecosystem may gradually shift to hills and mountains.