Hemlock trees are attractive bush-like plants which have proved very popular with amateur gardeners and landscapers. Unfortunately, the Hemlock tree seems to be particularly attractive to bugs and pests—it is rare to find one hemlock tree that does not have some kind of infestation.
Another problem is the Hemlock borer; this is decidedly a secondary pest, but can result in fungal infections, or attacks by other pests. A small, flat beetle, it bores into the pulp of Hemlock trees, and causes the bark to peel away from the trunk.
Bag worms are the larvae of moths, and they form cone-shaped bags which sometimes look like pine cones. They emerge during the spring and eat plant material before attaching themselves to a twig.
Scale insects are small pests that feed on hemlock trees, and cause a waxy substance to transfer to hemlock needles, giving them a mosaic appearance.
Hemlock Trees and their Pests
Perhaps the biggest pest of Hemlock trees is the Hemlock woolly adelgid. With no natural enemies except the ladybird, Hemlock woolly adelgid was introduced from the orient, and spread quickly from the U.S. into most of the American continent. This pest sucks sap from the branches and stems of Hemlock trees, and can kill a tree in less than 5 years after first infestation. It is easy to spot when a tree has been infested with the woolly adelgid, as it produces a white "‘puff" at the base of the hemlock’s needles, which is sometimes mistaken for a fungal infection.
Encourage the predatory ladybird beetle Pseudoscymnus tsugae. This beetle eats the woolly adelgid. Experiments have shown that these creatures can significantly reduce the amount of pests which attack the Hemlock tree. If you choose this method, be careful of spraying insecticides and anti fungals into the tree.
Controlling the woolly adelgid can be done by coating Hemlock trees in insecticide, until the spray is literally running off of the leaves. Spraying must be done both over and under leaves and branches. It is possible to use horticultural oils and soaps which are non-chemical; horticultural oil can be sprayed on Hemlock trees from fall to spring, and will kill adults before they lay eggs.
The Hemlock borer tends only to attack weakened hemlock trees, or ones which are infested by something else, so the best method of preventing these pests from attacking is to keep a healthy tree, and minimize pest attacks.
Bagworms can be prevented by encouraging birds to feed on the larvae; the bags can be manually gathered from the tree and burned, and insecticides can be sprayed when the larvae are smaller.
The ladybird beetle enjoys eating these insects, and lacewings will also eat the scale insect. Insecticides can also be used, although it requires a great deal of effort, as several sprays are necessary.
The best way to prevent all of these pests from attacking your hemlock trees is to keep them healthy, well watered and fertilised, and minimise stress and damage to the trees.