Never use a pesticide unless you see a pest. Anybody spraying herbicides around your patch? Yellowing in the interior is not unusual in fall. If the tips yellow you got problems. Spider mites were a problem this summer, but they're toast by now. Mulch them this fall and if it does not rain be sure to water them in November. Best And Happy Yardening, Nancy.
The issue is water - in a year when heat and drought have reached record levels, lack of water is killing trees and shrubs at a record level. No amount of chemicals will make up for H2o. Water them. They need a good soaking going into the winter. Best Nancy