The emerald ash borer (EAB), Agrilus planipennis, is a highly destructive metallic green beetle that originated in northeastern Asia. Since its detection in North America in 2002, this invasive pest has caused the widespread death of tens of millions of ash trees (Fraxinus spp.) across the continent. North American ash species lack the natural defenses found in Asian ash, allowing EAB populations to grow unchecked. To combat this ecological threat, scientists have studied which organisms consume the pest, from local wildlife to specialized organisms imported from the beetle’s native range.
Native North American Predators
Woodpeckers are the most significant native North American predators of the Emerald Ash Borer. Species like the Red-bellied, Downy, and Hairy woodpeckers actively forage for EAB larvae and pupae hidden beneath the ash tree bark. Woodpeckers are known to remove up to 38.5% of the larvae in some infested trees, leaving “flecking” patterns on the bark as they strip away the outer layers to reach their prey.
Generalist predators also contribute to mortality, though their impact is less substantial. Small mammals, such as shrews, may consume larvae located close to the soil line or prepupae exposed near the base of the tree. Other generalist insects, like praying mantises or spiders, may capture the adult beetles during their flight and feeding periods. However, these native predators are insufficient to control the EAB population because the beetle reproduces quickly and spends most of its life cycle protected beneath the bark.
Specialized Biological Control Agents
The most effective biological controls are specialized parasitic wasps introduced from the EAB’s native range in Asia through U.S. government biocontrol programs. These tiny, stingless wasps co-evolved with the Emerald Ash Borer, allowing them to locate and attack the pest’s hidden life stages. The primary larval parasitoid is Tetrastichus planipennisi, an endoparasitoid that lays its eggs directly inside the EAB larva.
Multiple wasp larvae develop within a single EAB host, killing the host upon emergence. Another crucial agent, Oobius agrili, targets the egg stage, laying a single egg inside an EAB egg deposited on bark crevices. Two species of Spathius wasps, S. agrili and the cold-tolerant S. galinae, are ectoparasitoids that lay eggs on the outside of the EAB larva, which the hatching wasp larvae then feed upon. These specialized agents, particularly T. planipennisi and O. agrili, have shown success in reducing EAB density and slowing the overall rate of ash tree decline.
Pathogens and Environmental Factors
Pathogens and environmental stresses contribute to EAB mortality. Specific microbial agents, such as entomopathogenic fungi, function as natural enemies by infecting and consuming the beetle. Fungi like Beauveria bassiana have been recovered from EAB larvae and are being studied as potential biocontrol agents, causing disease.
Environmental factors, particularly extreme cold, cause high mortality during the overwintering stage. EAB larvae typically produce antifreeze compounds, allowing them to survive temperatures down to approximately -30°C. However, extended periods below this threshold, common in northern latitudes, can kill a significant portion of the prepupal population and slow the beetle’s spread.
Susceptibility Across the EAB Life Cycle
The success of any predator depends on the EAB’s life stage and location within the ash tree. The egg stage, laid in bark crevices, is vulnerable to the specialized egg parasitoid Oobius agrili and generalist surface insects. Once hatched, the larva enters the inner bark (phloem), creating serpentine galleries where it is heavily protected.
The protected larval stage is mainly targeted by woodpeckers, which excavate the larvae, and the larval parasitoids (Tetrastichus and Spathius), which penetrate the bark with their ovipositors. The pupal stage, occurring in a chamber excavated in the outer sapwood or bark, is generally safe from most predators. The adult beetle is vulnerable during its emergence and while in flight, making it susceptible to generalist aerial predators like birds, bats, and spiders.