Boxwood Pests


Laef_Minor

 

Leafminer 

The boxwood leafminer, Monarthropalpus flavus, is actually a gall midge and not a leafminer. It is a serious insect pest of boxwood. A high population can defoliate and kill weak boxwood. Injury is caused by the larvae (maggots) feeding in the leaf, which result in premature leaf drop. Most cultivars of B. sempervirens and B. microphylla are susceptible to boxwood leafminer.

 

What does leafminer damage look like?

The larvae of boxwood leafminer feed on the parenchyma tissue between the top and bottom of the leaf. The resulting damage appears as an irregular oval swelling on the leaf. There may be a slight blistering of the leaf on the lower surface with a yellowish or brown discoloration. The new foliage will not show this blistering effect until late summer. The female, a true bug, enters the leaf through the stoma, a natural opening in the leaf surface used for the exchange of air. Early signs are holes on the lower leaf surface after the female deposits her eggs. These punctures are visible if the leaves are closely inspected. Leafminers prefer the protected part of the boxwood, the lower and innermost new leaves. Shake the branches of the shrub in late April to detect flying adults. Damage from high infestations results in premature leaf drop.

 

What does leafminer damage look like?

 

The larvae of boxwood leafminer feed on the parenchyma tissue between the top and bottom of the leaf. The resulting damage appears as an irregular oval swelling on the leaf. There may be a slight blistering of the leaf on the lower surface with a yellowish or brown discoloration. The new foliage will not show this blistering effect until late summer. The female, a true bug, enters the leaf through the stoma, a natural opening in the leaf surface used for the exchange of air. Early signs are holes on the lower leaf surface after the female deposits her eggs. These punctures are visible if the leaves are closely inspected. Leafminers prefer the protected part of the boxwood, the lower and innermost new leaves. Shake the branches of the shrub in late April to detect flying adults. Damage from high infestations results in premature leaf drop.


Understanding the leafminer life cycle

 

Leafminers produce a single generation each year. They overwinter as translucent yellowish-green, partly grown larvae inside the leaves. During the early warm days of spring they grow rapidly into yellow-orange-colored pupae. They time their pupation so that the adults emerge from the underside of the leaf as flies in late April, usually when Weigela begins to bloom. The adult fly is a little shorter than 1/2" (13 mm) long and resembles an orange mosquito. The flies emerge over a three-week period, leaving noticeable white pupal skins which hang down from the undersurface of the leaf. It is easy to notice the yellow-to-orange-colored adults flying around from leaf to leaf, particularly if the plant is disturbed. The females quickly begin laying small white eggs deep into the leaf tissue from the underside of the new spring leaves. The entry points, tiny egg-laying ruptures, can be seen on the underside of the leaf. One female will deposit about 30 eggs, often dying within hours of laying her last eggs. The eggs will develop for three to four weeks. The resulting larvae will feed inside the leaf all summer, fall, and early the following spring. Most plant damage occurs in fall and late winter. During feeding, the larvae use a microscopic hook to rupture the leaf cells. The plant responds by sending more biomass into the leaf, which gives the larvae more to eat.

                             

  Leaf_Minor_Damage 

How to control leafminer

 

Control is necessary in cases when persistent or intolerable damage. The leafminer is most susceptible right after the adult emerges and before she lays her eggs. The adults emerge over a three-week period, but each lives for only two days. Control measures should be scheduled when adults appear, usually late April to early May using a contact insecticide. Effective control for the larvae in summer, fall, or early spring comes from foliar-absorbed pesticides such as: abamectin, azadirachtin, dimethoate, imidacloprid, malathion, or spinosad. Systemic insecticides, applied as a soil drench (imidacloprid) may be used against the first instar of the larvae in mid-June. There must be adequate soil moisture during and after application of the soil drench to obtain best results. Surprisingly, birds (nuthatches, warblers, tufted titmice, and chickadees), are able to hear the faintly audible crinkling or rustling of the boxwood leafminer pupae. The pupae actively wiggle while feeding within the boxwood leaf, and are particularly energetic during emergence. Birds peck away at the underside of the boxwood leaves in order to gain access to the pupae inside. While cold temperatures do not cause any mortality to the over-wintering larvae, they will dry up and die through lack of moisture during hot, dry summers.


Mites

The mite is not an insect, a member of the Arachnida, a class to which spiders belong. The boxwood mite, Eurytetranychus buxi, is a rather common and widespread pest. Mites are very inconspicuous. Early damage is also not obvious and the problem is often overlooked until high populations and extensive damage have occurred. The mite is a serious problem on most B. sempervirens cultivars, particularly those grown in sunny locations. It also attacks cultivars of B. microphylla var. japonica.

Mites

What does mite damage look like?

The mite feeds on the upper and lower leaf surfaces. Depending on how large the population is, it will produce varying degrees of leaf stippling. On close examination, the damaged leaf appears as if someone took a small needle and made tiny white scratch marks. This damage is a result of the mites piercing the epidermal cells then sucking out the juices of the new leaves. In the absence of the green chlorophyll pigments, the surface of the leaf appears dusty or pale green. The mite damage is most apparent on the second- and third-year leaves. Viewed from a short distance, the damaged plant takes on a light silvery cast.


Understanding the mite life cycle

The mite overwinters as a yellowish green egg on the underside of the boxwood leaf. The eggs hatch in April and May. The adult is period-sized, yellowish-brown, has eight long legs, and closely resembles a miniature spider. Mites favor hot, dry conditions. For example, at 70o F (21oC), mites complete their life cycle, egg to adult, in as little as ten days. The population increase is greatest in early summer. During a hot and dry summer, it is common to have five to eight generations of mites. This can result in a rapid build-up of populations and extensive damage.

 

How to control mites

Many species of insects and other mites feed on boxwood mites. The presence of mites can be determined by lightly beating a small branch over a white sheet of paper. The mites will fall onto the paper and can be seen moving around. It is important to delay control measures until mites reach a damaging level. Damage occurs at about 25 mites per “beat.” This 25-mite level permits beneficial predatory insects and mites to become established. Overwintering eggs can be destroyed with a delayed dormant oil spray on the underside of the leaves. Insecticidal oil and soap are effective against light populations when summer control measures are necessary. Heavy summer populations require residual or foliar-absorbed pesticides, which include: bifenthrin, dicofol, dimethoate, fluvalinate, hexythiazox, lambda-cyhalothrin, or malathion.


 Psyllid

The most common insect that attacks B. sempervirens and its cultivars is the boxwood psyllid, Psylla buxi.

 

What does psyllid damage look like?

The nymphal insect feeds in the spring when boxwood produce new leaves. It consumes leaf cells, causing the young leaves to become distorted and cupped inward. The damage is quite conspicuous. This condition is unsightly and will stop growth at the shoot apex for about two years. Large populations of psyllid can kill the youngest of leaves. The remaining damaged young leaves cannot receive their normal amount of sunlight. As a result they do not manufacture the expected amounts of carbohydrates (food) for the plant. Generally, these weakened leaves abscise after one year.


Understanding the psyllid life cycle

There is one generation of psyllid per year, overwintering as orange eggs beneath the bud scales. The c" (3 mm) long light green nymph emerges in mid- to late-April, coinciding with the emergence of the spring foliage. The nymphs produce a white waxy stringlike secretion while they feed within the cupped leaves, until the adults appear in June. The adults do not feed, but fly around depositing eggs into the bud scales during June and July.


How to control psyllid

Low populations are common, and injury is usually not serious enough for control measures. Control is used to reduce the population and its damage to a level that will not affect the growth and overall health of the boxwood. Correct timing of the control measures effectively reduces the populations. 
Use insecticidal oil or soaps to control nymphs when detected in April and May. Care should be used to obtain complete coverage of the leaves with these materials. A residual foliar-absorbed pesticide such as acephate, azadirachtin, bifenthrin, carbaryl, cyfluthrin, dimethoate, or fluvalinate, may be used to control adults found in June.

Psyllid

 

Scale1

Scale

There are a variety of armored or soft scale insects that attack boxwood. They include: the wax scale, which has common names that include Japanese, Indian and ceriferus scale (Ceroplastes ceriferus); oystershell scale (Lepidosaphes ulmi); California red scale (Aonidiella aurantii); Oleander scale (Aspidiotus nerii); peony scale (Pseudaonidia paeonia); greedy scale (Hemiberlesia rapax); euonymus scale (Unaspis euonymi); San Jose scale (Quadraspidiotus perniciosus); cottony maple scale (Pulvinaria innumerabilis); cottony cushion scale (Icerya purchasi); and the Lecanium scales (Lecanium). Many of these scales occur only in the southern region of the United States, and are of particular concern in California and the states bordering on the Gulf of Mexico. For the majority of boxwood, grown in more temperate northerly regions, scales are only occasional problems. However in these isolated cases, scale damage can have fatal results particularly if the boxwood is already weak. Perhaps most notable, the wax scale attacks boxwood almost everywhere. While most common in the southern United States, it is found along the East Coast with a northern limit near New York. As a result of this large distribution and its potential for damage, attention here will focus on the wax scale.


 



Understanding the scale life cycle

The wax scale has only one generation per year. In very warm climates, two generations per year are possible. It overwinters as an adult female, which can be expected to lay about 1,000 eggs. The mature female is about a" (8 mm) in length and has a soft, bright white, waxy covering. The eggs are laid in April or May and hatch in June.
The emerging crawlers will settle on the twigs and stems, but not on the leaves. Soon after they molt, the nymphs begin producing wax which creates a cameo appearance. As wax production continues, the body is quickly and fully covered with wax by the end of July. At this stage, effective chemical control is not possible.



How to control scale

Correct timing of treatments will provide superior results. The eggs hatch in early June and the emerging crawlers are quite active. This is the best time to apply controls that should cover all of the trunk, stems, and branches. Foliar sprays of insecticidal oil, insecticidal soap, or residual insecticides may be used.

Scale