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A Trissolcus japonicus parasitoid wasp lays eggs inside brown marmorated stink bug eggs at the USDA-APHIS Quarantine Facility in Corvallis, Oregon. Photo courtesy of Christopher Hedstrom

A Trissolcus japonicus parasitoid wasp lays eggs inside brown marmorated stink bug eggs at the USDA-APHIS Quarantine Facility in Corvallis, Oregon. Photo courtesy of Christopher Hedstrom

Brown marmorated stink bugs show no signs of stopping their advance in the Pacific Northwest. But last fall, Oregon launched the first attempt to stop the voracious invasive pest with the help of a longtime foe — a tiny wasp with a taste for its eggs.

Scientists have studied the parasitoid wasp — Trissolcus japonicus under strict quarantine for about 10 years to see if it could be safely deployed as a biological control on brown marmorated stink bug without risks to native stink bugs or other unintended consequences. Signs were promising, but after T. japonicas was discovered in the wild in Maryland and then in the Pacific Northwest, Oregon Department of Agriculture officials gave the go ahead to experiments to rear and relocate the wasps found in the wild.

“We certainly haven’t released anything from quarantine, we are moving around the field population,” said Chris Hedstrom, biological control specialist for ODA. “The USDA restricts interstate movement, but since it’s already here, it’s up to us if we want to move it. And since BMSB is all over the Willamette Valley and beyond, we figure that the wasp is going to spread on its own without our help.”

So far, the effort is quite small, just tens of wasps released in three different spots, Hedstrom said. But Oregon State researchers plan to study the wasps this summer to measure their impact on BMSB and nontarget insects. Hedstrom is optimistic that the findings from this first field experiment — coupled with ongoing lab studies — will support a petition to have T. japonicus approved nationally as a biological control.

It’s good timing — as the first significant crop damage from BMSB was reported in Oregon last year as well. Near Yakima, Washington, the population has grown twentyfold every year, but largely in urban areas, not orchards, said Peter Landolt, entomologist with the U.S. Department of Agriculture down the road in Wapato.

“So far, to me, it looks like it’s showing up and multiplying like crazy in urban areas and has a risk of being a problem at the orchard-urban interface. Whether it goes on from there to be a general orchard apple pest, we’ll see,” Landolt said.

Outbreak response

Growers have a lot more tools to respond to BMSB now, thanks to research to identify effective insecticides that started with the first major Mid-Atlantic outbreak in 2010. Now, researchers are focused on strategies that can fit into, rather than disrupt, existing integrated pest management systems.

“Our original project was to put out the fire and identify insecticides that were effective so our growers would able to mitigate the problems caused by these bugs. But of course, they are all broad spectrum and disruptive to our IPM systems, so we ended up with secondary pest problems,” said Tracy Leskey, entomologist for the U.S. Department of Agriculture in West Virginia who headed the first Specialty Crop Research Initiative-funded effort. “We’ve been playing catch-up over the last six years, but now we are making progress.”

Spraying for BMSB is complicated by the fact that it’s an unpredictable pest, a generalist that may suddenly invade an orchard after months of population growth on different hosts. Leskey said that growers in the Mid-Atlantic are seeing success using traps placed near orchard borders to inform spraying decisions. “It’s certainly a border-driven pest. Using the trap as a mechanism to detect their movement into crops is critical,” she said.

Better traps are the key to making such a strategy work. Landolt had a breakthrough on traps last year by combining a big sticky trap designed for large wasps with pheromones. But he’s continuing to study BMSB pheromones to improve the traps, because the long-lived bugs use a wide variety of pheromones for different purposes.

“What I can do for you is detection and monitoring,” he said at the Apple Crop Protection Research Review in Yakima in January. “You need to know about it as early as possible if you want to do something about it.”

Other areas of research

Ongoing research is focused on reducing the insecticides needed to control stink bugs through methods such as using border-only sprays and attract-and-kill methods as well as encouraging natural enemies to reduce the population across the landscape, Leskey said.

Landolt is working on a trap that mimics the tight conditions BMSB seeks over winter. The idea is that instead of seeking warm refuge in people’s homes, the stink bugs could be drawn to housing boxes with pheromones and then discarded. In lab tests, he’s seen promising results and, if it works, it could combat the nuisance aspect of the BMSB problem.

“It’s one of the few pests that we have that’s an agricultural and urban pest. After it insults you in your orchard, it comes to taunt you in your house in the winter,” Leskey said.

Similarly, pheromones could be used to bait stink bugs in the orchards — attracting them to border row trees for example — which would reduce the amount of insecticide required to manage an outbreak, Leskey said.

Then, there is the possibility of excluding BMSB from orchards completely. Washington State University researchers are testing exclusion cages — netting that completely surrounds a tree or a block — to keep out BMSB and other large pests such as codling moths, said Betsy Beers, a WSU entomologist.

Use of such netting over orchards is already popular in some places to prevent hail damage or sunburn, she said, but it’s relatively new as a non-insecticidal pest control. Initial data shows that it did keep out the pests, but it also kept out desirable insects such as lacewings and syrphid flies, and the experimental trees saw a spike in wooly apple aphids, Beers said at a presentation at the Apple Crop Protection Research Review.

Beers is also leading an effort to survey Eastern Washington for native parasitoid wasps to serve as a baseline before T. japonicus is potentially released in the state. That could be done as early as 2018, she said, but the Washington State Department of Agriculture hasn’t made any approval decision yet.

All the various control strategies are going to w important as the BMSB population continues to grow and spread, but biological control — through both encouraging native predators such as praying mantis, lacewing larva, and parasitoid wasps and introduction of T. japonicus — is the ultimate goal, Beers and Leskey agreed.

“We all agree that, ultimately, the long-term solution is going to be based on biological control because we need methods that can work across the landscape and really reduce populations in these unmanaged areas,” Leskey said. 

– by Kate Prengaman