Alien versus Predator

In the last decades, invasive emerald ash borer beetles decimated native ash tree populations across North America. LEFT: An adult emerald ash borer. (Photo courtesy of F.W. Ravlin, Michigan State University) CENTER: An emerald ash borer larva in an ash log. (USDA Forest Service photo by Debbie Miller) RIGHT: Young emerald ash borer adults emerge from holes in an infected ash tree. (USDA Forest Service Photo by Debbie Miller)

Hundreds of tiny, invasive emerald ash borer larvae gnaw under the bark. Their zig-zagging paths across the tree’s delicate tissues mark its slow death. This ash tree, like tens of millions of others before it, appears to be doomed.

The larvae suddenly stop.

If they could, they might scream.

Parasitic wasps erupt from the larvae’s bodies. Many of the emerald ash borers are dead – and the remaining could be next.

This seemingly sci-fi horror scene is real life. It’s playing out in forests near you. And it may help save native ash trees from extinction.

Toby Petrice is a USDA Forest Service research entomologist who specializes in biological control of invasive species. His current work involves parasitoid wasps that attack emerald ash borer larvae and eggs to control their populations. (Photo courtesy of Toby Petrice)

Truth is stranger than fiction.

“These are endoparasitoids, so they are like little aliens in a way. They're either laying their eggs inside of another insect or its eggs and consuming them from the inside out,” said Toby Petrice, a USDA Forest Service research entomologist who specializes in biological control of invasive species.

Biological control efforts use natural enemies of invasive species to control their damage to native forests. The endoparasitoids Petrice works with are natural enemies of emerald ash borer, killers of large ash trees across North America. Like the emerald ash borer, these parasitoids are from Northeast Asia where they feed on emerald ash borers in its native environment.

“The host insect can look perfectly fine for a while,” said Petrice. “Then, all of a sudden, all these little parasitoids chew their way out of it.”

Erupting from ash borer larva.

Tetrastichus planipennisi, consumes and kills its host much like the creatures in the Alien movies. T. planipennisi is an endoparasitoid, meaning it lays its eggs inside the host’s body. The parasitoid does this using its modified stinger to drill through the tree bark, stab into the emerald ash borer larva, and lay its eggs.

LEFT: Some parasitoid wasps Petrice works with use their modified stingers to drill through the tree bark and lay eggs on emerald ash borer larvae. (Photo courtesy of F.W. Ravlin, Michigan State University) CENTER: The endoparasitoid wasp larvae develop inside the emerald ash borer larva host. (USDA Forest Service Photo by Debbie Miller) RIGHT: When ready, the wasp larvae kill the ash borer larva and erupt from its carcass. (USDA Forest Service Photo by Debbie Miller)

T. planipennisi also injects venom into the larva. This venom doesn’t kill the host larva but inhibits the larva’s immune system defenses, leaving it incapable of attacking the developing wasp. This way the host larva’s immune system cannot attack the wasp’s developing progeny inside of it.

With a safe, plentiful, and defenseless host, the parasitoid eggs hatch inside the emerald ash borer larva and eat it from the inside out. The host larva lives for a little while, but the young T. planipennisi larvae eventually consume so much of the internal hemolymph and tissues that the larva dies.

But the process gets even crazier.

The wasp larvae time the death of the host larva so it dies about the time the wasp larvae mature enough to emerge from it. They just chew their way out of it and pupate.

“It's pretty freaky when you think about it,” said Petrice. “You're glad you're not an insect, right?”

Mummifying ash borer eggs.

LEFT: An egg parasitoid wasp stabs into an emerald ash borer egg and deposits its own. (Photo courtesy of F.W. Ravlin, Michigan State University) RIGHT: The parasitoid wasp larvae develop inside the emerald ash borer eggs. (USDA Forest Service Photo by Debbie Miller)

Petrice’s team also works with an egg parasitoid called Oobius agrili. It parasitizes the eggs of the emerald ash borer, which are tiny - only 1 millimeter long.

O. agrili wasps use their modified stingers to drill into the emerald ash borer eggs, which are hidden in bark crevices, and lay their own egg. The parasitoid eggs hatch inside the ash borer eggs and consumes their contents. But they do not always emerge from the host eggs right away.

As the days get shorter, many parasitoid larvae start entering diapause. Diapause is like hibernation. The insect goes into a dormant state, and that's how it overwinters.

O. agrili overwinters as a larva inside the emerald ash borer egg that it already consumed. It even changes the outside of the egg.

“Something chemically happens to the eggshell where it seems to be harder and thicker,” said Petrice. “It's basically a mummy for the parasitoid, a little protective shell.”

O. agrili and T. planipennisi are only a couple of the parasitoid wasps Petrice’s team uses to control emerald ash borer populations. His team also works with ectoparasitoid wasps, including Spathius galinae, which is native to the Russian Far East. They also work with a native parasitoid wasp species, Phasgonophora sulcata, that lives inside an ash borer larva for up to two years before eating it alive and turning the empty host’s body into a mummy that protects the parasitoid until it becomes an adult.

Even with all the parasitoid wasp helpers and up to 80% reduction of emerald ash borer populations in some instances, biological control alone is not always enough to ensure the survival of native ash trees.

Persisting challenges.

Petrice injects a tree with emamectin benzoate, a pesticide, to protect it against emerald ash borer infestation. Protecting ash trees from emerald ash borers requires a combination of protection measures. (Photo courtesy of Toby Petrice)

The goal of biological control is to reduce insect densities below damaging levels. Most of the time, it seems to work.

But sometimes the emerald ash borer population increases faster than the parasitoids because there's a lag time. When lots of healthy ash trees are available, emerald ash borers can experience a rapid population boom. The parasitoids can’t always reproduce fast enough to catch up.

“If the densities of emerald ash borer larvae in trees get too high, they damage the trees so much that they are unable to recover,” said Petrice. “If you have a rapid increase in emerald ash borer population levels, you still get tree mortality even though the parasitoids are reducing the beetle population.”

That’s why other management strategies may need to be integrated with biological control to protect ash trees.

Joining forces.

Petrice’s team works with scientists from Michigan State University and the University of Kentucky to incorporate parasitoid wasp biological controls with insecticide treatments. The strategy is to use insecticides to protect a portion of the trees and reduce the overall emerald ash borer population in a forest stand, while relying on parasitoids to regulate populations in untreated trees.

Another strategy is to develop trees that are resistant to emerald ash borer. This is being led by Forest Service scientists in Delaware, Ohio, who are propagating trees from cuttings collected from trees that have survived emerald ash borer attack. These trees are then bred to produce progeny that are resistant to emerald ash borer. Although, these trees display levels of resistance, emerald ash borer could still attack them. Parasitoids could play an important role in reducing these attacks.

“Our hope is that by integrating parasitoids with these strategies, parasitoids will help modulate emerald ash borer populations when susceptible trees that are not treated with insecticides, or resistant trees get attacked,” said Petrice.

For now, his team observes how the sci-fi horror plots play out in real life at laboratories and field sites across the northern United States. And roots for the good guys to win.

These parasitoid wasp larvae recently consumed an emerald ash borer larva. These wasp larvae will soon pupate, their next stage before becoming adult parasitoid wasps. (USDA Forest Service photo by Debbie Miller).

Learn more

To learn more about the devastating impacts of emerald ash borer on tens of millions of North American ash trees and learn what you can do to help, visit the USDA Animal and Plant Health Inspection Service’s emerald ash borer site.