Biology and Life Cycle of Emerald Ash Borer
The emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is a wood-boring beetle indigenous to countries in northeastern Asia. In 2002, this invasive buprestid was identified as the killer of ash tree (Fraxinus spp.) in southeastern Michigan and nearby Windsor, Ontario. Scientists now estimate that EAB was introduced during the early 1990’s from infested solid-wood packing materials such as pallets and crated used in international trade. The spread of EABresults from transport of infested ash nursery stock, logs, and firewood, as well as natural dispersal. As of 2009, Lower Michigan, Ohio, Indiana, and areas of eastern Ontario are considered generally infested with EAB. More isolated infestations are also known in Upper Michigan, Illinois, Pennsylvania, Maryland, Missouri, Virginia, West Virginia, Wisconsin, and the Montérégie region of Quebec. Researchers and land managers predict that the presence of EAB in North America places at risk 16 species of native ash trees. Where EAB is native, however, it is considered only a minor and periodic pest of endemic ash species.For this reason, virtually no literature on EAB biology, ecology, natural enemies, and management options was available when it was discovered in North America. Scant Chinese literature was found and translated (USDA FS NCRS 2005); these include brief descriptions of A. planipennis distribution and biology (Chinese Academy of Science 1986, Yu 1992, Hou 1993, Xu 2003, Gao et al. 2004), and a report on A. planipennis management after species of ash from North America were planted in China (Liu 1966).
After the discovery of A. planipennis in North America, we initiated research on the biology, prevalence, host interactions, and natural enemies of this buprestid in China and Michigan. This has also involved developing methods to rear A. planipennis in the laboratory and field.
Knowledge of A. planipennis biology has provided us with more understanding of usefulness of various management tools for this invasive pest in North America. It has also allowed us with to develop management tools, which are currently being test in the field.
Life Cycle. In Michigan, emerald ash borer (EAB) completes one generation every one or two years. Eggs are laid from mid June and well into August. Female EAB deposit their eggs individually on ash trees, between layers of outer bark and in cracks and crevices of the trunk and major branches. EAB eggs hatch in about two weeks, depending on temperature.
The new larvae tunnel through the bark to the cambial region and feed on phloem. Phloem is a thin layer of tissue beneath the outer bark and conducts sugars and other nutrients throughout the tree.
Larvae etch galleries in the outer sapwood during feeding. These galleries are typically S-shaped (serpentine) and packed with larval frass or waste. At high larval densities, individual galleries overlap due to overcrowding and are less defined.
As EAB populations increase over the years after initial infestation, larval development becomes more synchronized, and the majority of larvae mature during September. These fully-grown larvae spend the winter folded inside a small pupation cell constructed in the outer sapwood or outer bark. Younger larvae spend the winter in feeding galleries in the phloem and outer sapwood and may feed another summer before reaching the adult stage.
During April and May of the next year, the overwintering mature larvae will pupate inside their pupation cells and gradually transform into adults.
After one to two weeks, these new adults chew D-shaped exit holes in the bark and emerge from the ash trees. Adult emergence starts in late May and peaks in June, however, some adults may continue to emerge throughout the summer. EAB adults are capable of flight upon emergence and spend most of the day feeding on ash leaves up in the ash canopy.
These are active, sun-loving beetles, and after emergence they start mating after about one week and laying eggs in about three weeks. The best time to observe these bright green metallic beetles in the field is during mating and egg-laying, which occurs during the afternoon (3PM to 7PM) on sunny, warm days from mid June through mid July when they are found crawling on and hovering around the trunks of ash trees. Females live about two months and males about one month. On average, females lay about 55 eggs during their lifetime, but some individuals may lay more than 150 eggs.
Dispersal. We studied the dispersal potential of EAB using flight mills, which allowed us to measure the distance EAB adults flew. We found that mated females flew further than unmated females and males. The average distance flown by mated females was about 3 km, however, 20% flew >10 km and 1% flew >20 km. These findings demonstrate one of the reasons that eradication of EAB in North America has been unsuccessful.
Signs and Symptoms. In forested areas, EAB tend to attack upper trunks and limbs of larger trees, gradually weakening them by destroying phloem. This severely weakens the trees, facilitating mass attack by EAB on the main trunks, resulting in tree death. It has been estimated that this process requires about five years when EAB populations have reached outbreak levels. Some trees die more quickly depending on age, health, species of ash, and EAB population density in the area.
The best early sign of EAB infestation in an area is woodpecker feeding on the main upper limbs and large branches of ash trees. Woodpeckers are important predators of EAB living under the bark of ash trees. Evidence of woodpecker feeding is readily observed because they remove patches of bark from trunks while scavenging, resulting in light-colored (orange-pink) patches of bark along the usual grey weathered ash trunks. Over the years of EAB attack, woodpecker feeding can be observed lower and lower on the ash trunks.
Other signs and symptoms of attack include weak and thinning ash crowns, branches with yellowing leaves, epicormic shoots or suckers on ash limbs and trunks, bark splits, and D-shaped exit holes on the trunk. If large numbers of EAB exit holes are present on the main trunk, the tree will not survive except possibly through stump sprouting. Ash regeneration by stump sprouting occurs more commonly when young ash trees are top-killed by EAB or when the trees are felled and the stumps are retained.
Liu HP, LS Bauer, DL Miller, TH Zhao, RT Gao, LW Song, QS Luan, RZ Jin, CQ Gao. 2007. Seasonal abundance of Agrilus planipennis (Coleoptera: Buprestidae) and its natural enemies Oobius agrili (Hymenoptera: Encyrtidae) and Tetrastichus planipennisi (Hymenoptera: Eulophidae) in China. Biological Control 42: 61-71.
Liu HP, LS Bauer, RT Gao, TH Zhao, TR Petrice, RA Haack. 2003. Exploratory survey for the emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae), and its natural enemies in China. Great Lakes Entomologist 36: 191-204.
- Leah S. Bauer , US Forest Service, Northern Research Station, Research Entomologist
- Therese M. Poland , US Forest Service, Northern Research Station, Research Entomologist
- Deborah L. Miller , US Forest Service, Northern Research Station, Entomologist
- Keith N. Windell , US Forest Service, Technology and Development Center, Mechanical Engineer
Last Modified: 01/27/2010