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Borers Attack Stressed Local Trees

GARDEN TIPS – written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written June 18, 2015

The discovery of holes in a tree’s trunk or branches usually means it has been the victim of a boring insect attack. While it is alarming to discover an increasing number of trees with significant borer damage, it is not unexpected. Most borers take advantage of trees weakened by drought stress, injury, insects, or disease. Several years of extremely dry winters along with last summer’s extreme heat has stressed local trees and shrubs, making them vulnerable to attack. Some of the dastardly culprits attacking local trees include the:

Ash Borer (a.k.a lilac borer) – The adult is a moth that looks like a yellowjacket. It primarily attacks ash, lilac, and privet. The moth lays its eggs on the bark. They hatch into small larvae that tunnel into trunk and branch wood, weakening it. The larvae pupate and emerge from the tree in May or June leaving noticeable 1/4″ exit holes.

Redheaded Ash Borer – The adult is a “longhorned” reddish brown beetle with an elongated body and long antennae. It also resembles a yellowjacket because of the yellow to white horizontal bands on its back. While called an “ash borer” it attacks a wide variety of trees including ash, linden, oak, and others. Like many other borers, it lays its eggs on the bark of stressed or dying trees. These hatch and then eat their way under the bark and tunnel into the wood as they mature. There may be more than one generation of these borers a year with adult beetles emerging from spring through summer and leaving 1/4″ exit holes.

Bark Beetles – There are a number of different types of bark beetles and one or more of these are attacking local stressed arborvitae and other evergreens. Typically, the adults are little, .08″ long, brown beetles. What they lack in size they make up for in number. They feed directly under the bark of trees and shrubs, creating serpentine paths as they eat. Their feeding can girdle trunks and branches, cutting off the tree’s access to water and nutrients. Their exit holes are pencil-point sized.

Other borers that commonly attack landscape trees in this area are the bronze birch borer, the peach borer, and the locust borer.

Unfortunately, pesticide applications are not very effective for borer control in attacked and dying trees. Sprays made to the bark surface will not kill any borers residing under the bark or within the wood. For sprays to be effective they would need to be applied when the adults emerge. Timing of sprays is critical and they may need to be reapplied if the insect emerges over a span of several months or has several generations a year.

There are some systemic insecticides that are applied as a drench to the base of trees and taken up into plant tissues. These are only effective on some flatheaded borers, like the bronze birch borer, that spend most of their time feeding in tissue just beneath bark. They are not effective in controlling borers that eat mostly in tree wood.

WSU Extension experts indicate that the best control for any borers is to keep your trees healthy and vigorous to prevent attack. This is sage advice, but too little too late for attacked and dying trees.


written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

Have you noticed the ghostly forms of dead birch trees around the area? Birch trees may be a thing of the past… if the bronze birch borer (bbb) has anything to say about it. This devastating boring beetle is attacking and killing birch trees in home and commercial landscapes throughout our region, state and country.

Unlike many newer tree boring pests, the bronze birch borer (Agrilus anxius)is native to North America. It has been around for years. So why is it a problem now? It’s probably because birch trees aren’t well adapted to urban landscape situations, especially in our climate and soils. Birches grown in adverse “urban” conditions become stressed, especially as they grow older. Stressed trees are weaker and more subject to attack by the bronze birch borer. Winter drought, extremely hot summers, improper watering techniques, poor soil conditions, and lack of protective snow cover during the winter are all factors in weakening and making birches prime candidates for attack by the borer.

The City of Spokane urban forester, Jim Flott, notes that it’s not a good idea to plant any type of birch in Spokane. That’s because all species of birch can be attacked by the bronze birch borer… and are attacked and killed in the Spokane area. However, certain birches are more prone to attack than others. If you simply must have a birch, avoid the European white birch (Betula pendula), the Jacquemonti birch (Betula jacquemontii), and Young’s weeping birch (Betula pendula ‘Youngii’). Your best bet is to plant a river birch (Betula nigra) or a Heritage river birch (Betula nigra ‘Heritage’) that have very low susceptibility to the borer or the Whitespire birch senior (Betula platyphylla japonica ‘Whitespire’ with moderate susceptibility. Like many gardeners, I adore birches. I have three of Heritage river birch in my yard. They’re planted in a triangular pattern to give the feeling of a “clump” or grove, but planted far enough apart to allow all three room to grow. Ten years after planting they’re still healthy and thriving… knock on sound wood.

Let’s learn a little more about this infamous pest. The adult insect is a slender, metallic beetle about 3/8 of an inch long. It belongs to the buprestid or metallic wood boring group of beetles. However, it’s unlikely that you’ll see the adult. You’re more likely to find the creamy white legless larvae or “flatheaded borer” tunneling in the wood beneath the bark. When mature the larvae are about one-half inch long.

It’s surprising that such relatively small beetles and their larvae are responsible for the demise of large birch trees. The adults only feed on leaf margins, causing little significant damage. It’s the larvae that create the problem. They feed mostly just underneath bark in the sap conducting (phloem) tissue, tunneling back and forth in an irregular criss-crossing pattern. It’s this feeding beneath the bark that causes the most damage. The bbb first attacks smaller branches (about 3/4 inch in diameter), girdling them and then moves into larger branches.

How can you tell if a suffering birch has been attacked by the bbb? The first hint is that the tree starts to die back from the top down. As the attack progresses, the bark on branches will become ridged and bumpy caused by the larvae feeding beneath. Cutting into the bark, the winding, zig-sag tunnels are evident. Eventually, D-shaped holes will appear on the bark. This is where the adult beetles exit. Untreated, a tree under attack will die within several years.

If your tree is already attacked, you’ll want to consider use of chemicals to control the borer. WSU recommendations are aimed at controlling the adult beetle and hatching larvae to prevent them from laying eggs and reinfesting a tree. Local tree care companies and arborists recommend the use of a soil drench or trunk applied systemic insecticide to fend off the bbb. One of these materials, imidacloprid, is also available to home gardeners in the Bayer Advanced Garden Tree and Shrub. It’s mixed with water and applied to the soil close to the trunk. These treatments can be pricey, but are worth it if you want to try to save your tree. The only problem is that you ‘ll need to apply any type of insecticide, spray or drench, on an annual basis to protect your tree, plus take measures to improve the health of the tree.

If your birch tree hasn’t been attacked by borers yet or you want to plant a birch in your landscape, there are some things you can do to help your tree resist attack.

– Keep your birch as healthy as possible. Water correctly in the summer. Develop a deep root system by watering deeply at least once a week…moistening the soil to a depth of 18 to 24 inches. Avoid watering for short periods every day…. this leads to a shallow root system that’s more vulnerable to drought and temperature extremes. Water birch trees in the late fall and winter, when the soil is dry and temperatures are mild.

– Spread bark mulch or compost in a two to four inch layer over the root system to help retain soil moisture and to protect the roots from extreme hot and cold temperatures.

– Remove dead or infested wood in birch trees at any time, but if you must remove healthy live wood don’t prune in late spring through early summer. Female bbbs are attracted to fresh wounds created by pruning. The best time for pruning is late winter or early spring. Don’t remove large amounts of live wood… birch trees should never be topped!

– The native habitats of birches are shady, cool, moist, wooded areas. If possible, when planting a birch tree in the landscape, place it where it will receive semi-shade and have semi-moist soil. “Hot” situations next to pavement, in courtyards, close to light-colored or stone structures will stress a birch, making it more vulnerable to be attacked by the bbb. Landscape beds on the north and east sides of homes are probably the best locations. One of the worst places to plant them is in the middle of the lawn with full exposure to intense sun and drying wind. Oops!

Published: 8/21/2004 2:18 PM


written by Marianne C. Ophardt WSU Extension Faculty for the Tri-City Herald, Kennewick, WA It seems like it wasn’t that long ago that I wrote a column about why all the birch trees were dying, but it was actually several years ago. With your forbearance I’m going to address the topic again, because more birch trees are failing and dying in area home landscapes. The two main factors in their failure are stress and the bronze birch borer. Birch trees are not well adapted to our climate. When placed in a harsh urban environment, especially in a region with very hot summer temperatures, they suffer from heat stress and often fail to thrive. They can also often suffer from winter drought in our region. If you want to plant a birch tree, there are some things you can do to help promote a healthy, vigorous tree. First, site it where it will receive full sun most of the day, but be protected from the heat of the afternoon sun and heat. The east and north sides of a building are best. Second, birches like to have cool roots. They’ll do better if mulched with a shredded bark, wood chips, or coarse compost. These organic mulches moderate soil temperatures and keep the soil moist… essentials for a happy birch tree. Definitely, do not mulch your birch with a rock mulch. Yet another ‘must’ is to provide the tree with enough water during the spring, summer, fall, … and even the winter. Adequate water is a key factor in birch tree health. During the summer be sure the trees are watered deeply, moistening the soil to a depth of 12 to 18 inches once a week. If a tree is situated in a lawn, shallow watering every day will not provide the water that’s needed. A deep soaking in the root zone is required. Weekly deep watering is not necessary once the trees go dormant, but the birch roots still need some moisture if the temperatures are mild and the soil isn’t frozen. In our region, natural precipitation during fall and winter months is usually not adequate for birch trees. This has been especially true during the past several winters that were very mild and extremely dry. (Yes, this means hauling out the hoses once the irrigation water has been shut off. ) The other factor in the demise of many birch trees in our region is the bronze birch borer. Like a vicious animal can sense your fear, bronze birch borers can sense that a birch tree is under stress and vulnerable to attack. This boring beetle is a native pest that attacks birch trees. It’s a problem throughout this country and Canada where native birches grow in woodlands and in urban landscapes where native and non-native birches are planted. It’s not a new pest problem, records of its destruction date back to the late 1800

s and early 1900

s. Birch trees that are subject to some sort of stress are prime candidates for attack by the bronze birch borer. Stress can be brought about by extreme heat, drought, old age, soil compaction, physical injury, severe pruning, poor growing conditions, or improper planting. All efforts should be made to plant birches correctly in a favorable location and to promote good growth. One of the first symptoms of a bronze birch borer infestation is dieback at the top of the tree… where the leaves on the top branches start to yellow and then die. The dieback is progressive from the top of the tree downwards. However, this can also be a symptom of tree decline due to drought stress, root problems, trunk injury, or other poor cultural conditions. The key to determining the presence of the dreaded bronze birch borer is lumpy bark. That’s because the larvae feed under the bark creating extensive winding galleries. The tree responds to the ‘wounds’ created by the borer’s feeding by producing callus tissue that results in bumps and lumps beneath the bark. After the larvae mature and turn into adults, the beetles chew their way out. They leave behind a hole that’s D-shaped and about 3 to 5 mm wide. However, these characteristic holes are usually only observed in badly infested trees. The bronze birch borer kills trees by damaging the conducting tissues (phloem) under the bark that transport carbohydrates up and down the tree. Because of this disruption, the tree roots aren’t able to get the ‘food’ or energy they need to survive and they starve to death. Once the roots die, the top of the tree is unable to get water and the tree starts to die. It’s a slow, but progressive death. If more than 50 per cent of the crown is affected with dieback, it’s unlikely the birch tree can be saved. We’ll talk later about control measures that can be taken if the tree is just beginning to show signs of a bronze birch borer infestation. One of the best ways to control bronze birch borer is to plant birch trees that are resistant to attack by the borer. Unfortunately, the most popular types of birches are highly susceptible to attack. This includes the whitebarked Himalayan or Jacquemonti birch (Betula jacquemontii), the European white birch (Betula pendula) with its graceful pendulous branches, and Young’s weeping birch (Betula pendula ‘Youngii’). The Whitespire birch (Betula playtyphylla japonica ‘Whitespire’), known for its resistance, is moderately susceptible to the borer. The river birch (Betula nigra) and the Heritage river birch (Betula nigra ‘Heritage’) are the only birches known to be fairly resistant (but not immune) to attack. Once a tree is attacked by bronze birch borer what steps can be taken to control it? In the past, chemical insecticide sprays were applied to the bark and leaves of the tree as preventatives to kill adult beetles before they have a chance to lay eggs after emergence and to kill young larvae before they enter the tree. Once the larvae are beneath the bark, external surface spray applications are not effective. Because beetle emergence occurs over several weeks or more, repeated sprays are needed. This is costly and not extremely effective. Because the larvae feed on the phloem tissues just below the bark, systemic insecticides have been found to be somewhat effective in the control of existing bronze birch borer infestations. These systemics are applied by injecting them into the tree trunk or by applying them to the soil for absorption by the roots. Some of these materials can only be applied by licensed commercial applicators, but one product is available for application by home gardeners. That product is Bayer Advanced Tree & Shrub Insect Control Concentrate containing an insecticide called imidacloprid. However, it’s expensive and some price conscious gardeners with large trees have found that it may be less costly to have a commercial applicator apply the same chemical. Injection into the trunk is preferred by some applicators, but this should only be used to control a severe existing bronze birch borer infestation. Repeated use of trunk injection as a preventative measure can be harmful to the tree’s vascular system and compromise tree health. Keep in mind that relying exclusively on the use of chemicals for control of the bronze birch borer is not environmentally sound gardening and it’s expensive. If you want to plant a birch tree, select one of the moderately resistant species. If you have a birch tree in your landscape, keep it as healthy and vigorous as possible. That’s what I’ve done! My three ‘’Heritage’ birches are vigorous, attractive, and free of bronze birch borer… knock on wood. Special Note: It’s important to note that while use of systemic insecticides does seem to provide control of bronze birch borer in birch trees, these chemicals are generally not effective for control of other types of borers that bore deeper into the wood of trees and don’t stay primarily in the area of the phloem just under the bark.

Published: 7/9/2005 11:46 AM



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