Answer: I think you are finding the grubs of arborvitae weevils in your soil. Arborvitae weevils are small and black, covered with metallic green scales and fine short hairs. Their larvae (grubs), which are white to pink with brown heads, attack arborvitae roots from June or July to midwinter or the following spring. Then they emerge from the soil to feed on leaves from May to July. Notches appearing in the margins of arborvitae leaves probably means these weevils are at work. The adults usually are active at night and hide in soil and trash during the day. Adult weevils will "play dead" when disturbed, folding their legs and dropping off plants to the ground. The larvae, or grubs, are in the soil and feed on plant roots.
If this is what you have, you can use a Pyrethroid based insecticide. Pyrethroid insecticides are contact and stomach acting and tend to be very broad-spectrum being active against a wide range of pest insects. Be sure to apply according to label directions.
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