Please Help! :)....AP Bio Chapter 22?

An orange grower discovered that most of his trees were infested w/ destructive mites. He sprayed the trees w.insecticide, which killed 99% of the mites. Five weeks later, most of the trees were infested again, so he sprayed again, using the same quantity of the same insecticide. This time, only about half the mites were killed. Explain why the spray did not work as well the second time.