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Pest problems are an inevitable part of modern day agriculture. They occur because agro-ecosystems have created less stable natural ecosystems which otherwise govern ecological forces that regulate potential pest species in natural ecosystems. Raising crops in a monoculture thus provides a food resource cycle that allows pest populations to achieve far higher densities than they would in natural environments. A certain cultivation practice can also make the physico-chemical environment more favourable for pest activity, for example, through irrigation or the warm conditions found in glasshouses. New cultivars or new crops introduced into a certain area or country may provide food resources for potential pests. Also, the use of broad spectrum insecticides can destroy natural predators that help keep pests under control. In these scenarios, new pest problems arise or existing pests become more serious and cause significant damage to crops, biodiversity and landscape valued at billions of dollars per annum. New strains of plant insect-pests may arise to overcome varietal resistance in crops.