The emerald ash borer (Agrilus planipennis) is a green beetle native to Asia and Eastern Russia. Outside its native region, the emerald ash borer (also referred to as EAB) is an invasive species, and emerald ash borer infestation is highly destructive to ash trees in its introduced range. The Emerald Ash Borer was first discovered in America in June 2002 in Michigan. It is believed to have been brought to America unintentionally in ash wood which was used to stabilize crates during shipping.
Adults – The adult beetle is dark metallic green, bullet-shaped and about 8.5 millimetres (0.33 in) long and 1.6 mm (1⁄16 in) wide. The body is narrow and elongated, and the head is flat with black eyes. Adults begin to emerge from the trunks of ash trees after the accumulation of 400-500 growing degree days base 50 °F (GDD). Peak adult emergence occurs at ~1000 GDD. After emergence, adults fly into the ash canopy where they feed on leaves throughout their lives. EAB adults start mating one week after emergence, and females begin laying eggs 2–3 weeks later. A typical female will lay approximately 100 eggs during her 2-month life; 56% of these eggs on average will be female. In the field, EAB adults are readily observed mating and egg-laying on the trunks of ash trees on warm, sunny afternoons and evenings. The adults of both sexes are strong fliers.
Eggs – A female EAB may lay >100 eggs in her lifetime, depositing them individually or in groups on the bark along the trunk and portions of the major branches. Eggs are laid in areas where the bark is rough, and between bark layers or in bark crevices. Eggs are approximately 1.0 mm long x 0.6 mm wide and creamy white when laid; fertile eggs gradually turn amber after a few days. The eggs hatch after about two weeks.
Larvae – Newly hatched larvae bore through the bark to the phloem and outer layer of new sapwood where they feed until the weather gets too cold in the fall. There are four stages of larval development (instars). As they feed, the larvae create long serpentine galleries filled with frass, which enlarge in width as they grow. Larvae are creamy white, and dorso-ventrally flattened. When fully mature, fourth-instar larvae are 26 to 32 mm long. Their head is mostly retracted into the prothorax with only the dark brown mouthparts visible. The prothorax is enlarged, with the mesothorax and metathorax more narrow. Larvae have 10 bell-shaped abdominal segments and a pair of small brown structures called urogomphi, which are characteristic of all larvae in the genus Agrilus.
Overwintering larvae, pre-pupae, pupae, and adults In the fall, mature fourth-instar EAB larvae excavate pupal chambers in the sapwood or outer bark where they fold into overwintering “J-shaped larvae”. In the spring, the J-shaped larvae shorten into prepupae then shed their cuticle to become naked pupae. Pupae are initially creamy white, but the eyes turn red and the body begins to darken as they develop. To emerge from ash trees, adults chew D-shaped exit holes through the bark and are capable of immediate flight upon emergence. EAB larvae that are immature as cold weather arrives in the fall will simply overwinter in their larval gallery. Larger larvae complete development the following spring, whereas smaller larvae may require another summer of feeding to complete development.
Effect on trees
The most significant damage to a tree by the emerald ash borer takes place when the insect is in its larval stage. The larvae feed on the conductive tissue of the tree. This tissue is what transfers the nutrients and water from the roots to the leaves, and when this is disturbed, the tree begins to die. At the onset of winter, the larvae relocate to the bark of the tree, effectively cutting off the tissue more. This ultimately results in the death of tree. This can take place over a number of years, and the first noticeable sign is usually some die back in the crown of the tree. The tree will usually be dead by the following year or soon after. In areas where the insect is invasive and has no natural predators, it can and usually does have a devastating effect on the local ash tree population.
Prevention of EAB is possible by the use of a systemic insecticide into the base of the tree. This treatment can prevent damage to the tree for up to two years. Note that application must be reapplied every two years. Soil injections are another option for the prevention of EAB. These insecticides are injected directly into the soil surrounding the base of the tree, and are then transported through the rest of the tree via the roots. In order for these treatments to have the greatest effect soil must be moist when applied. Water logged or dry soils will result in less of the insecticide to be absorbed into the tree. There are two insecticide spray treatments that can be used as well. The first is a spray which is applied to the trunk and absorbed through the bark. This treatment is less invasive to the tree and soil, however if the tree has thick bark absorption is slow and limited. The second spray treatment is a protective cover spray, which is applied to the branches and trunk of the tree. This treatment kills adult beetle and newly hatched larva; however it will not kill eggs.
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