Can Caging Orchards Protect Apples?
When brown marmorated stink bug hit the Mid-Atlantic region, it wreaked havoc with the tree fruit industry. Growers were forced to abandon effective integrated pest management programs and resort to frequent sprays of broad-spectrum insecticides to protect their crops.
Elizabeth Beers and her colleagues at Washington State University are working to make sure that doesn’t happen again in Washington’s valuable apple and pear orchards.
Beers, stationed at the university’s Wenatchee Tree Fruit Research and Extension Center, is investigating whether IPM tactics – specifically exclusion and biological control – can keep the voracious invasive insect at bay.
“The brown marmorated stink bug is still in the process of invading our state,” she said. “About half the counties in Washington have had a confirmed detection, and growers are on the threshold of where they’re getting damage. Based on what happened the eastern U.S., that’s going to get steadily worse.”
To prevent that, Beers and her graduate students, Adrian Marshall, Joshua Milnes and Jim Hepler, are testing whether the shade netting growers already use to protect their apples from sunburn can also protect it from stink bugs.
“We’re looking at physical exclusion,” Beers said. “Literally putting the trees in a cage.”
The idea isn’t as crazy as it sounds. As apple orchards increasingly move toward high-density, trellised plantings, the fruit is less shaded by foliage and more susceptible to sunburn, so growers are increasingly encasing large swaths of apples in netting that provides 20 percent shade. The opening in that netting are about 2 millimeters by 5 millimeters.
“Will that netting exclude stink bugs?” Beers asked. “Will it exclude other direct pests, like codling moth, because you can never have too many controls for codling moth. And are there unintended effects of the netting?”
The team already has answers to some of those questions, and yes, there are unintended effects. While the netting does seem to keep the stink bugs out (and help a little with codling moth) it also keeps out beneficial insects that control other pest species.
“We’re seeing outbreaks of secondary pests, like woolly apple aphid, due to the exclusion of natural enemies,” she said.
The team is trying to introduce natural enemies into the enclosures, but haven’t have much success yet. They are also testing insecticide-infused netting, which works very well but indiscriminately, and testing a variation of exclusion tactics that puts a barrier between natural vegetation where the stink bugs come from and the orchards.
Beers and her colleagues are building big bug walls.
“What I like about the idea is that it uses the behavior of the insect to target it,” she explained. “Some insects may just fly around a barrier. We believe stink bugs, when they fly into something, they cling to it then crawl up to get over it.”
So researchers are sewing flaps into the net wall that traps stink bugs as they make that climb, and looking at lining those pockets with the insecticide-infused netting to make the traps even more effective.
Both full enclosures and the orchard border walls have been tested for one season but it was a low-pressure year for stink bugs so their effectiveness was hard to measure. (One thing the team did learn was that successfully erecting a 12-foot-tall screen wall in a windy environment requires engineering and construction expertise not typically found in research entomologists.)
Unleash the Parasitic Wasps
“I think the long-term solution to suppressing brown marmorated stink bug will be biological control,” Beers said. “A survey looking for parasitoids of brown marmorated stink bug in 2015 detected the samurai wasp, (Trissolcus japonicus), an exotic egg parasitoid in Vancouver, Washington. Since then, there’s been more surveys and more detections in Washington and around the U.S.”
Native parasitic wasps don’t much care to attack brown marmorated stink bug eggs, but several Asian species do. Some of those species are being tested in quarantine to see if they are exclusive or preferential parasitoids of the brown marmorated stink bugs or if they’ll also attack native stink bugs, some of which are beneficial.
But that research is in quarantine laboratory conditions, so in Washington, researchers are looking at the parasitic wasps in the wild.
“It’s not a species-specific parasitoid, but it clearly shows a preference for brown marmorated stink bugs,” Beers said. “And all the native species we’ve tested it against are all pests, too.”
Beers believes there won’t be a silver bullet to managing the brown marmorated stick bugs – or codling moth, or spotted-wing drosophila, or pear psylla, or any of the other pests they study at the Wenatchee Tree Fruit Research and Extension Center. Instead, classic integrated pest management will continue to provide the best option for long-term pest control that protects crops, consumers and the environment.
“Unfortunately, when you go through an invasion, the first priority is control damage by any means possible,” she said. “And that’s usually broad-spectrum insecticides that are unfriendly to IPM. We want to avoid that and find IPM solutions to these pest challenges.”