Posted on August 1, 2019
The emerald ash borer (EAB) is a non-native, invasive insect that was first discovered in North America in 2002 in Michigan. It is native to eastern Russia, northern China, Japan, and Korea. EAB utilize ash (Fraxinus spp.) as their primary hosts. However, emerald ash borer was found attacking and developing in white fringe tree (Chionanthus virginicus) in Ohio and has most recently been confirmed as able to feed and develop successfully on cultivated olive (Olea europaea). In fact, Cipollini et al. 2017 note that the success of EAB development in cultivated olive is higher than in more resistant native Fraxinus spp. hosts, such as Manchurian ash (F. mandshurica); however, EAB does not develop as successfully in olive as it does in favored North American ash species. In Massachusetts, the primary host trees are white ash (Fraxinus americana), green ash (Fraxinus pennsylvanica), and black (or brown) ash (Fraxinus nigra). As a component of Massachusetts forests, the highest percentages of ash are located in Berkshire County, but these trees are also found in forested areas throughout the state. Ash is also a popularly planted tree in the urban environment.
Since its initial detection in Michigan, it has spread to at least 30 US states and 2 Canadian provinces, including bordering communities in New York, Connecticut, Vermont, and New Hampshire. It was first detected in Massachusetts in 2012 by the Massachusetts Department of Conservation and Recreation (DCR) on a purple panel prism trap, when a single adult beetle was found in Dalton, a town in Berkshire County. These traps are 3-sided, purple, and hung within the canopy of ash trees as a method of detecting EAB. They are baited with plant volatiles, chemicals which mimic those released by stressed ash trees. Emerald ash borer is attracted to those chemicals. Since then, detections have been made by the DCR and others in Essex County (North Andover; phone call from a concerned citizen) in 2013, Suffolk County (Boston; trap detection by the Arnold Arboretum) in 2014, Worcester County (Worcester; visual survey by Asian Longhorned Beetle ground surveyors) in 2015, in Hampden County (Wilbraham; phone call from a private arborist) in 2016, and in Norfolk County in 2017 (Brookline; 6 adult beetles were caught in 2 green panel traps deployed by the community and baited with host plant volatile chemical and pheromone – sexual attractant chemical – lures; and Dedham where exit holes and galleries were detected in trees). Additional county detections in Massachusetts include in Hampshire County (2017) in the cities and town of Easthampton, Northampton, and South Hadley and in Bristol County (2018) in Raynham, MA. The Bristol County detection was made by the MA Department of Conservation and Recreation using green funnel traps and lures. Detections in further towns have been made using visual survey, calls from private citizens, and “trap trees”. A trap tree is created by girdling the bark of an ash tree, causing it to release volatile chemical compounds that are attractive to adult emerald ash borer (like those used in the trap lures). Female EAB will lay their eggs on these trap trees, which are cut in the winter and then have their bark peeled and removed to look for developing larvae below.
The DCR, MA Department of Agricultural Resources (MDAR), and United States Department of Agriculture (USDA) also monitor aggregations of Cerceris fumipennis, a native, ground-nesting and solitary wasp that nests in hard-packed, sandy soils exposed to direct sunlight, such as those found in baseball diamonds and dirt roads. This wasp can be used to monitor for the presence of emerald ash borer in a practice known as biosurveillance. This particular species of wasp is incapable of stinging people. These wasps hunt native Buprestidae, or the jewel beetles/flat headed borers, and bring them back to their subterranean nests where they will lay their eggs on the paralyzed beetles. In areas where EAB exists, they have also been observed bringing the invasive beetle back to their nests. DCR has detected emerald ash borer in a Cerceris fumipennis aggregation in Pittsfield, MA in 2013 (1 adult beetle) and 2015 (1 adult beetle) using this method. MDAR has detected EAB in a new county (Middlesex) in the town of Newton using this method in 2016 along with a find in Boxford, MA in Essex County, adding to the list of towns known to have EAB.
Like all beetles, the emerald ash borer undergoes complete metamorphosis (is holometabolous) with four life stages: egg, larva, pupa, and adult. Adults are a dark green metallic color and approximately ½ inch long. They are the only Agrilus species in North America known to have a bright metallic coppery-red (may also appear purple) dorsal surface of their abdomen. (The wings must be lifted to see that feature.) Larvae are white, 1 to 1.25 inches long at maturity, have a small brown head, a pair of brown pincers at the end of the abdomen, and have bell-shaped abdominal segments. The fourth instar larvae overwinter in a pre-pupal stage in a J-shaped position. Pupae are present in the spring and look like cream-colored adults that begin to darken as they develop. The larval and pupal stages are found beneath the bark of their host trees, as the larvae feed on the nutrient and water-conducting tissues of the plant. Adults emerge in May and June and mate and lay tiny, flat, oval-shaped eggs that are initially whitish-yellow in color and turn reddish-brown as they develop. Eggs are difficult to see as they are approximately 1/32 of an inch and laid in cracks and crevices of the bark. On average, females can lay 55 eggs in their lifetime, but some have been observed laying more than 150 eggs. Adult emergence creates D-shaped exit holes in the bark.
Although adult emerald ash borer will conduct some maturation feeding (on ash foliage), it is the larval stage of this insect that causes the most amount of damage to ash trees. The larvae feed in the nutrient- and water-conducting tissues beneath the bark, which can lead to the eventual girdling of the tree. This damage can be seen as S-shaped galleries when bark is removed. Vertical splits or cracks can also form in the bark over these larval galleries. Heavily infested trees exhibit canopy dieback, beginning at the top of the tree. Some ash trees will push water-sprouts or epicormic shoots from their base or branches. D-shaped exit holes are created by the emerging adults. “Blonding,” or evidence of woodpecker feeding activity on emerald ash borer larvae, may be visible on the bark. This occurs as the woodpeckers forage for the insects, leading to bark removal. This can be a very visible sign that there may be emerald ash borer present. Emerald ash borer will attack healthy ash trees, although adults may prefer to feed on or lay eggs on stressed trees. When emerald ash borer populations are high, small trees can die within 1-2 years of initial infestation, while larger trees may take 3-4 years before succumbing to this pest.