Numerous cultivars of Chinese juniper (Juniperus chinensis) grow as evergreen shrubs or trees across U.S. Department of Agriculture plant hardiness zones 4 to 9, where they are prized for the year-round interest they provide, the variety of available forms and mature sizes and their relatively low maintenance requirements. Planting a Chinese juniper in an appropriate site and providing it with good cultural care typically allows this plant to thrive, although a handful of different pests may occasionally prove problematic.
Webworms and Bagworms
The juniper webworm (Daeborneris marginella) attacks most Chinese juniper cultivars. The adult webworm is a moth that is active in early summer and lays eggs on juniper foliage. Eggs hatch and young larvae feed as leafminers within juniper foliage before emerging to continue feeding on foliage, protecting themselves by spinning silken webs around the feeding site. Bagworms, also the caterpillar of a moth, feed on junipers and other plants, causing a loss of foliage, slowed growth and even plant death if an infestation is heavy. Bagworms and webworms have naturally occurring enemies that often control these pests well unless disrupted by dusty conditions or broad-spectrum, persistent pesticides. Prune off infested parts or remove the bags the pests create and destroy or dispose of them.
Multiple species of mites, small arachnids that look like tiny, moving dots to the naked eye, use piercing mouth parts to feed on Chinese junipers, causing yellow stippling on foliage. Juniper foliage may turn brown and drop off. Mite feeding is often accompanied by indicative webbing. There are several mite predators and parasites that usually sufficiently suppress mite activity unless disrupted by a pest or dust. Occasionally spraying the Chinese juniper foliage with a forceful water spray effectively knocks mites and unsightly webbing off of the plant and alleviates the dry, dusty conditions that these pests prefer.
A juniper scale (Carulaspis juniperi) infestation looks similar to a mite infestation, with needle discoloration and possible dieback. The scales appear as small, immobile, white or gray bumps on juniper needles measuring no more than about 1/8 inch in diameter. Juniper scales produce a sugary, shiny-looking substance, honeydew, as a byproduct of feeding. Ants are drawn to honeydew and will protect the scales against their natural enemies. An unsightly black fungus known as sooty mold grows on honeydew. If ants are present, you must address them to allow natural scale predators and parasites to curb scale activity. To control ants, regularly rinse honeydew and dust off the juniper with a forceful spray of water and apply a sticky trapping material to the juniper stem to capture ants attempting to climb the tree.
Voles are small, short-tailed rodents that tend to eat the bark and underlying cambium layer on a Chinese juniper in winter when preferred food supplies are limited. If vole feeding occurs around most of the circumference of a juniper stem, it can girdle the plant and kill it. To avoid problems with voles, keep snow and mulch away from the base of the juniper, thin out the branches selectively if it is a low-growing, dense and spreading cultivar, and address any other possible cover for the voles near the juniper like thick mulch or tall grass.
- Clemson University Cooperative Extension Service: Juniper Diseases and Insect Pests
- University of Illinois Extension: Chinese Juniper
- University of Illinois Extension: Voles
- University of Illinois Extension: Juniper Webworm
- Rutgers University, New Jersey Agricultural Experiment Station: Landscape Plants Rated by Deer Resistance
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