What is Biological Control? How is it Used to Manage EAB?

Biological control—often shortened to “biocontrol”—Is a process by which predators, parasitoids, or pathogens (collectively referred to as natural enemies) reduce insect pest populations. The services provided by natural enemies are often manipulated by humans to make natural enemies more effective at reducing populations of certain pest insects.

What Are Parasitoids?

Parasitoids are insects which feed on, and subsequently kill, their hosts as they develop from larvae to adults. Parasitoids are distinctly different from parasites in that parasitoids kill their hosts while parasites do not. Insect parasitoids differ from insect predators in that parasitoids kill one host in the process of developing from a larva to an adult while predators kill multiple prey items over the course of their lifespan. Parasitoid adults usually do not kill prey for food as predators do, instead parasitoid adults have short lifespans and often rely on resources such as nectar from flowers to survive until all their eggs are laid. There are thousands of parasitoid wasp and parasitoid fly species, all of which lay their eggs on their hosts or insert their eggs into the body of their hosts. When the eggs hatch, the parasitoid larvae will feed on and eventually kill their host. When the parasitoid larvae are large enough, they will pupate and then later emerge as adults. The adult parasitoids will disperse to find new hosts and repeat this process.

Native woodpeckers are the primary predators of emerald ash borer (EAB) in North America and are one of the most important sources of EAB biological control. Additionally, multiple species of native parasitoids have “learned” to search for and attack EAB eggs or larvae. In areas without EAB, these native parasitoids will attack native beetles that feed on phloem (inner bark) or bore into wood. Some of these native borers, such as two-lined chestnut borer (Agrilus bilineatus) and bronze birch borer (Agrilus anxius) are related to EAB. These insects are secondary pests, however, only colonizing trees that are very stressed or dying. Many native parasitoid wasps probably attack several native species of phloem- or woodborers. However, we often know relatively little about these native parasitoids, including how they find their host insects and which species of native borers they can attack.

Although woodpeckers and native parasitoids kill some EAB larvae, they have not been able to suppress EAB populations to prevent trees from dying. Because of the enormous ecological and economic impact posed by EAB to North American ash trees, scientists searched for additional parasitoid species from the native range of EAB in Asia to release in North America. The goal of these releases was to increase the number of EAB eggs and larvae killed by parasitoid wasps and hopefully slow the rate at which ash trees were killed by EAB.

Since 2007, four species of non-native parasitoid wasp species from Asia have been introduced into North America to support biological control of EAB. The sections below describe the biology and effectiveness of woodpeckers and parasitoid wasps in managing EAB; risks and benefits of introduced parasitoid wasp species on EAB management; procedures to rear and release EAB parasitoids; and highlights of current research to improve the efficacy of biological control for managing EAB

What is Biological Control? How is it Used to Manage EAB?

Many woodpecker species feed on emerald ash borer larvae. In Michigan and Ohio, the downy woodpecker, the hairy woodpecker, and the red-bellied woodpecker have all been observed to feed on EAB larvae. Woodpeckers generally ignore small young EAB larvae that are present during most of the summer, but they can be voracious predators of large EAB larvae.

Woodpecker predation begins in the fall and continues through the winter until the following spring. By late spring or early summer, most EAB will have completed development and emerged from trees. Observations of woodpeckers during the winter by citizen scientists have led some to assume that local woodpecker populations increase in areas with high EAB densities. However, woodpeckers nest and raise chicks during the summer when EAB are generally not available. Thus, while many people will notice woodpeckers preying on infested ash trees near their homes during the winter, whether woodpecker densities increase in response to EAB invasion is doubtful.

Woodpeckers leave a distinctive hole in the outer bark when they attack an EAB larva. These holes are often the first evidence that EAB is present in a local area. Woodpeckers can find and prey on EAB larvae even in newly infested trees with very low densities of EAB. In areas that have not yet been invaded by EAB, checking the upper portion of the trunk of ash trees for woodpecker holes in the winter can be helpful in detecting new infestations.

As an EAB infestation progresses over 2-4 or more years, larval density within ash trees builds and the density of woodpecker holes also increases. Heavily infested ash trees may be nearly covered by woodpecker holes from previous years, as well as the current year. On large ash trees, woodpeckers often rip off flecks or chunks of thick outer bark on the trunk to get down to the phloem or outer sapwood so they can pull out the EAB larvae. This process, called blonding, can be impressive. Blonding exposes the light colored “blond” and bark fragments may surround the base of the tree.

In a given tree, the proportion of EAB killed by woodpeckers in a year may range from 0 - 90% of the larvae within a tree. Unfortunately, by the time woodpeckers prey on the late-stage larvae or prepupae, the insects have completed their feeding, which means the damage to the tree has already been done. Also, woodpeckers tend to prey heavily on EAB larvae in a few ash trees but may largely ignore other infested ash trees. Thus, while woodpecker predation remains the highest source of EAB mortality in North America, ash trees continue to die and EAB continues to spread.

What Parasitoids Prey on EAB?

Parasitoid wasps are tiny wasps that cannot sting humans and are so small that they are difficult to see. Parasitoid wasps lay eggs on or inside their prey - in this case, EAB eggs or larvae within the ash trees. When the eggs hatch, the larvae feed on the prey as they grow, killing the prey. When wasps mature and become adults, they will lay more eggs which will produce more wasps—ultimately leading to suppressing the EAB population.

Native Parasitoids:

Several species of parasitoids native to the United States have been recorded parasitizing EAB larvae. For example, five wasp species in the genus Atanycolus (for example Atanycolus cappaerti) can parasitize EAB larvae. Other native wasp species known to parasitize EAB larvae include Phasgonophora sulcata and Leluthia astigmata. Balcha indica, a parasitoid native to Asia inadvertently introduced into North America years ago, also attacks EAB larvae.

These parasitoids lay their eggs either next to EAB larvae or within EAB larvae during the spring and summer. Once the parasitoid eggs hatch, the tiny wasp larvae begin feeding on the EAB larva. After an EAB larva is consumed, the parasitoids will pupate and emerge as adults, or enter diapause (a dormant state) so they can overwinter. Some species form cocoons and spend the winter within the tree. Other species overwinter within the EAB larva, or as “naked” larvae or pupae within the EAB feeding gallery. After overwintering, these parasitoids complete development and emerge as adults to repeat this process.

All native parasitoid species other than P. sulcata are external parasitoids, which means their larvae consume EAB larvae from the outside in. In contrast, P. sulcata is an internal parasitoid which means adults of this species lay their eggs inside the body of an EAB larva. When the eggs hatch, the wasp larvae feed on the EAB larva from the inside out.

Although we now know that there are many species of native parasitoids that can lay eggs and develop on EAB as hosts, parasitism rates by native wasps are often low and may account for less than 5% of the EAB larvae within an infested ash tree. However, EAB parasitism rates by native parasitoids can vary widely. For example, researchers have found parasitism rates by Atanycolus cappaerti in ash trees that ranged from 9 - 71% in Michigan.

In the EAB's native range within Asia, parasitism rates by Asian parasitoid wasps are much higher than parasitism rates by native species in North America. For this reason, parasitoid species that specialize on EAB in its native range were of considerable interest and evaluated for release in the United States. Scientists conducted studies with native phloem- and woodborers in laboratories. They found no significant effects of these non-native wasps on native, non-target insect species. Following approval by federal agencies, four non-native parasitoid species were introduced to the United States to help manage EAB populations.

Risks and Benefits of Introducing Non-native Parasitoids to Manage EAB in North America:

Non-native parasitoid wasps have been introduced to North America to manage damaging non-native invasive pest insects like EAB. Parasitoids from the native range of a given pest have co-evolved with their host insects for millions of years and are typically better adapted at finding and parasitizing the invasive pest species.

In the case of EAB, scientists identified parasitoids that attack and kill EAB in its native range in Asia. These species were studied initially in China, and later in quarantine laboratories in the US to identify their life cycle and to develop rearing methods for the parasitoids. Perhaps most importantly, scientists needed to know if these parasitoid species posed a risk to native insects if they were released in the U.S.

Introducing any non-native species to a new region has many potential risks that must be carefully evaluated. For the EAB parasitoids, one risk is that these species could parasitize other wood-boring beetle species that are closely related to EAB but native to North America. This could eventually result in non-native parasitoid wasps preferentially attacking native wood-boring beetles instead of EAB. This would obviously result in poor biocontrol of EAB and could affect populations of native, non-target phloem- or wood-boring insects.

Because of these risks, scientists conducted host range studies and safety evaluations to determine if the non-native parasitoids were able to successfully attack native wood-boring beetle species and if so, whether they displayed a strong preference for EAB over native woodborers.

All introduced parasitoids of EAB were studied to assess potential impacts on forest ecosystems and non-target, native phloem- or woodboring beetles. Some of the non-native parasitoids released in the United States could develop in native woodborers. However, these same parasitoid species demonstrated a strong preference for EAB when provided with a choice of EAB or native woodboring beetle larvae. Results of these studies indicated the impacts to non-target woodboring beetles in North America from the introduction of these Asian parasitoids is likely to be minimal.

Results from these studies were reviewed by the North American Plant Protection Organization (NAPPO), the Biological Control Review Committee and United States Department of Agriculture (USDA) Animal Plant Health Inspection Service (APHIS) to assess the risks and benefits of releasing non-native EAB parasitoids into the United States. Once a “finding of no significant impact” was determined, requests were made to each state for approval to release one or more species. Environmental release permits must be issued before any releases occur.

Although there are some risks associated with introducing non-native parasitoids into North America, the potential benefits must also be considered. Ideally, parasitoids will eventually be able to slow EAB population growth and the rate of ash mortality over a broad geographical area. Once parasitoids are established, their local density should increase, leading to greater mortality of EAB. Moreover, parasitoids that can successfully disperse can become established in new areas without requiring further releases or additional management costs. While highly effective systemic insecticides can be used to protect ash trees from EAB, these applications must be repeated over time. Most property owners and land managers can treat only a limited number of ash trees in any given year with insecticide. If parasitoids disperse and actively seek out EAB larvae in untreated ash trees, they could contribute to long-term management of EAB across North America.

Introduced parasitoids:

Oobius agrili is a parasitoid of EAB approved for release in North America in 2007. O. agrili was selected for release because parasitism rates of EAB eggs by O. agrili average 44% in its native range of northeast China. Additionally, host-choice studies determined that O. agrili consistently preferred to use EAB as a host over other related native wood boring beetles.

O. agrili is a tiny black wasp (less than 1 mm in size) which parasitizes EAB eggs. O. agrili differs from the other EAB parasitoids because it feeds on the eggs of EAB and not EAB larvae. In late June in the northern United States, O. agrili adults begin laying their eggs on EAB eggs that are in the cracks and crevices of ash tree bark. Adult O. agrili females generally lay 1 egg per EAB egg. The wasp larvae hatch and feed within EAB eggs and either pupate and emerge as adults to lay more eggs, or the larvae remain in EAB eggs over the winter and emerge as adults in the following summer. Therefore, O. agrili may have 1 or 2 generations per year. O agrili is also unique among the currently released parasitoid species in that no males of this species exist in North America. Female O. agrili can lay eggs without mating and all eggs will emerge as females. O. agrili has now been released in over 31 states in the United States. Studies conducted in Michigan have found that EAB parasitism rates by O. agrili initially increased slowly from 1 to 4% over the period of 2008 - 2011 then accelerated and increased to 28% in some locations by 2014. This suggests that O. agrili may become more effective as a biological control agent of EAB over time.

Tetrastichus planipennisi, a small black parasitoid wasp of EAB larvae, ranges in size from 3 - 4 mm and is native to China and eastern Russia. Unlike O. agrili that parasitizes EAB eggs, T. planipennisi parasitizes EAB larvae. Female wasps use their ovipositor - the egg laying part of the wasp's abdomen - to insert their eggs through ash tree bark and into the EAB larvae in tree branches on the trunk. They may lay 5 - 12 eggs inside an individual EAB larva. When the eggs hatch, the offspring begin feeding on EAB larvae from the inside out. T. planipennisi is therefore an internal larval parasitoid like the native species P. sulcata. After T. planipennisi have consumed their host, they either emerge as adult wasps and attack more EAB larvae or overwinter as pupae or larvae and emerge as adults the following spring.

Researchers found that in Asia, EAB parasitism by T. planipennisi averaged 22 %, indicating that this species could be an effective biological control agent of EAB in North America. Prior to its introduction, host suitability studies were conducted to determine if T. planipennisi would parasitize larvae of native woodboring insects in North America. Researchers found that T. planipennisi did not parasitize any species besides EAB. These results led to approval for T. planipennisi release in the United States in 2007. Studies in Michigan found that parasitism rates of EAB by T. planipennisi ranged from 1 - 6 % in 2008 to 2011. By 2014, rates as high as 30 % had been recorded. However, T. planipennisi has a small ovipositor which means that the wasps are unable to reach EAB larvae in trees with thick bark. Thus, T. planipennisi is effective at parasitizing EAB larvae in smaller ash trees with diameters less than 12 cm but will not parasitize EAB in larger trees.

Two species of larval parasitoids in the genus Spathius have been released for biological control of EAB in North America. Spathius agrili, which is native to China, was approved for release in the United States in 2007. Researchers found that while S. agrili can parasitize two native wood boring beetles that are closely related to EAB, parasitism rates of these species were significantly lower than EAB. Because of these and other studies, regulatory officials determined that S. agrili would have minimal non-target impacts in North America. Because S. agrili does not survive cold winters in the northern United States, S. agrili is now only released south of the 40th parallel in the United States. This species is collected less frequently than the other introduced parasitoids which suggests that it may not be an effective parasitoid of EAB in North America.

Spathius galinae is native to the Far Eastern region of Russia. Scientists determined S. galinae parasitism rates of EAB were as high as 60% in its native range. Prior to its introduction into North America, scientists tested if S. galinae would parasitize 15 other wood boring insects and found that the only other species besides EAB parasitized by S. galinae was the gold-spotted oak borer (Agrillus auroguttatus), an invasive wood-boring beetle of oak trees in California. However, parasitism rates of the gold-spotted oak borer by S. galinae were much lower than parasitism rates of EAB. As a result of these studies, S. galinae was approved for release in the United States in 2015.

S. galinae is a brown and black parasitoid wasp with patterned wings that is approximately 4.5 mm long. Spathius galinae lays its eggs on the surface of EAB larvae that hatch and feed externally on the larvae as they develop. Each female S. galinae may lay 2 - 15 eggs on the surface of a single EAB larva. When feeding is complete, the wasps pupate and then emerge as adults and search for additional hosts. S. galinae has multiple generations per year, which means this life cycle can be completed multiple times over the course of the spring and summer. Unlike S. agrili, S. galinae can survive cold winters and is therefore suitable for parasitizing EAB in northern states. Recent studies conducted in Michigan and the northeastern United States showed that S. galinae alone contributed to a 31 - 57% reduction in EAB populations during an outbreak phase. However, other studies in this same region have found much lower parasitism rates (8 - 14%) which suggests S. galinae parasitism rates are highly variable across infested ash trees. Because S. galinae is a larger wasp with a longer ovipositor (the egg laying part of the wasp's abdomen) than T. planipennisi, it can parasitize EAB larvae within larger ash trees with thicker bark (i.e., trees with diameters greater than 12 cm). A recent study found that S. galinae prefer to parasitize EAB larvae in larger trees while T. planipennisi prefers to parasitize EAB in smaller trees. This means the two species should not compete for hosts and will be able to coexist.

How are Exotic Parasitoids Reared and Released?

In 2009, an EAB biological control facility in Brighton, Michigan, was created and is managed by the USDA Animal & Plant Health Inspection Service (APHIS). This facility rears the four species of introduced EAB parasitoids for widespread release.

Oobius agrili is reared in eggs laid by adult EAB beetles. To acquire these eggs, adult beetles emerge from harvested ash trees. Adult beetles are placed in rearing containers with ash leaves grown in a greenhouse facility. Beetles feed on the leaves, then mate and lay eggs. In nature, adult EAB beetles lay their eggs in cracks and crevices between layers of ash bark. In the laboratory, EAB adults lay eggs on filter paper that is placed over a layer of screening which simulates the bark cracks and crevices of ash trees. EAB eggs laid on filter paper are exposed to O. agrili adults for parasitism. Sheets of filter paper with parasitized EAB eggs are then placed inside small plastic cups with a screen over the top. The plastic cups are hung upside down on infested ash trees to allow the O. agrili adults to emerge and disperse once they have matured.

To rear the other three EAB parasitoid species, EAB eggs are applied to the bark of small ash tree cuttings (often referred to as “bolts”). When the EAB larvae have hatched and are feeding inside the bolts, adult parasitoids are exposed to the bolts to parasitize the larvae. Bolts are attached to infested ash trees so that when parasitoids emerge, they have nearby EAB larvae to parasitize.

The Future of EAB Biological Control

Although the combined efforts of woodpeckers and parasitoids have partially reduced EAB abundance in North America, EAB continues to expand its range and ash trees continue to die. Scientists are hopeful that over time, parasitoid populations will increase to levels that can suppress EAB population to densities that allow ash trees to survive and reproduce. To improve the efficacy of biological control for EAB, scientists are researching ways to improve the process for rearing and recovering introduced parasitoids, as well as evaluating their impacts on EAB populations and ash health.

Researchers are also studying the climactic conditions that influence when parasitoids emerge and search for hosts so that parasitoid releases can be better coordinated with the EAB life stages that they parasitize. Other studies are evaluating how interactions between non-native parasitoids and native parasitoid species will affect EAB biological control. Scientists are also looking at effects of integrating biological control and systemic insecticide treatments to better manage EAB. Results from these and many other studies should lead to greater establishment of EAB parasitoids and higher parasitism rates, helping to protect ash species across North America.

References:

Bauer, L.S., Duan, J.J., Lelito, J.P., Liu, H. and Gould, J.R., 2015.Biology of emerald ash borer parasitoids. In: Van Driesche, RG; Reardon, RC, eds. Biology and control of emerald ash borer. Morgantown, WV: US Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team: 97-112. Chapter 6., pp.97-112. https://www.fs.usda.gov/research/treesearch/49294

Duan, J. J., J. R. Gould, B. H. Slager, N. F. Quinn, T. R. Petrice, T. M. Poland, L. S. Bauer, C. E. Rutledge, J. S. Elkinton, and R. Van Driesche. 2022. Progress toward successful biological control of the invasive emerald ash borer in the United States, pp. 232-250. In: Van Driesche, R. G., R. L. Winston, T. M. Perring, and V. M. Lopez (eds.). Contributions of Classical Biological Control to the U.S. Food Security, Forestry, and Biodiversity. FHAAST-2019-05. USDA Forest Service, Morgantown, West Virginia, USA. https://bugwoodcloud.org/resource/files/23194.pdf