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Bagworms are white larva that eat the needles of many evergreen tree species, including the cedar. Most infestations begin in the late spring or early summer, but they are only noticed once the worms construct bags that hang down from the branches. They lay up to 800 eggs in each bag that later hatch into moths. Killing the bagworms involves killing any active larva and the eggs inside the bags.
Put on gardening gloves and place a ladder next to the cedar tree. Climb on the ladder carrying a trash bag. Locate any existing bagworm bags, which closely resemble pine cones and hang downward from the cedar branches.
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Grasp the tops of the bags where they connect to the tree limb and pull them down from the limb. Place the bagworm bags into the trash bag. Remove all bagworm bags from the tree, repositioning the ladder as needed.
Spray the entire trunk, all limbs and needles of the cedar tree with an insecticidal spray in mid-June. Choose a spray that states it is effective at killing bagworms. Spraying before mid-June will not kill the worms.
Destroy the collected bagworm bags by burning them in a fire pit.