Junipers may not be the most attractive plants on the planet, but when it comes to pest control, there aren’t that many critters to deal with. Junipers are unusually tough plants that are drought tolerant and resistant to most pests. From time to time, however, certain insects and diseases can raise their ugly heads and create a need for control, or at least management. Learning about potential problems and their solutions is the first step.
When properly sited, junipers usually take care of themselves. However, sometimes stuff happens. Late-season frost, extremely cold winters, reflective heat from buildings, rock mulch, excessively wet soils, poor/excessive drainage, shade, competition from other trees and shrubs and soil disturbance are among the many factors responsible for damage to junipers. These maladies are abiotic in nature, that is, influences that are not related to living organisms such as insects or mites, fungus or bacteria. Abiotic causes are commonly associated with weather extremes, poor growing conditions, vandalism, or encouragement of the other landscape plants at the expense of the junipers.
Determining the exact cause of a problem can be difficult. In many cases, it is helpful to take thorough notes of the site and review recent weather conditions, and then inspect ailing plants for the presence or symptoms of pests or diseases such as the ones discussed below.
Bagworms are a major pest of most conifers and many broadleaf trees including juniper. In recent years, bagworm populations have been on the rise. Fortunately, bagworms are easy to identify. Early in the season, tiny caterpillars hatch from eggs and emerge from the bags in which they overwintered. They immediately begin to feed and construct a cocoon-like structure around themselves. These bags look pretty much the same throughout the season, but expanding in size as the season progresses.
Hand removal of these bags prior to egg hatch in the spring can effectively reduce numbers on small trees. Once eggs have hatched, applications of acephate, Bacillus thuringiensis, bifenthrin, carbaryl, permethrin or spinosad should help eliminate these unwanted pests.
Spruce spider mites
While not insects, spider mites produce similar insult to the health and vigor of juniper trees and shrubs. Spruce spider mites are early season pests, so it is important to begin monitoring for damage in April. Mites can be detected by placing a clipboard holding a white sheet of paper under a branch and rapping the branch a couple of times with your gloved hand or a medium-size stick. If present, mites will drop from the needles and be visible on the paper as tiny moving yellow to reddish dots.
Spruce mites develop in cool, moist weather, rapidly building in number. As the stress of summer begins, damage becomes more and more evident as they continue to feed. The first sign of mites is a faint yellowish discoloration of the needles. As damage progresses, infested areas turn yellow or brown and appear dried out. Once established, a fine webbing becomes apparent, which mites use for transport from one needle to another, as well as to protect themselves from predators. If numerous mites are detected, then applications of abamectin, acephate, bifenthrin, etoxazole or hexythiazox should help reduce their numbers. Thorough coverage is important, and re-treatment is often necessary. By midsummer, infestations are generally in decline.
Spittlebugs rarely injure junipers, but their spittle-like masses can be unsightly. Spittlebug eggs overwinter on the host plant and hatch in the spring. Nymphs insert their sucking mouthparts into stem tissue and remain stationary. Plant sap is excreted as they feed, producing a frothy protective mass that surrounds the nymphs. Masses develop mainly on new growth in May and June. Brownish, mottled adults remain on foliage through the summer. There is usually one generation per season.
Control of spittlebugs is rarely needed, although the nymphal spittle masses can be a nuisance. Natural enemies can help reduce spittlebug numbers. In extreme cases, where foliage is yellowing, an insecticide may be necessary. Spittlebugs can usually be dislodged with a vigorous jet of water.
Juniper scales appear as circular, off-white, flattened scales on needles. Female scales overwinter on needles under their protective covers. Under heavy infestations, branches and twigs turn yellow or die, and entire plants can be killed. There is a single generation each year.
Sound cultural practices are important when managing juniper scale. Prune out heavily infested and dead branches. Monitor for newly hatched crawlers in June, and if abundant consider applying insecticidal soap, a horticultural oil spray or an insecticide such as acephate, bifenthrin, carbaryl or permethrin. Systemic insecticides containing a neonicotinoid, such as imidacloprid, can also be effective when applied in May as a foliar spray.
Cedar apple rust
Perhaps the easiest disease in the landscape to diagnose is cedar apple rust. An interesting disease involving two different woody species, cedar apple rust appears most visibly as golf ball-sized fruiting bodies that are bright orange to rust in color. In early spring, brown, medium-sized, warty looking galls hang from the branches of cedars and junipers, ready to burst into color. Following a rain during the warmer days of mid to late spring, the galls produce long gelatinous “spore horns” that release teliospores in the air. When the spores land on the leaves of susceptible apple or crabapple trees, they produce an infection that results in a small yellow blemish that grows into a small gall on the leaf. The following spring, the gall produces short, brown, threadlike, fruiting structures on the underside of the leaf, which release aecial spores that are often blown back to the juniper, causing new galls to form on foliage whips.
Though cedar apple rust can cause quite a bit of defoliation and loss of vigor in apple and crabapple trees, cedars and junipers are rarely injured to the point where fungicide applications are justified.
Phomopsis twig blight
Phomopsis is largely a concern of newer juniper foliage; however, in advanced infections it can spread to and sometimes kill stem tissues. At first, infected needles turn light green but rapidly change to the characteristic red-brown color of dead shoots and ultimately turn ash gray. Lesions on larger stems frequently develop into cankers, but the stems are usually not girdled. In most cases the fungus does not spread far below the cankers.
Spores produced in fruiting bodies (pycnidia) formed on needles and stems the previous year are the most important source of inoculum early in the growing season. Spores are dispersed through rain splash in late spring and early summer. When junipers in the landscape become heavily infected, they are usually viewed as unsightly by customers because of the numerous dead branch tips. If a sufficient number of branch tips die, the entire tree or shrub may succumb.
Fungicide applications may be warranted on high-value specimens. If needed, applications of propiconazole, mancozeb or thiophanate-methyl made every 10 days during wet periods in spring and summer will reduce the severity of the disease.
Cercospora needle blight
Cercospora blight is readily distinguished from phomopsis by its overall symptoms and appearance. The branches of Cercospora infected trees usually will be devoid of foliage near the base, but have healthy foliage on the tips. In contrast, Phomopsis infected trees will have dead tips.
Early symptoms include bronzed leaf tips on shorter, middle-aged branches. Soon after they become entirely brown and die. Usually all needles of a branchlet are affected. The infected foliage of branchlets usually dies before the end of the growing season. In many situations the affected branchlets will drop from trees in late fall, resulting in the typical appearance of Cercospora infected trees – the extremities of the branches bearing healthy foliage and the inner regions devoid of growth.
Because susceptible foliage is usually not infected before early to midsummer, fungicide applications can be highly effective if applied prior to this time. A second application in mid to late summer is appropriate for heavy infections and/or high-value trees. Liquid copper and mancozeb are among the recommended products for Cercospora needle blight control. Thorough canopy coverage is essential for effective control. Be sure to read and follow all label directions.
Editor’s note: This article was originally published in July 2013 and has been updated.
John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Nebraska. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.