Number of species that evolved resistance to a minimum of one insecticide had gone beyond 500 by the year 1990. It takes on an average 10 years for a species to develop resistance to a particular drug (1786, 7 September 2001, Science's Compass).
The evolution is caused by the change in traits of an insect that were susceptible to a particular insecticide. As a natural adaptation to the drug by genetic variation in its offspring, it supports the survival of that particular species. It appears as a genetic directional change to overcome the lethal influence of the drug. Thus evolution triggered by excessive human activities gives way to undesirable and more resistant type of insects that would put forth a new challenge that is much more difficult to overcome. I believe human activities need to have controlled commercialisation to reduce the speed of such evolution.