You might have to make a repeat application if damage continues. With severe infestations, the entire plant is defoliated and evergreens will die. When possible, handpick and destroy bags in winter to reduce the number of eggs the following year. If you use a synthetic insecticide, make sure that only the spruce is treated and that there is no drift, especially onto flowering plants. The poison will kill bees and other insects.
Fertilizer is most effective when plants are growing most vigorously.
Then it gets used by the plants and doesn't lay on the soil where it can wash into rivers and streams. October is also a good time, unless we have an early freeze, in which case it might get washed away. The only way to really know if your lawn requires regular fertilizing is to have a soil test done through the Cooperative Extension Service or a garden center. Amend the soil according to test results. The Oregon grapeholly I bought four years ago has failed to thrive.