Invasive emerald ash borer has destroyed millions of trees – scientists aim to control it with tiny parasitic wasps | Kiowa County Press
Kristine Grayson, University of Richmond
Emerald ash borer (Agrilus planipennis) is a deceptively attractive metallic green adult beetle with a red abdomen. But few people actually see the insect itself – just the trail of destruction it leaves under the bark of ash trees.
These insects, native to Asia and Russia, were first discovered in Michigan in 2002. Since then, they have spread to 35 states and have become the most destructive and costly invasive wood-boring insect in the world. history of the United States. They have also been detected in the Canadian provinces of Ontario, Quebec, Manitoba, New Brunswick and Nova Scotia.
In 2021, the US Department of Agriculture stopped regulating the movement of ash trees and wood products to infested areas because the beetles spread rapidly despite quarantine efforts. Today, federal regulators and researchers are pursuing a different strategy: biological control. Scientists believe tiny parasitic wasps, which prey on the emerald ash borer in their natural range, hold the key to tackling this invasive species and bringing ash trees back to North American forests.
I study invasive forest insects and work with the USDA to develop easier ways to rear the emerald ash borer and other invasive insects in research labs. This work is essential to discover and test ways to better manage forest recovery and prevent future epidemics. But while the emerald ash borer has spread uncontrollably in nature, producing a constant laboratory supply of these insects is surprisingly difficult – and developing an effective biological control program requires a lot of target insects.
The value of ash trees
Researchers believe that the emerald ash borer probably arrived in the United States on wood packaging material imported from Asia in the 1990s. The insects lay their eggs in crevices in the bark of ash trees; when the larvae hatch, they tunnel through the bark and feed on the inner layer of the tree. Their impact becomes apparent when the bark is peeled off, revealing dramatic feed tracks. These channels damage the vascular tissues of trees – internal networks that carry water and nutrients – and ultimately kill the tree.
Before this invasive pest appeared on the scene, ash trees were particularly popular for residential development, accounting for 20-40% of trees planted in some Midwestern communities. The emerald ash borer has killed tens of millions of trees in the United States with an estimated replacement cost of US $ 10 to 25 billion.
Ash wood is also popular for wood used in furniture, sports equipment, and paper, among many other products. The ash lumber industry produces over 100 million board feet per year, valued at over $ 25 billion.
Why the quarantines failed
State and federal agencies have used quarantines to control the spread of several invasive forest insects, including Asian longhorned beetles and Lymantria dispar, formerly known as 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 detected, regulations generally require that wood products be heat treated, debarked, fumigated or shredded before they can be moved.
The federal quarantine against the emerald ash borer began with 13 counties in Michigan in 2003 and has grown exponentially over time to cover more than a quarter of the continental United States. drink.
However, females of the emerald ash borer can fly up to 12 miles per day for six weeks after mating. Beetles are also difficult to trap and are usually not detected until they have been around for three to five years – too late for quarantines to work.
Next option: wasps
Any biological control plan raises concerns about unintended consequences. A notorious example is the introduction of cane toads to Australia in the 1930s to reduce beetles on sugarcane farms. The toads did not eat the beetles, but they spread quickly and ate a lot of other species. And their toxins killed the predators.
The introduction of species for biological control is strictly regulated in the United States. It may take two to 10 years to demonstrate the efficacy of potential biological control agents, and obtaining a permit for field trials may take an additional two years. Scientists must demonstrate that the released species specializes on the target pest and has minimal impacts on other species.
Four species of wasps from China and Russia that are natural enemies of the emerald ash borer have gone through the approval process for field release. These wasps are parasitoids: they deposit their eggs or larvae in or on another insect, which becomes an unsuspecting food source for the growing parasite. Parasitoids are excellent candidates for biocontrol as they typically exploit a single host species.
Selected wasps are tiny and do not sting, but their egg-laying organs can penetrate the bark of ash trees. And they have specialized sensory abilities to find emerald ash borer larvae or eggs that will serve as their hosts.
The USDA is working to rear massive numbers of parasitoid wasps in labs by providing lab-grown emerald ash borer as hosts for their eggs. Despite COVID-19-related disruptions, the agency produced more than 550,000 parasitoids in 2020 and released them to more than 240 sites.
The goal is to create self-sustaining ground populations of parasitoids that sufficiently reduce populations of emerald ash borer in the wild to allow replanted ash trees to grow and thrive. Several studies have shown encouraging initial results, but securing a future for ash trees will require more time and research.
One obstacle is that laboratory-grown ash borers need fresh ash logs and leaves to complete their life cycle. I’m part of a team working to develop an alternative to the long and expensive process of collecting logs: an artificial diet that the beetle larvae can eat in the lab.
The food should provide the right texture and the right nutrition. Other leaf-feeding insects readily eat artificial wheat germ foods, but species whose larvae digest wood are more difficult. In the wild, the emerald ash borer feeds only on ash species.
In today’s global economy, where people and products move rapidly across the globe, it can be difficult to find effective management options when invasive species become established over a large area. But the lessons learned from the emerald ash borer will help researchers mobilize quickly when the next forest pest arrives.
Kristine Grayson, Associate Professor of Biology, University of Richmond
This article is republished from The Conversation under a Creative Commons license. Read the original article.