Pesticide-resistant species have caused this increase. Today, nearly 1,000 major agricultural pests are immune to pesticides. For example, in the former Soviet Union, Colorado potato beetles, resistant to all available chemicals, decimated the region's staple food. Resistance is, simply, life adapting. Repeated pesticide use kills the vast majority of the pest population but in turn gives the resistant few a distinct advantage.
Resistance is largely a result of pesticide overuse.
And resistance can happen very quickly. DDT, first introduced in the 1940s initially worked very well, but in 1947, resistance began to appear in various pests. The long term result of this is an increasing cost of production and declining yields. Luckily, pesticides are not the only option. Crop rotation, intercropping, and the release of beneficial insects all work well to prevent pests.