Emerald Ash Borer Background

Emerald Ash Borer is an invasive pest of North America. It is believed to have first been introduced from Asia in the 1990s and it has spread to at least 25 states in the UNited States and has also been observed in parts of Canada. It is an extremely destructive plant of ash trees and is considered a major threat to ash trees in North America. 

Emerald Ash borer is a wood boring and phloem feeding pests, but unlike most wood boring beetles, Emerald Ash Borer can infest healthy trees in addition to stressed trees. It is also able to kill healthy trees within just a few years. It is also particularly dangerous in North America, because there are no known natural predators. It is considered a major threat to ash trees, comparable to historical forest epidemics like dutch elm disease and chestnut blight in the 20th century. 

Current management strategies include traps, and public awareness campaigns about the risks of transporting fire woo, which is the primary mode of transportation for Emerald Ash Borer. APHIS also has a program underway to develop biological control options for this pest.

Emerald Ash Borer infestations can cause serious economic damage to lumber and nursery industries if untreated. Severe infestations can permanently alter ecosystems in some areas of the United States. Ash trees are also a very popular landscape tree. It has been estimated Kovacs et. al. (2009)  that Emerald Ash Borer has the potential to cause $10.7 Billion to state and local governments and homeowners to replace dead ash trees. Emerald Ash Borer damage could also cause additional losses through cooling costs and negative property values.

Emerald Ash Borer Host Range

In North America, Emerald Ash Borer can infest all types of ash trees in the Fraxinus genus including green ash, white ash, black ash, pumpkin ash, blue ash, and other native species. While in some instances, it has been observed on other types of trees, it is only considered a major pest of ash trees.

Emerald Ash Borer Life Cycle

Emerald Ash Borer can have either a one or two year lifecycle. Lower levels of infestations on healthy, vigorous ash trees tend to support a two year lifecycle, but high levels of infestation on stressed trees can have a one year lifecycle.(Cappaert et al. 2005).

Eggs

Emerald Ash Borer eggs can be very difficult to detect. They are usually found on the southwestern side of trees in areas where the bark is rough, cracked or near branch crotches. Eggs can be laid 1-23 at a time, but 1 egg alone is common. However, females can lay between 60-90 eggs in their lifetime, depositing most of them individually on the bark.

Larvae

Larvae emerge from the eggs and bore into the tree, where they feed on phloem for most of the season. As they feed the larvae create long, serpentine trails which are filled with frass. These trails enlarge as the tree grows. Larvae develop through 4 instars before entering the outer bark and overwintering as prepupae.

Pupae

Pupal development begins in spring, and typically takes about 4 weeks, depending on temperature. After the pupae transform into adults, they take 1-2 weeks to emerge through exit holes in the bark.

Adults

Adults are capable of flight and adults begin to mate soon after emergence. Females may take 3 weeks of maturation and feeding before mating. Adults feed on foliage and cause minimal damage to trees. They are most active on warm, clear days.

The Emerald Ash Borer Emergence Growing Degree Day model can be used to predict adult emergence. The model uses a lower threshold of 50F and an upper threshold of 100F. It can be calculated with daily high and low temperatures, using the sine model to estimate growing degree day units, or the Pest Prophet app can be used to calculate GDD accumulation from hourly temperature data for a specific field.

Using Jan. 1st as a default “biofix” date, or starting point for accumulation, the first adult emergence occurs at approximately 550GDD (F) and peak at approximately 856 GDD (F).

Emerald Ash Borer Identification

Emerald Ash Borer can resemble other similar species so it is important to have a positive identification before considering management strategies. Eggs are approximately 1 mm long X 0.6 mm wide and oblate shape. They are creamy white but turn amber before hatching.

Emerald Ash Borer larvae are creamy white and flattened. They are 26-32 mm long in the fourth instar, with small brown heads. Prepupae appear similar to larvae.

Adults are metallic green in color with dark emerald green elytra. The head is flat with a vertex shield-like structure.

Emerald Ash Borer Management

Cultural

Diversifying landscape plantings can help reduce the impacts of Emerald Ash Borer infestation. Landscapes should be composed of no more than 30% of any one family, 20% of any one genus and 10% of any one species. Native species are also preferable to non native species, because they will require less maintenance, and are less likely to be stressed due to water or fertilization limitations. 

Biological Control

There has been some biological control research done for Emerald Ash Borer, mostly in China, from where the Emerald Ash Borer is native. There are three known non-native biological control insects: Spathius agrii is a larval ectoparasitoid, Oobius agrii is an egg parasitoid, and Tetrastichus planipennisi is a larval endoparasitoid. These three insects all attack Emerald Ash Borer in China, and are small wasps. They have been released in the United States and are currently being monitored for establishment. Further research is being done on how to maximize the effectiveness of these parasitoids and efforts are being made scale up the rearing and releasing methods.

Chemical Control

Chemical control of Emerald Ash Borers is not recommended for large scale control due to cost and effectiveness. However, for some individual trees or small landscape populations there are a number of chemical spray options that can be effective. 

Most of the products available to homeowners contain imidacloprid as the active ingredient, which is a systemic insecticide that can be applied as a soil drench around the base of the tree. These applications should be made in spring to be most effective. These should be repeated each year. Always check the label and consult with a professional before making insecticide treatments, as there can be environmental and regulatory concerns.

Trees that are already severely infested, with over 50% canopy loss, should not be treated, as it is too late for effective chemical control.