The emerald ash borer (Agrilus planipennis) is a deceptively attractive metallic-green adult beetle with a red abdomen. But few people ever see the insect itself – only the trace of the destruction it leaves under the bark of ash trees.
Native to Asia and Russia, these insects were first discovered in Michigan in 2002. Since then, they have spread to 35 states, becoming the most destructive and costly invasive wood-boring insect in US history. They were also discovered in the Canadian provinces of Ontario, Quebec, Manitoba, New Brunswick, and Nova Scotia.
In 2021, the U.S. Department of Agriculture stopped regulating shipments of ash and wood products in infested areas as the beetles spread rapidly despite quarantine efforts. Now federal agencies and researchers are pursuing a different strategy: biological control.
Scientists believe that tiny parasitic wasps that prey on emerald ash borer in their home range hold the key to containing this invasive species and returning the ash trees to North American forests.
I study invasive forest insects and work with the USDA to develop simpler methods of raising emerald ashborers and other invasive insects in research laboratories. This work is critical in discovering and testing ways to better manage forest restoration and prevent future outbreaks.
But while the emerald ashbore has spread uncontrollably in nature, it is surprisingly difficult to produce a constant laboratory supply of these insects – and developing an effective biological control program requires many target insects.
Adult emerald ash borer beetles are approximately 0.5 inches long (Photo not to scale) Wikimedia Commons
Combating the emerald ash borer: the value of ash trees
Researchers believe the emerald ash drill probably made it to the US sometime in the 1990s with imported wood packaging material from Asia. The insects lay their eggs in the crevices of the ash trees; When the larvae hatch, they pierce the bark and feed on the inner layer of the tree.
Their effects are seen when the bark is withdrawn and dramatic traces of food become visible. These channels damage the trees’ vascular tissue – internal networks that carry water and nutrients – and eventually kill the tree.
Before this invasive pest emerged, ash trees were particularly popular for housing developments, which made up 20 to 40 percent of the trees planted in some Midwestern communities. Emerald ash borer has killed tens of millions of U.S. trees with an estimated replacement cost of $ 10 billion to $ 25 billion.
Ash is also popular for sawn timber, which is used in furniture, sports equipment, and paper, among other things. The ash industry produces over 100 million boardfoot annually, valued at over $ 25 billion.
Fighting the Emerald Ash Bore: Why Quarantines Failed
State and federal agencies have used quarantines to control the spread of several invasive forest insects, including Asian hardwood beetles and Lymantria dispar, formerly known as the gypsy moth. This approach aims to reduce the movement of eggs and young insects hidden in lumber, nursery plants, and other wood products.
In counties where an invasive species is discovered, regulations typically require that wood products be heat treated, stripped of bark, fumigated, or chipped before they can be shipped.
The state quarantine for emerald ash moth began in 13 Michigan counties in 2003 and has grown exponentially over time to cover a quarter of the continental US quarantine.
However, female emerald ash borer can fly up to 12 miles per day for up to six weeks after mating. The bugs are also difficult to catch and usually aren’t spotted until they’ve been around for three to five years – too late to quarantine.
Fighting the Emerald Ash Bore: Time for Wasps
Any bio-control plan raises concerns about unintended consequences. A notorious example is the introduction of cane toads in Australia in the 1930s to reduce beetles on sugar cane farms. The toads did not eat the beetles, but they spread quickly and ate many other species. And their toxins killed predators.
The introduction of biological control species is strictly regulated in the United States. It can take two to ten years to demonstrate the effectiveness of potential biological control agents, and it can take another two years to obtain a field trial permit. Scientists need to demonstrate that the species released specialize in the target pest and have minimal impact on other species.
Four species of wasp from China and Russia that are natural enemies of the emerald ash borer have passed the approval process for release in the wild. These wasps are parasitoids: they lay their eggs or larvae in or on top of another insect that becomes an unsuspecting source of food for the growing parasite. Parasitoids are great candidates for biological control because they typically exploit a single host species.
The wasps selected are tiny and do not sting, but their egg-laying organs can penetrate the ash bark. And they have special sensory abilities to find larvae or eggs of emerald ash borer to serve as their hosts.
An emerald green ash borer larva in the wood (left); Tetrastichus planipennisi, a parasitic wasp that hunts ash borer; and wasp larvae that grew and ate the ash borer.United States Department of Agriculture
The USDA is working to raise huge numbers of parasitoid wasps in laboratory facilities by providing laboratory-grown emerald ashworms to host their eggs. Despite COVID-19 disruptions, the agency produced over 550,000 parasitoids in 2020 and released them in over 240 locations.
The goal is to create self-sustaining field populations of parasitoids that will reduce the natural emerald ash borer populations to such an extent that newly planted ash trees can grow and thrive. Several studies have shown encouraging initial results, but securing a future for Eschen will require more time and research.
One hurdle is that lab-grown emerald ash burs require fresh ash stems and leaves to complete their life cycle. I am working in a team to develop an alternative to the time-consuming and costly collection of round wood: artificial nutrition that the beetle larvae can eat in the laboratory.
Freshly cut ash logs await processing to collect newly emerging adult emerald ashborers laying eggs for the laboratory colony. Anson Eaglin / USDA
The food must have the right texture and nutrition. Other leaf-eating insects like to eat artificial food made from wheat germ, but species whose larvae digest wood are more picky. In the wild, the emerald ash borer feed only on species of ash.
In today’s global economy, with people and products moving rapidly around the world, finding effective management options can be difficult when invasive species take hold in a large area. But the findings from the emerald ash drill will help researchers mobilize quickly when the next forest pest arrives.
This article was originally published on The conversation B.and Kristine Grayson toot the University of Richmond. read this Original article here.
