Boxwoods (Buxus spp) are small evergreen shrubs that are commonly seen utilized as hedges and border plants. While they are quite hardy and are adaptable in several climatic zones, it is not uncommon for the plants to be afflicted with common boxwood shrub pests. While many of the unwanted pests are benign, in some cases, controlling boxwood insects is paramount to the continued health of the plant. The following article contains information regarding common boxwood pests and treating bugs on boxwoods.
Boxwood Shrub Pests
Boxwoods are generally easy care shrubs that can be grown in either full sun or shade and are predominantly used for small to medium sized hedges. Despite their ease of care, many insects thrive on boxwood bushes.
The most detrimental pest of boxwoods is the boxwood leafminer. It is a small fly that is indigenous to Europe but is now found throughout the United States. Both adults and their larvae cause serious damage to the boxwood foliage in the form of blistering and discoloration.
The adult leafminers are about 0.1 inches (0.2 cm.) long and fragile looking. They are orange-yellow to red. In May, the tiny (0.125 inch (0.3 cm.) long) larvae become orange colored pupae and emerge as a fly. Adults mate and then the female lays her eggs deep inside the leaf tissue. Eggs hatch three weeks later and the larvae slowly grow as they munch away on the inside of the leaf.
Controlling boxwood leafminer insects begins with selecting a more resistant variety initially. Some cultivars with various resistance are:
- ‘Varder Valley’
- Buxus microphylla var. japonica
If it’s a little too late for that, you can reduce the population by pruning prior to adult emergence or after eggs are laid.
Some insecticides can be used, but control is difficult, as the application needs to be timed with the emergence of the adults. Sprays containing bifenthrin, carbaryl, cyfluthrin, or malathion can all be used to treat these insects on boxwood bushes.
Eurytetranychus buxi is a spider mite – the boxwood mite to be exact. These boxwood shrub pests feed on the underside of the leaves, leaving them stippled with tiny white or yellow spots. Both European and American boxwoods are susceptible to boxwood mite. Japanese boxwood is a bit more resistant. High nitrogen fertilizer applications coincide with large populations of boxwood mites.
As with other types of spider mites, these pests overwinter as eggs on the underside of the leaves. They then hatch in May with another generation on the way in 2-3 weeks. Since this means multiple generation per year, treating these bugs on boxwoods is imperative as early in the season as possible. The mites are most active in the spring and early summer and at their worst when conditions are dry and dusty. Complete defoliation can occur if the infestation is heavy.
To treat boxwood mites, you can try and wash them from the plants with a stream of water. Also, horticultural oil is effective. For an aggressive treatment, apply products containing abamectin, bifenthrin, malathion, or oxythioquinox in the first two weeks of May to get a jump on the population.
Another common insect marauder is the boxwood psyllid (Cacopsylla busi). While this is a less serious pest than the above mentioned, it can still wreak plenty of havoc on your boxwoods. The damage is purely cosmetic with cupping of leaves and affected twig growth. The psyllid afflicts all boxwoods, but the American boxwood is most susceptible.
Like the spider mite, the boxwood psyllid overwinters as a tiny, orange egg which hatches in the spring when the buds of the plant open. The nymphs begin feeding on the plant right away. At this stage, the insects damage the plant, causing the leaves to cup. The cupping provides a hiding place for the psyllid as well as protection. They become winged adults by early June and then mate. The females lay their eggs between the bud scales of the boxwood to overwinter until the next spring. There is one population per year.
To control psyllids, apply the same insecticides mentioned above in early May when the young have hatched.
Additional Insects on Boxwood Bushes
The aforementioned are the three most common insect invaders on boxwoods, but there are other damaging pests as well.
Boxwoods are susceptible to parasitic nematodes, which cause leaf bronzing, stunted growth and general decline of the shrub. There are several types of these nematodes. American boxwood is resistant to root-knot nematodes but tolerant of stunt nematodes.
Once you have nematodes, you have them. The goal is to minimize the population as much as possible. Grow plants that are unaffected by nematodes to reduce the population and be consistent with care – fertilize, mulch and water on a regular basis to keep the overall health of the plant stable.
Less damaging, but no less irritating, on occasion are infestations of scale, mealybugs, and whiteflies. Scale and whitefly are both sucking insects that cause various marring on the leaves of the boxwood but are otherwise fairly benign.
Mealybugs exude honeydew, which is attractive to ants, so you are likely to have at least two infestations to deal with. Mealybugs are difficult to control with insecticides. Naturally occurring predators and parasites can aid in controlling the population. Also, application of insecticidal soap, narrow-range oil, or even a forceful stream of water can reduce populations.
Various types of caterpillars may also pose problems with boxwood shrubs.
Boxwood Leafminer: Identification, Damage & Control
Boxwood leafminer (Monarthropalpusi flavus) is a common and destructive pest that causes significant damage to boxwoods here in the Dayton area, although the symptoms are often mistaken for winter injury rather than insect infestation.
Since boxwoods are such a popular shrub, it’s important to control leafminers so they don’t spread to neighboring plants and properties.
Underside of healthy boxwood leaves
Preventing Pest Attacks on Your Boxwoods
Boxwood shrubs are vulnerable to several pests that can cause major damage to the plant. These pests can cause the leaves to turn yellow or brown and allow diseases to take over. In large numbers these pests can kill the plant. Here are the major pest problems, and some ways that you can help to prevent an attack on your boxwoods.
There are a few different types of nematodes, all of which feed in the same way. Nematodes are in the soil and will embed themselves into the root system of boxwood. They will make themselves at home and block any nutrients from getting through the root. Some plants will try to grow new roots on their own and get nutrients that way, but the nematode attacks will likely migrate to those roots as well. Prevention of nematode is really the only way to keep them from harming your boxwood. Buy your boxwood from a reputable nursery as they take care to ensure that nematodes are not an issue. Also make certain that the area you plant in does not already have nematodes. There are testing kits and centers that you can send soil samples to verify. Be certain to plant your boxwood in soil that is well draining. If you plant the American Boxwood, your plant is already resistant to a couple different species of nematodes however planting such plants as marigolds and grass can help cut down the nematode population as they are resistant to all types of nematodes.
Boxwood mites very closely resemble a spider. They are very small, only around 1/64 inch. Prevention of this little critter can be done organically by introducing lady bugs to the boxwood. Ladybugs will eat up all of the little bugs on your plant. You can use insecticides as well however insecticides will also kill all of the preventative insects that are on the tree as well. Use them as a last resort.
Boxwood Leaf Miner
The Boxwood leaf miner is a very small fly larva that digs into the leaves of boxwood. They stay in the leaf almost all year and then emerge an orange colored fly. This small fly has of a life of about three weeks. The only time that you can effectively prevent an attack on your plant is to spray insecticide when the adult flies have emerged. Once they have laid their eggs on the leaves then it is too late to do anything about this insect. A small population of the leaf miner will allow your boxwood to survive, however you will need to trim off the infected branches and be sure to watch for any flies to emerge so that you can spray for them.
Arming yourself with knowledge about boxwood pests and their prevention will help to ensure that your boxwood has a long pest free life. It is also a good idea to find out different diseases that affect boxwoods as most are easy to prevent if you know enough about them.
Summer Tree Care: Identifying Common Tree Insects in Summer, Part 3
This is the third part of a series on summer tree insects. This article examines boxwood leafminer, and cooley spruce gall adelgid.
As spring transitions into summer, temperatures gradually rise, and plants enter the next phase of their development. This period coincides with the appearance of numerous insects, many of which infest vulnerable trees and shrubs. When infestations occur, they can be detrimental to plant health. The following discusses some of the insects that commonly infest plants in summer, and how they impact their hosts.
Boxwood Leafminer (Monarthropalpus flavus)
Boxwood leafminer is a small midge that feeds on the leaves of boxwood. It is considered the most serious pest of boxwood. During its larval stage, boxwood leafminer feeds on the upper and lower portions of the plant’s leaves, causing significant blistering and discoloration. As temperatures cool, the larvae cease feeding, and overwinter in the blisters. The adults resemble yellow to orange-red mosquitoes.
Boxwood leafminer consumes the leaves of American boxwood, littleleaf boxwood, and common boxwood. It may also be found on English and Japanese boxwood, albeit with less frequency. The slower growing English varieties exhibit greater resistance to the insect than the American varieties.
Symptoms of Infestation
The larvae feed between the upper and lower parts of boxwood leaves. This causes blisters to form on the underside of the leaves. Blistering may not become apparent until late summer. Leaves infested by boxwood leafminer are often stunted, appearing smaller, and dropping prematurely. Yellow or brown splotches form on infested leaves. When boxwoods are severely infested, the leaves will become laden with blisters. Heavy infestation can result in significant defoliation, and tree mortality.
- When planting, consider selecting resistant cultivars. Cultivars of English boxwood, such as Buxus sempervirens ‘Pendula’, ‘Suffruticosa’, ‘Handworthiensis’, ‘Pyramidalis’, ‘Argenteo-varigata’, and ‘Varder Valley’ exhibit an increased resistance to the insect.
- Maintain plant vigor through sound cultural practices. Ensure plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around susceptible plants to improve soil quality, moderate soil temperature, and retain soil moisture.
- Encourage the development of natural predators such as green lacewings, and spiders. This will limit the density of boxwood leafminer populations.
- Prune boxwoods before adults emerge, or just after the adults lay their eggs in May. This will help reduce leafminer populations.
- Rake and dispose of fallen leaves to reduce the potential for leafminer outbreaks.
- Insecticide applications can be administered to control leafminer populations. Begin treatment in mid-April to early May, when the adult flies can be observed hovering around boxwoods. Perform a second application in mid summer.
- Foliar systemic insecticides are effective when leafminers are present in the leaf blisters.
- Soil applications of dinotefuran or imidacloprid are effective at controlling leafminers. Dinotefuran is diffused more rapidly into the soil, allowing for faster control.
Cooley Spruce Gall Adelgid (Adelges cooleyi)
Cooley spruce gall adelgid is a small black insect that is native to North America. During the nymphal stage of the insect’s life cycle, the nymphs crawl into the expanding buds of host plants, and proceed to feed on the elongating needles. The persistent feeding of the nymphs promotes abnormal plant cell development in the buds. This causes the formation of galls on several spruce trees, as well as needle distortion and discoloration on Douglas-fir. Severe infestations can deform trees, and inhibit their growth.
The most common host plants for cooley spruce gall adelgid are Colorado blue spruce, and Douglas-fir. The pest also infests Engelmann spruce, Sitka spruce, and Oriental spruce.
Symptoms of Infestations
On spruce twigs, the feeding of the nymphs induces the formation of galls on the branch tips. When infestations are severe, the resulting bud deformation can distort the tree. Galls range from ½ an inch to three inches in diameter. The gall formed on spruces is an elongate growth shaped like a pineapple. Galls are initially green, but turn brown by late summer. Galls may persist for several years. Galls are not formed on Douglas-fir, but feeding by the species on the needles may cause needle discoloration, and needle distortion, with infested needles becoming bent or crooked. Infested needles often drop prematurely. As the insects feed on the needles, they secrete a substance called honeydew. Sooty mold often develops on the honeydew. This causes the infested branches to assume a blackened appearance. On Douglas-fir, the waxy masses may be observed on infested needles in early spring. Once the eggs hatch, the nymphs will also feed on developing cones. Significant damage to the cones can inhibit seed production.
- On small spruce trees, remove and dispose of the green colored galls during June or July, prior to the adults’ emergence. This will help to limit adelgid populations.
- To manage the overwintering females, registered insecticides can be applied on spruce and Douglas-fir from mid-September to early October.
- Applications of dormant oil may be administered to control the overwintering nymphs. Applications should be performed before new growth emerges in early spring, or in early fall after the first frost has occurred.
- Encourage the development of natural predators, such as lacewings, assassin bugs, and lady beetles.
Size & form
A broadly rounded evergreen shrub reaching 3 to 4 feet high and wide
Tree & Plant Care
Best in part shade, but tolerant of full sun with adequate soil moisture. Plants in deep shade will be more open and loose.
Avoid windy sites.
Prune as needed, can be sheared and shaped in early spring. Avoid late summer pruning. New growth will not harden off for winter.
Remove heavy snow cover to avoid winter damage.
Disease, pests, and problems
Winter frost cracking during sudden temperature drop, volutella, phytophthora, boxwood psyllid, leafminer, mites.
All plant parts are poisonous.
Disease, pest, and problem resistance
Deer and rabbit resistant
Native geographic location and habitat
Bark color and texture
New growth is angular and green. Mature stems is tan to light brown.
Leaf or needle arrangement, size, shape, and texture
Opposite. Small, 1/2 inches to 1 /12 inches oval to oblong leaves with smooth-margins.
Leaves are dark glossy green above and yellowish-green below. Leaves have a malodorous fragrance.
Winter sun can cause bronzing.
Flower arrangement, shape, and size
Early spring, pale green to yellow to creamy white flowers are inconspicuous in auxiliary clusters.
Fruit, cone, nut, and seed descriptions
Fruit is a 1/3 inch, dehiscent capsule that matures to brown.
Cultivars and their differences
North Star® common boxwood (Buxus sempervirens 'Katerberg'): A densely globe-shaped habit reaching 2 to 2 1/2 feet high. requires little pruning to retain shape.
Schmidt common boxwood (Buxus sempervirens 'Schmidt'): An upright evergreen reaching 5-7 feet high and 4 feet wide.
Vardar Valley common boxwood (Buxus sempervirens 'Vardar Valley'): Low-growing, flat-topped mound reaching 2-3 feet high and 4 to 5 feet wide. Excellent dark bluish-green foliage.
When a Replacement is Needed
While most Boxwood problems can be solved (or prevented in the first place), there are times when replacement plants may be needed. In particular, if your Boxwood was planted in the wrong location, then it may never thrive and will always be prone to problems due to its poor health.
Some other plants that are quite similar to Boxwoods and could be utilized in its place include the Boxleaf Euonymus, the Inkberry Holly, or the Winterberry Holly. Of course, exactly which plant will be ideal for your location will require a consultation with a plant expert. The last thing that you want is to get stuck in another cycle of plant problems that could be prevented simply by planting the right plant in the right location.
Plant Diseases That Can Affect Japanese Boxwood
While the Japanese boxwood is a hardy plant, it can be damaged by various plant diseases and pests, including boxwood psyllid, boxwood leaf minor, boxwood mite, nematodes and phytophthora root rot.
Boxwood blight is an awful fungal disease that can affect boxwood shrubs. It is caused by Calonectria pseudonaviculata. First reported in the mid-1990s in the UK, boxwood blight has since spread to Europe, New Zealand, and North America.
A plant suffering from boxwood blight can be infected in all of its aboveground parts. The first signs of blight are dark leaf spots. These end up forming brown blotches.
Infected leaves will show white spots on their undersides after experiencing high-humidity. Narrow black streaks will also appear on the green stems, and fuzzy white masses will develop from these stem cankers during periods of high-humidity.
Japanese boxwood shrubs with blight will experience rapid defoliation starting from the bottom branches and moving upward over time. This can kill young plants and cause older plants to lose the ornamental value they once had.
The best way to deal with boxwood blight is to prevent it from entering the landscape. You’ll want to buy your boxwood plants from nurseries that participate in a blight compliance agreement and are considered to be reputable.
You’ll always want to avoid shearing boxwoods when they’re wet. This can help reduce the probability of spreading diseases. Make sure all of your shearing tools are clean and disinfected.
If you have infected plants, make sure you remove any of the debris from pruning operations from the property. Don’t compost this material. If you have a property that contains historically important and large boxwoods, don’t introduce new boxwoods into the landscape.
If planted in compacted and poorly draining soils, root rot can develop. Root rot is caused by a fungus, the typical culprits of which are Phytophthora, Rhizoctonia, Pythium, or Fusarium fungi. All of these fungi will thrive in overly wet soil and can become a problem for other plants if you transplant infected plants.
Plants that are plagued with root rot won’t be able to absorb nourishment or moisture from the soil. When suffering from this disease, plants can look similar to those that are inflicted with mineral deficiencies, stress, and drought.
If you notice that the leaves are discolored, wilting, or stunting, you might have a case of root rot. You’ll notice shoots and foliage dying back and soon after the whole plant can die. If you pull a plant suffering from root rot out of the ground, you will notice that the roots are soft, brown, and unpleasant smelling instead of white and firm.
The best way to deal with a root rot problem is to avoid having one in the first place. The most important thing is to plant the shrubs in well-draining soil and to not overwater your plants. Boxwood can recover from root rot, but only if the issues with soggy, wet soil are fixed promptly.
Leaf spot is a disease that interrupts photosynthesis and therefore weakens shrubs and trees. While this disease will most likely not harm your plant seriously, it should be taken seriously if there is moderate leaf loss in a 2-4 year period. If for several consecutive growing seasons there is leaf loss, your Japanese boxwood can become more susceptible to diseases and pests and its growth can be reduced.
If your shrubs are infected with leaf spot disease, there are a number of things you can do to reduce the occurrence of the disease in the years to come. They include:
- Avoid overcrowding your plants, space them at least three feet apart
- Prune your shrubs to improve air circulation and increase light penetration
- Ensure mulch and organic debris is pulled back from the stem’s bark where it enters the ground
- Rake up and destroy leaves that have fallen before the first snowfall
- Water shrubs at the base and be sure to avoid splashing water on the leaves
- Reduce shrub stress by proper watering techniques and mulching
- Don’t fertilize unless a soil test recommends it.
Japanese boxwoods are tough shrubs, but all plants have diseases they can fall prey to. Knowing ahead of time what kinds of diseases boxwoods can face can ensure that you can prevent problems to the best of your ability and stop them early on in their process before they become too problematic.
Pests: Mites & Leafminers
If you’re looking for a generally pest-resistant shrub, the Japanese boxwood is a good choice. However, there are two pests that you should still keep an eye out for when growing Japanese boxwood. These are boxwood mites and boxwood leafminer.
Boxwood mites feed on the undersides of the leaves. You’ll need a magnifying glass to see these tiny, sap-sucking bugs. Their presence results in brown marks or flecks right on the leaves.
Boxwood leafminers can result in raised areas, brown splotches, and blistering on leaves.
Sometimes the leaves of young Japanese Boxwood plants look yellow-gold in the spring. This is called “bronzing” and is a result of harsh winter conditions. Bronzing is generally decreased after the plant has established its roots in the landscape for a few years.