The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle from northeastern Asia that has caused widespread destruction to ash trees in North America. First detected in Michigan in 2002, this metallic green insect likely arrived in the United States via wood packaging materials from Asia. EAB larvae feed on the inner bark of ash trees, disrupting nutrient flow and leading to tree death within two to six years. This invasive pest has killed tens of millions of ash trees, posing significant ecological and economic threats across the continent.
Native Predators of the Emerald Ash Borer
North America’s native wildlife has begun to adapt to the emerald ash borer, with woodpeckers emerging as significant predators. Species like the Hairy, Downy, and Red-bellied Woodpeckers actively forage for EAB larvae beneath the bark of infested ash trees. These birds are effective at extracting larger EAB larvae during fall and winter. Heavy woodpecker damage on ash trees can often indicate an EAB infestation.
Beyond birds, some native insect species, particularly parasitic wasps, also opportunistically attack EAB. While not specifically evolved for EAB, these generalist parasitoids target native wood-boring beetles, including Atanycolus species that parasitize EAB larvae. Although these native predators contribute to EAB mortality, their impact alone has not been sufficient to control widespread populations or prevent extensive ash tree mortality.
Introduced Biological Control Agents
Recognizing the limitations of native predators, scientists have introduced specialized non-native natural enemies from EAB’s native range in Asia as part of biological control programs. Since 2007, four species of parasitic wasps have been approved for release in North America: Tetrastichus planipennisi, Spathius agrili, Oobius agrili, and Spathius galinae. These tiny, stingless wasps were selected after extensive research to ensure they specifically target EAB and pose minimal risk to non-target insect species.
These introduced wasps are reared in specialized facilities and then released in infested areas. Tetrastichus planipennisi and Oobius agrili have shown increasing populations and establishment in various regions. Spathius agrili has had lower establishment success in colder northern regions, but Spathius galinae, a more recently introduced species, shows promise due to its cold tolerance and ability to parasitize larvae in larger trees.
How Natural Enemies Attack EAB
Natural enemies employ distinct strategies to target emerald ash borer at different life stages. Woodpeckers use their strong beaks to chip away bark, creating distinctive holes as they excavate EAB larvae and pupae from galleries beneath the tree’s surface. Woodpecker predation occurs from fall through spring.
Parasitic wasps use a different approach. Oobius agrili, a tiny wasp less than 1 mm in size, targets EAB eggs, injecting its own egg inside the host egg, which prevents the EAB egg from hatching. Larval parasitoids like Tetrastichus planipennisi drill through the bark to lay their eggs directly inside EAB larvae. Spathius agrili and Spathius galinae also penetrate the bark, but they lay their eggs on the outside surface of the EAB larvae, paralyzing them first. The developing wasp larvae then consume the EAB host, killing it.
The Role and Limits of Natural Control
Natural enemies, both native and introduced, play an important role in managing emerald ash borer populations, but they cannot eradicate the pest entirely. Woodpeckers can cause significant EAB mortality in individual trees, sometimes consuming a high percentage of borers in a single infested tree. However, woodpecker populations do not increase fast enough to keep pace with widespread EAB infestations.
Introduced parasitic wasps contribute to EAB mortality by targeting eggs and larvae, reducing the pest’s reproductive success. For example, Tetrastichus planipennisi can achieve parasitism rates as high as 30% in some areas, and Oobius agrili can parasitize over 50% of EAB eggs in certain trials. Despite these successes, factors such as the EAB’s high reproductive rate, its ability to quickly spread, and the vast scale of the infestation limit the overall effectiveness of natural control. While these natural enemies can help reduce EAB populations and slow the rate of ash tree decline, they are not a standalone solution, and other management strategies, such as insecticide treatments for high-value trees, are often still necessary.