Emerald Ash Borer: Biology and Life Cycle

The Emerald Ash Borer, Agrilus planipennis Fairmaire, commonly referred to as 'EAB', is an invasive wood-boring beetle native to Asia. The beetle’s first North American populations were confirmed in the summer of 2002 in southeast Michigan and in Windsor, Ontario. EAB was likely introduced to the area in the mid-1990’s in ash wood used for shipping pallets and packing materials in cargo ships or shipping containers.

The Emerald Ash Borer is a member of a family of insects called metallic wood-boring beetles. Emerald ash borers feed on and eventually kill all native ash trees. Slowing their spread is imperative.

  Description

Adult

Adult Emerald Ash Borers are elongate, cylindrical, slender beetles that can grow to be approximately ¼ to ½ inches long and 0.1 inches wide. They can be recognized by a metallic green sheen with a coppery-red abdomen that is hidden under the wings. Males are densely covered with setae, a stiff structure resembling a hair or a bristle, on their thorax.

Egg

Adult females lay eggs that are 1 mm in diameter on the bark of ash trees. Eggs are white when laid, and turn amber as they develop.

Larvae

Larvae are cream-colored, with bell-shaped body segments. Their size varies as they feed and grow under the ash tree’s bark, but reach a length of 1½ inches long.

Life Cycle

Adult females lay eggs that are 1 mm in diameter on the bark of ash trees. The eggs hatch within 15 days into their flattened, segmented larval state. Larvae chew their way through the bark and tunnel into the tree's inner cambial layer. By tunneling through the phloem, they disrupt the tree's transportation of nutrients. The larval stage is the longest within the life cycle, lasting about 300 days. This generally occurs between June to April of the next year.

Emerald ash borers have four larval stages. In late summer and early fall, larvae in their final stage enter the layers of bark and prepare a chamber where they remain during the winter months as pre-pupae. Larvae can reach 23 to 26 mm in length. The larvae have a pincer-like appendage called a urogomphi, paired "horns" at the posterior tip of the abdomen of larvae, attached to the last abdominal segment.

The overwintering physiology of the emerald ash borer prepupae allows them to accumulate glycerol in high concentrations along with other antifreeze agents. This behavior contributes to the emerald ash borer's ability to tolerate cold temperatures. The larvae complete development within their pupation chambers in the summer.

  Diet

Emerald Ash Borers are folivores, an insect that feeds on leaves, in the adult phase, and lignivores, an insect that feeds on wood, in the larval phase. As adults, Emerald Ash Borers feed upon the leaves of ash trees, leaving behind noticeable ridges along the leaf edge. Upon hatching on the bark, larvae chew their way inward and feed upon the phloem and cambial region of the tree. This action prevents nutrients from photosynthesis from being transported and eventually leads to the tree's death.

After maturing into adults within their chambers, fully developed adults chew their way out from their chamber within the bark. This process generally occurs on days with high temperatures and begins in mid-May, with adult activity peaking in June and July.

  Management, Treatment And Prevention

Integrated pest management involves pest monitoring and the use of multiple control strategies (cultural control, plant resistance, biological control and pesticides) to mitigate losses from insect pests.

Monitoring and Surveying

Several methods are being used in survey efforts for EAB. Large purple prism traps are used in government-sponsored survey programs for EAB and are very visible to the public in parks and recreation areas.

Prism traps are pre-coated with an insect-trapping adhesive. Lures are attached to the trap and are effective in the field for 20 days as an attractant. The chemical component of the lure has gotten better in recent years but still has room for improvement. Prism traps are hung over sturdy branches in the mid to lower canopy of ash trees of at least 8 inches in diameter before EAB emergence is expected.

In addition to the prism traps, girdled “trap trees” also have been used for locating EAB, but this method is time-consuming and, therefore, expensive. Other traps that have been developed included green funnel and prism traps, but these are not widely used in the U.S.

Cultural Control

One of the main cultural methods for preventing the spread of EAB is NOT MOVING INFESTED FIREWOOD, LOGS OR NURSERY STOCK to uninfested areas. Much of the rapid spread of EAB outside of its original detection sites near Detroit, Michigan, was due to direct, human-assisted movement of these products. Larvae of EAB are hidden underneath the bark of living trees or boards cut from infested logs where they can be transported easily into non-EAB-infested areas.

Another cultural control method is TIMELY REMOVAL OF EAB-INFESTED TREES and then chipping the trees to a small size - less than 1 inch - on each of two sides or burning the trees that were removed. This will kill EAB and help prevent further spread. Many cities are proactively removing and replacing diseased or unhealthy ash trees. This has been done to increase diversity of tree species with new plantings to prepare for the arrival of EAB.

Plant Resistance

In North America, all native species of Fraxinus are susceptible to EAB, although some species are preferred more than others. For example, blue ash (F. quadrangulata) is a less-preferred species. However, researchers have observed that ash trees native to Asia have reduced larval tunneling and only stressed trees (for example, from drought) are colonized.

Ash trees in the native range of EAB may be more resistant because their natural defenses have co-evolved throughout time. Researchers are studying Asian ash species as a possible source of resistance genes against EAB. Identification of resistant ash genotypes is important for reforestation and maintaining a market demand for ash in the nursery industry.

Biological Control

Biological control involves the use of natural enemies (predators or parasitoids) to control insect pests naturally. Biological control primarily is being targeted at EAB in forests. The EAB has no known predators other than woodpeckers that occasionally feed on larvae and kill about 30-50% of large EAB larvae.

Insecticides

Research has demonstrated that insecticides can protect individual ash trees from EAB effectively. Insecticides are recommended only if the EAB infestation is within 15 miles, or the ash trees are in an EAB-infested (or quarantined) area.

Soil Applied Systemic Insecticides

Systemic insecticides are applied to the soil as a drench or through an injection technique, absorbed by the roots and then translocated throughout the tree.

The best timing for soil injection and drenches is likely early to mid-May. A fall application also can be made as an alternative timing, but generally is not as effective.

Insecticide uptake and translocation may take up to 4-6 weeks in trees with trunks smaller than 12 inches in diameter. Larger trees with trunks greater than 12 inches in diameter require more time for uptake, so treatment should be initiated earlier.

Broadcast Foliar Sprays Applied to Trunk, Main Branches and Foliage

Insecticides can be sprayed on the trunks, branches and foliage to kill EAB adults as they feed on foliage and newly hatched EAB larvae before they bore into the tree. This technique does not kill larvae already feeding internally in the tree. They are available to homeowners and professional applicators, depending on the label restrictions.

For good control, non-systemic insecticide sprays need to have complete coverage and be properly timed for adult EAB emergence. Treatments would need to be repeated at least 2 times per year and as many as 4 times per year.

Further Information on Garden Pests:

 Groundhog Facts and Control
 How To Get Rid Of Ants
 All About Aphids and Their Control
 Voles — Both the Good and the Bad