What options do I have for treating my ash trees?
Several insecticide options are available to effectively treat landscape ash trees threatened by EAB. Products listed in Table 1 have been evaluated by university and government scientists in field trials. Keep in mind, however, that controlling insects that feed under the bark with insecticides has always been challenging. This is especially true with EAB because most of our native North American ash trees have little natural resistance to this pest. Effective control of EAB requires some care when selecting an insecticide product and application method to ensure the product is applied at the proper rate and time.
I know my tree is already infested with EAB. Will insecticides still be effective?
It is best to begin using insecticides while ash trees are still relatively healthy. By the time most people notice canopy thinning or dieback, EAB has already caused considerably injury to the vascular system of the tree. An effective insecticide may stop additional damage, but it cannot reverse damage that has already occurred and it takes time for trees to recover. Most insecticides used for EAB control act systemically the insecticide must be transported within the tree. In other words, a tree must be healthy enough to carry a systemic insecticide up the trunk and into the branches and canopy. Trees are damaged by EAB larvae feeding in galleries under the bark. These galleries injure the phloem and xylem tissue that plants use to transport nutrients and water. A few galleries have only a small effect on most trees. As the EAB population grows and more larvae feed on a tree, however, the galleries interfere with the ability of the tree to transport nutrients and water, as well as insecticides. As a tree becomes more and more infested, the injury becomes more severe. Canopies become thin because fewer leaves can be supported by the tree. Large branches or even the trunk can be girdled and killed by the larval galleries.
Multi-year studies have shown that if more than 50% of the canopy has been killed by EAB or if the canopy appears to be thin and carrying less than half as much foliage as it should, it is probably too late to save the tree. The ability of trees to recover from low to moderate EAB injury can vary, depending on the extent of the damage and which control options are used. Studies have also shown that if the canopy of a tree is already declining when insecticide treatments are initiated, the condition of the tree may continue to deteriorate during the first year of treatment. If treatment is effective, the tree canopy will usually begin to improve in the second year of treatment. This lag in the reversal of canopy decline probably reflects the time needed for the tree to repair its vascular system after the EAB infestation has been reduced.
My ash tree looks fine but EAB has been detected in the vicinity of my property. Should I start treating my tree?
Detecting new EAB infestations and identifying ash trees that have only a few larvae is very difficult. Ash trees with low densities of EAB larvae often have few or even no external symptoms of infestation. In addition, scientists have learned that most female EAB lay their eggs on nearby trees, i.e. within 100 yards of the tree from which they emerged. A few female beetles, however, appear to disperse much further, anywhere from 0.5 miles to 2-3 miles. Therefore, if your property is within 10-15 miles of a known EAB infestation, your ash trees are probably at risk. If your ash trees are more than 10-15 miles beyond an infestation, it is probably too early to begin insecticide treatments. Treatment programs that begin too early waste money and result in unnecessary use of insecticide. Conversely, treatment programs that begin too late will not be as effective. Remember, however, that new EAB infestations have been discovered every year since 2002 and existing EAB populations will build and spread over time. Quarantine maps found on the http://www.emeraldashborer.info website can help you stay up-to-date regarding locations of known infestations. You can use the links in this website to access specific information for individual states. When an EAB infestation is detected in a state or county for the first time, it will be added to these quarantine maps. Note, however, that once EAB has been found in a county, surveys by regulatory officials end. Similarly, once an entire state is declared to be infested, regulatory surveys may cease. Therefore, quarantine maps may or may not adequately reflect the current distribution of EAB in such areas. Personnel from city, county or state agencies sometimes continue to survey or monitor local EAB infestations. City foresters, county extension offices or state departments of agriculture may have information on local EAB distribution. There is no substitute for local knowledge and tree care professionals should actively monitor changes in the condition of local ash trees.
When is the best time to treat my trees?
As with any pest management effort, optimal timing is required to achieve best control. Two life stages of EAB are targeted by treatments: adult beetles and young larvae. Therefore, systemic insecticide applications should be made in time to allow for uptake and distribution of the insecticide within the tree to ensure adult beetles and very young larvae encounter the toxin. Non-systemic cover sprays, which are less commonly used, should be applied to foliage to target adult beetles, as well as the trunk and branches to help control newly hatched larvae. Thorough coverage is critical for achieving successful control. Adult EAB feed on ash foliage throughout their life span and females must feed on leaves for at least 14 days before they begin laying eggs. This provides a window of opportunity to control the adults before any new eggs or larvae are produced. The onset of adult beetle emergence begins from early May (southern Ohio) to early June (central Michigan) and peaks two to three weeks later. Beetle emergence may begin sooner at locales farther south or later in more northern areas. Regardless of location, emergence of adult EAB consistently begins at 450-550 growing degree days, based on a threshold of 50 ºF and a starting date of January 1. Beetles are most abundant at about 1,000 growing degree days. Cumulative growing degree days are tracked and posted on websites of many land grant universities as well as the NOAA website. First emergence of EAB also closely coincides with the period when black locust trees bloom. This phenological indicator is a reliable predictor of EAB emergence across a wide region, ranging from southern Michigan to Kentucky and Maryland.
Peak egg hatch and larval establishment occur between early June and mid-August, depending on location and weather. As a general rule, young larvae are more susceptible to insecticides than are older larvae.
Moreover, controlling young larvae prevents damage to the tree caused by older larvae that feed in larger galleries and thus injure more area on the tree. The efficacy of insecticide treatments will likely decline if they are applied later in the growing season when larger, more mature larvae are present. Consistent with this, MSU scientists found that imidacloprid trunk injections made in mid-May were 70% more effective against EAB than those made in mid-July.
For imidacloprid soil treatments, which require four to six weeks for uptake and distribution of the insecticide within the tree, applications should be made in mid-March to late April, depending on your region. Treatments should be applied on the earlier side of these schedules in more southerly locations and later side in more northerly regions. Soil applications of dinotefuran can be applied 2-3 weeks later than imidacloprid because it is more soluble and is taken up and transported through the tree more rapidly. Basal trunk sprays of dinotefuran move into trees even faster and can be made between late May and mid-June. Optimal timing for trunk injected products is just after trees have leafed out, typically from mid-May through early or mid-June. When treating larger trees, treat on the earlier side of the recommended timing, because large trees may require more time for uptake and transportation of the insecticide than small trees. Imidacloprid soil applications can also be made in fall, from mid-October to mid-November. However, this timing is less efficient and studies have shown that higher rates must be applied in the fall than in spring to achieve similar levels of control.Sometimes, a tree is not known to be infested until in late June or early July. Although late treatments are not optimal, there may still be some benefit to treating the tree if the treatment can be made promptly. Consider using a treatment approach that maximizes rate of uptake and within-tree distribution. Uptake of dinotefuran is faster than imidacloprid because it is more soluble. Basal trunk sprays with dinotefuran will be taken up faster than soil applications (see discussion below). Trunk injections will be taken up faster than soil applications, assuming the injections can be made under favorable conditions (e.g. adequate soil moisture, moderate humidity and air temperature). Even in a best case scenario, it will still likely take one to two weeks for the systemic insecticide to move throughout the tree.
I realize that I will have to protect my ash trees from EAB for several years. Is it worth it?
The economics of treating ash trees with insecticides for EAB protection are complicated and depend on several factors. Tree size, health, location and value should be considered, along with the cost of the insecticide and expense of application, the likelihood of success, and potential costs of removing the trees. Scientists, however, have compared costs of removing urban ash trees versus treating the same trees with emamectin benzoate, which provides two years of EAB control. Results consistently show treatment costs are much lower than removal costs. As treatment options continue to evolve, costs of treatment will likely change. It will be important to stay up to date on these options and management recommendations.
Benefits of treating trees can be more difficult to quantify than costs. Healthy landscape trees typically increase property values, provide shade and cooling, and contribute to the quality of life in a neighborhood. Landscape trees, especially mature trees, capture storm water, reducing potential pollution of streams and rivers. The economic benefits provided by trees increase with the size of the tree, as does the cost of removal. Hence, it may be particularly economical to treat larger trees. Many people are sentimental about their trees. These intangible qualities are important and should be part of any decision to invest in an EAB management program. It is also worth noting that the size of EAB populations in a specific area will change over time. Populations initially build very slowly, but later increase rapidly as more trees become infested. As EAB populations reach peak densities, a high proportion of the untreated ash trees in a given area will decline and die, usually over a 3-5 year period. Once untreated ash trees in the area succumb, however, the local EAB population will decrease substantially. Ongoing studies in southeast Michigan and northwest Ohio, for example, indicate EAB populations still persist but at much lower densities simply because few mature ash trees remain in this area. Young ash saplings in forests or woodlots will likely be colonized by EAB eventually, so landscape ash may continue to face some risk of EAB infestation. It seems likely, however, that surviving ash trees can be managed with less frequent treatments once the EAB invasion has passed. Studies on the dynamics of EAB populations and whether the intensity of insecticide treatments can decrease after the local EAB population has collapsed are underway in Michigan and Ohio.
What are the biological control measures for the Emerald Ash Borer?
Biological control (or biocontrol) is the practice of importing and releasing natural enemies from a pest’s native range to control the target pest populations in the area of introduction. Biocontrol has been used for over 100 years in the U.S. and has successfully controlled invasive plant and insect pests such as gypsy moth, winter moth, ash whitefly, eucalyptus longhorned borer, purple loosestrife, and Klamath weed. Because EAB is from northeast Asia, U.S., Chinese, and Russian scientists have been searching for EAB and its natural enemies in that region since 2003. In Asia, EAB population densities are relatively low due to the combined effects of EAB-resistance in Asian ash species, scarcity and patchiness of forests, and the EAB natural enemy complex. Exploration for EAB natural enemies in China, Russia, and Korea has yielded several hymenopteran parasitoids, and four species have been approved for release as biological control agents of EAB in the U.S and others are under consideration.
In January 2009, a Biological Control Production Facility became operational in Brighton, MI. As of February 2016, this facility has reared and released over three million EAB parasitoids in twenty-two states and two Canadian Providences.
Wooded areas at least 40 acres in size are preferred as parasitoid-release sites. Smaller release sites will require higher ash densities and ash corridors connecting the release sites to other wooded areas. Examples of ash corridors are rivers, ditches, highways, and fence rows. Use of these criteria will facilitate parasitoid reproduction, establishment, and dispersal to nearby areas. If possible, at least 25% of the trees should be ash, but a higher percentage of ash would be even better.
Low to moderate EAB-population densities are recommended for potential parasitoid-release sites. Unless there are many young trees in the vicinity, stands with dead and dying ash trees are not appropriate as release sites because ash and EAB may decline before the parasitoids become well established. The most accurate method of estimating EAB density requires felling and peeling the bark from ash trees to count EAB present under the bark and along the trunk. This direct estimate of EAB density, however, is difficult, labor intensive, destructive, and counter-productive in areas where EAB density is low. Therefore, we recommend using an indirect EAB-density estimate based on the signs and symptoms of EAB infestation in ash trees.
Overall, ash trees at a potential release site should be fairly healthy, with an average crown condition of 1 to 2 (healthy or mostly healthy) and only a few trees in condition classes of 4 to 5 (dying or dead). The presence of EAB must be confirmed at each potential release site.
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