Bagworms are a common pest of arborvitae, as well as cedars, spruce and junipers. This pest feeds on host plants, causing extensive damage to the tree. Entire trees can be defoliated by large populations of bagworms. If your arborvitae is infested with bagworms, it is important to get rid of the pest before extensive or repeated defoliation occurs.
Bagworms are caterpillars that spin silky, brown bags and place them on the branches of host trees. These bags are the place where the bagworm makes its home until it is time to feed, and are made of twigs and leaves. Bagworm caterpillars lay large numbers of eggs in their bags before they die. Once the eggs hatch in the spring, the larvae begins to feed on the tree and makes its own bag, which typically measures between 1 and 2 inches in length. Adult male bagworms are moths and female bagworm caterpillars never leave the bag.
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Arborvitae is a favored host of bagworms and often suffer serious damage from feeding. While in the caterpillar stage, bagworms feed for approximately six weeks. Bagworms eat the foliage of arborvitae, consuming entire leaves and leaving only the leaf veins. Heavy infestations of bagworms are detrimental to host trees. Arborvitae that suffer severe or complete defoliation for three consecutive years, often die. Bagworms are also deadly for arborvitae if other stress factor, such as drought, is present.
Arborvitae with light infestations of bagworms often survive if the homeowner hand-picks the bags and caterpillars from the tree. This is especially effective if done during the early spring, winter or fall before the eggs hatch. Drop bagworms into a bucket of soapy water after removal to kill them. Keep your tree healthy and stress free to avoid insect infestations. Provide plenty of water to your tree and avoid wounding the tree during pruning. A vigorous, healthy tree is less likely to be attacked by bagworms.
Arborvitae with heavy infestations of bagworms often require chemical control products to avoid severe defoliation and plant death. Apply chemical products during the months of May or early June for best results. Small bagworm larvae are especially susceptible to insecticides. Larger bagworms are much more difficult to eliminate. Insecticides with the active ingredients malathion, diazinon or Bacillus thuringiensis are available at your local garden center and are recommended for bagworm control. Always apply these products strictly according to the label directions.