Lilac Borer Pests: Learn How To Get Rid Of Lilac Borers

By: Teo Spengler

Lilac shrubs are flowering ornamentals beloved by gardeners for their fragrant, light purple blossoms. Naturally, lilac borer pests are not popular. According to lilac borer information, the larvae of ash borer moths damage not only lilac (Syringa spp.) but also ash trees (Fraxinus spp.) and privet (Ligustrum spp.). If you want more information about lilac ash borer symptoms or tips for managing lilac ash borers, read on.

Lilac Borer Information

Lilac borer pests (Podosesia syringae), also known as ash borers, are clear-wing moths. However, according to lilac borer information, adult females look more like wasps. The insects are found throughout the continental United States.

Borer larvae are what cause the lilac ash borer symptoms. The larvae are large, growing up to an inch (2.5 cm) long. They damage lilacs and other plants by feeding on the phloem and outer sapwood of trees and shrubs.

The principal lilac ash borer symptoms are the galleries they dig. These are extensive, even if only a few larvae are present on a tree, and cause significant damage to the plant. Generally, lilac borer pests attack the main trunk of a lilac. However, they can also dig tunnels in larger branches.

How to Get Rid of Lilac Borers

If you are wondering how to get rid of lilac borers, you are not alone. Most gardeners whose plants show signs of borer symptoms want to rid their yard of these pests. However, managing lilac ash borers isn’t easy.

Your best bet is prevention. Keep your shrubs and trees free of stress when they are young. The borers often are able to enter a tree when you cut the trunk with lawn equipment, so be especially careful. Also, take care to irrigate during dry periods.

While you can prevent an insect attack with insecticide sprays and pheromone traps in spring to catch the adult males, this will not help with borers already inside the plants. To prevent the issue, begin spraying the plants 10 days after you trap the males with pheromone. If you don’t use the traps, spray your plants in early May when lilacs are just finishing bloom. Repeat the spray three weeks later.

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Lilac borers, also called ash borers, are tiny pests that can kill your lilac trees. Immature lilac borer larvae burrow into the stems of your lilac tree and feed on the materials inside. Over time, the lilac borers will bore holes in the branches of your tree. Once the pests bore holes in your lilac tree, it will die. Lilac borers are most common in the Central Valley of California, but can infest trees throughout the state. If you notice lilac borers taking up residence in your tree, take action immediately to protect the health of your plant.

Cut off any infested branches with gardening shears. Wear gloves to protect your hands.

Place lilac borer pheromone traps around the trees in April or May. Follow the package directions for proper placement. Borer pheromone traps capture any male borers present in your lilac tree, which helps prevent any more larvae from being produced, according to the Iowa State University Extension.

Spray an insecticide designed to destroy borers on the trunk of your lilac tree. Because insecticides do not kill borers already in the branches of your tree, apply the product 10 days after you capture the first borers in the pheromone traps.

Prevent future infestations by applying a layer of mulch around your tree and by watering it well during dry spells. Protecting your tree from damage caused by lawn equipment can also help protect your lilacs from a borer infestation.

  • Pheromone traps are a chemical-free way to control lilac borers, but set them out as soon as you notice the pests. You are more likely to get rid of them this way if there are only a few pests to contend with.
  • If you have ichneumon wasps in your yard, don't spray insecticide right away. These types of wasp can destroy lilac borers for you, but insecticide will kill them also. Wait to see if they take care of the problem before applying insecticide.
  • Read the directions on your insecticide and use it only as the package directs. It is against the law to use insecticide for any purpose other what is stated on the label.
  • Extensive damage might require you to cut down your lilac tree and burn it. Burning the branches prevents the spread of borers to other trees and plants in your yard. Check local city ordinances for information about burning.

Sara Ipatenco has taught writing, health and nutrition. She started writing in 2007 and has been published in Teaching Tolerance magazine. Ipatenco holds a bachelor's degree and a master's degree in education, both from the University of Denver.

Emerald ash borer insecticide treatment considerations. Several insecticide products are available to homeowners for control of emerald ash borer (EAB). Since the presence and infestation level of EAB is quite difficult to determine at early stages of an infestation, insecticide treatments may be merited to mitigate damage by EAB. However, not all ash trees should be treated as some may be too extensively compromised or in poor condition to receive treatment. Tree location, value, and health, as well as the cost of treatment are all factors to consider. Due to the expense of yearly insecticide treatments, one should consider the value of a particular ash tree in relation to insecticide treatment costs before making any treatments. In addition, consider the health of each tree before treating. Research suggests that insecticide treatments are significantly more effective on EAB-infested ash trees with less than 50% canopy thinning. Insecticide treatments are not suggested for trees with greater than 50% canopy thinning. Trees with greater than 50% canopy thinning should be removed and handled in accordance with local guidelines. For a more detailed discussion on this topic, see University of Wisconsin Garden Facts XHT1215, Is My Ash Tree Worth Treating for Emerald Ash Borer.

Emerald ash borer insecticide treatment options. Insecticide products available for use by homeowners are summarized in Table 1. They include:

  • ACECAP 97 Systemic Insecticide Tree Implants (acephate)
  • Bayer Advanced 12 Month Tree and Shrub Insect Control II (imidacloprid)
  • Bayer Advanced 12 Month Tree and Shrub Protect & Feed (imidacloprid)
  • Bayer Advanced 12 Month Tree and Shrub Protect & Feed II (imidacloprid + clothianidin)
  • Bonide Annual Tree & Shrub Insect Control with SYSTEMAXX (imidacloprid)
  • Compare N Save Systemic Tree & Shrub Insect Drench (imidacloprid)
  • Ferti-lome Tree and Shrub Systemic Drench (imidacloprid)
  • Monterey Once a Year Insect Control II (imidacloprid)
  • Optrol (imidacloprid)
  • Ortho Tree & Shrub Insect Control Granules (dinotefuran)
  • Several other products containing imidacloprid are also currently available

Most of the products available to homeowners are systemic insecticides containing imidacloprid and are applied as soil drenches around the base of an ash tree. A few granular products are also available. Recent university research suggests that applications of imidacloprid should be made in spring to be most effective. Research also has demonstrated that soil applications of imidacloprid-containing homeowner products provide excellent EAB protection for ash trees that are less than about 47 inches in circumference [i.e., 15 inches in diameter at breast height (DBH)]. Due to differences in application rates and label restrictions, treatment by a tree care professional (e.g., arborist) may be the best option for larger trees. For best results, treatment of trees should begin before trees become infested. Lastly, insecticide treatments must be repeated each year to maintain the health of ash trees.

Be aware that many insecticide products available at hardware stores and garden centers look alike. Carefully check all product labels before purchase to make sure that you have selected the correct product/active ingredient. ALWAYS read and follow the pesticide label directions on the product that you select!

Finally, note that although ACECAP 97 Systemic Insecticide Tree Implants are available to homeowners, we do NOT recommend that homeowners use these because they require physically drilling into a tree during their application.

Bayer Advanced 12 Month Tree & Shrub Insect Control II (D)

Bayer Advanced 12 Month Tree & Shrub Protect & Feed (D or G)

Bonide Annual Tree & Shrub Insect Control with SYSTEMAXX (D)

Compare N Save Systemic Tree & Shrub Systemic Insect Drench (D)

Ferti-lome Tree & Shrub Systemic Drench (D)

This factsheet addresses some of the most frequently asked questions regarding the treatment of ash trees for emerald ash borer (EAB), and the removal and disposal of infested trees.

When should I consider treating my ash tree for EAB? Based on current research, EAB treatments are suggested only for ash trees located within 15 miles of a confirmed EAB site, or for trees located within a quarantined area. Insecticide treatments are not necessary for ash trees located outside of these areas. Even within the 15-mile radius, not all trees should be treated. Due to the expense of insecticide treatments for EAB, consider the value of a particular ash tree in relation to insecticide treatment costs before making any treatments. Proper use of EAB insecticides can help maintain the health of high value ash trees over time. Lower value ash trees are not ideal candidates for EAB insecticide treatments.

How do I know if my ash tree has value? Ash trees can be a valuable part of the landscape. A properly cared for ash tree can increase property value, provide environmental benefits such as runoff and erosion mitigation, and reduce electricity costs by shading a home. Determining tree value can be subjective. Qualities to consider when assessing value include (but are not limited to) a tree’s overall health, shape, location with respect to landscape design, and appearance through the seasons, as well as whether or not a tree provides shade. A healthy ash that is properly located in the landscape, has a nice shape and good fall color, and provides shade has value. An ash tree that is not healthy due to disease or insects, has poor shape or structural damage, is otherwise unattractive, or is in a bad location (e.g., near a power line) is of lower value.

How do I know if there are ash trees in my area that are infested with EAB? The Wisconsin Department of Agriculture Trade and Consumer Protection (DATCP) keeps track of EAB infestations in the state. Visit the Wisconsin DATCP Emerald Ash Borer Resource Guide website ( and view the interactive map. You can also contact your local county UW-Extension office to see if EAB has been found in your area.

How do I know if my ash tree has EAB? Symptoms of an EAB infestation can include canopy thinning starting in the upper portion of the tree, epicormic sprouting (i.e., formation of sprouts) along the trunk, bark splitting, and woodpecker damage. These symptoms indicate general tree stress, and can be due to EAB. However, they also can be caused by diseases or insects other than EAB. Specific signs of EAB include D-shaped exit holes (

3 /16 inch wide) in the bark of the tree, S-shaped larval tunnels and/or larvae (cream colored, up to 1½ inches long) beneath the bark, and adults (metallic green,

3 /8 inch long). Visit the UW-Madison Emerald Ash Borer in Wisconsin website ( for additional information on the symptoms and signs of EAB.

If I decide to treat my ash tree, will I have to treat every year? In most cases, yes. Most insecticides registered for EAB management require yearly applications to effectively protect a tree. Products containing the active ingredient emamectin benzoate, are labelled for two years of protection. Products containing emamectin benzoate are trunk-injected insecticides intended for use by professional insecticide applicators (e.g., certified arborists). Such products can effectively protect an ash trees if the tree is treated every other year.

Can I treat an ash myself or do I have to call an arborist? If your ash is smaller than 47 inches around the trunk at chest height [i.e., 15″ diameter at breast height (DBH)], you may be able to treat your ash tree yourself. University of Wisconsin Pest Alert XHT1181 (“Homeowner Guide to Emerald Ash Borer Insecticide Treatments”) provides a list of products currently available for homeowner use. If you decide to treat your own trees, be sure to read and follow all label instructions of the insecticide that you select to ensure that you use the product in the safest and most effective manner possible.

In some situations, hiring a certified arborist to treat your ash tree may be more desirable. Professionals have access to specialized application equipment and additional insecticides not available to homeowners. They are also trained to measure trees accurately, and assess the overall health of trees. The Wisconsin Arborists Association website ( has a list of certified arborists in the state.

Note that the University of Wisconsin does not endorse any insecticide products, and does not recommend any professional products over those available directly to homeowners.

Am I allowed to treat an ash tree in my yard between the sidewalk and street? The answer to this question varies from municipality to municipality. In many cases, municipalities have treatment or removal and replacement plans already in place. Contact your local town, village or city to determine an appropriate strategy for protecting your sidewalk trees.

How much does it cost to treat an ash tree for EAB? A single tree that is 32 inches around at chest height (approximately 10″ DBH) can be treated with a granular or liquid soil drench homeowner product for about $20-35/year. Larger trees will require a larger amount of product and costs will be higher. Arborist treatment costs vary depending on tree size and location, the insecticide selected, and the application method. Other arborist-specific site visit charges may apply as well. Consult at least two arborists in your area to discuss treatment options and costs. To make an accurate comparison among service providers, make sure you know what insecticide will be used, the method of application, and how often treatments will be made. An arborist will not be able to determine the exact cost of treatment for your specific ash tree without a site visit, but an arborist should be able to provide you with a cost estimate for a typical ash tree.

Do I have to remove my ash tree if it is infested with EAB? Applying protective insecticide treatments to a healthy ash tree to prevent an EAB infestation is the best strategy for managing EAB. However, if a tree becomes infested and the infestation is detected early, you may be able to treat your ash tree to prevent further damage, and help the tree recover. Research suggests that insecticide treatments are significantly more effective on EAB-infested ash trees with less than 50% canopy thinning. Insecticide treatments are not recommended for trees with greater than 50% canopy thinning these trees should be removed. Trees that become infested with EAB and are not treated will ultimately die and will need to be removed.

Lilac Borer/Ash Borer

Authors as Published

Eric R. Day, Manager, Insect Identification Lab, Virginia Tech

This publication is available in a PDF file format only.

A factsheet on the lilac borer/ash borer, an ornamental pest.

Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

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