The samurai wasp is about the size of a sesame seed, but it's a natural predator of the brown marmorated stink bug, an invasive species from Asia. Photo by Penn State
June 6 (UPI) -- For homeowners, the brown marmorated stink bug can be a smelly nuisance. For orchard growers, the invasive insects are a very expensive problem.
Researchers are hopeful they're close to a natural remedy: the samurai wasp, Trissolcus halyomorphae.
Thanks to the work of Hillary Peterson, a Penn State doctoral student, researchers in Pennsylvania are beginning to test the wasp's potential as a natural weapon against the brown marmorated stink bug.
Adult samurai wasps hunt for the eggs laid by brown marmorated stink bugs. The stink bug eggs are usually arranged in clusters of 28 eggs. Unlike other parasitoids, which may test a couple eggs, the samurai wasp lays its own eggs inside the majority of the eggs in the cluster. As the wasp's larvae develop, they eat the stink bug egg, effectively killing the would-be stink bug. When they've fully developed, the tiny wasps chew their way out of the egg and continue the cycle.
According to USDA regulations, scientists aren't allowed to release or test invasive species in the field, even potentially beneficial ones, unless they're already present. Peterson, who trained in the art of identifying parasitoid wasp species at the Smithsonian after earning her undergraduate degree at the University of Maine, helped confirm the presence of Trissolcus halyomorphae in Pennsylvania through her ongoing research.
The wasp's presence is good news for orchard growers. Last year, apple and peach growers lost $35 million in crop damages caused by the brown marmorated stink bug.
Unlike native parasitoids, which prefer to lay their eggs inside the eggs, larvae or bodies of other native species, the samurai wasp evolved alongside the brown marmorated stink bug in Asia. The wasp only has eyes for the insect responsible for spoiling apple and peach crops across the Keystone State.
But just because Peterson found a few wasps doesn't mean the stink bug problem is solved.
"We're still in the potential solution folder, we can't yet put the wasp in the absolute solution folder," Peterson told UPI.
Now that Peterson and other researchers know the wasp is in Pennsylvania -- as well as nine other states -- they're conducting tests to determine the species' effectiveness at curbing the stink bug population.
"Finding and identifying the wasps in Pennsylvania was just one small component of my larger project," Peterson said. "Now, my hope is to get a colony going."
While some researchers are testing how well the wasps are able to find the stink bug's egg clusters in the wild, Peterson is working to collect and hatch samurai wasp eggs.
To procure samurai wasps, Peterson is using bait in the form of freshly laid stink bug egg clusters.
"You take these eggs and go out in the field and you put them where you'd naturally find them," Peterson said. "Then, you go back and get them before they hatch."
Once the eggs are back in the lab, you wait for them to hatch.
"Most of the time stink bugs hatch, but sometime they don't," she said. "When a stink bug fails to emerge, you wait for a potential parasitoid to develop and hatch."
The wasps take longer than stink bugs to develop before emerging.
Once she has a sustainable colony of wasps, Peterson will be able to conduct a variety of tests to measure the insect's stink bug-fighting capabilities. She and other researchers also need to confirm that the introduction of the samurai wasp won't disrupt the local ecosystem.
Early tests suggest the samurai wasp isn't interested in attacking the eggs of native stink bugs.
"The risks for releasing parasitoids are normally really low because you only usually release a specialist, not a species that feeds on a variety of insects," Peterson said.
Peterson believes strongly in integrative pest management, the use of biological remedies to curtail crop pest populations. She thinks growers see the value in the approach, too.
"The growers are very welcoming and let me come into their orchards and put my eggs out," she said. "They trust me and are really interested in the research."
Because the brown marmorated stink bug is so tough, it's difficult for growers to simply spray their way out of the problem. The large amount of pesticide required to kill the pests can cause collateral damage -- the loss of beneficial predator populations.
Some orchards have seen the emergence of other pests, including the San Jose scale and woolly apple aphid, after aggressive pesticide treatments intended for stink bugs also killed off important predators.
By the end of this summer, Peterson hopes to not just have a healthy wasp colony to fuel further experiments, but also more growers interested in her work.
"I'm planning to go talk to more growers at meetings and have some brochures to put out about best practices for integrative pest management," she said.