Native predators might be developing a taste for Chinese food.
Perdue University Extension
Brown marmorated stinkbug, an invasive insect from Asia, swept over the Mid-Atlantic states’ fruit crops like a tsunami in the fall of 2010, causing millions of dollars in damage to peaches and apples and leaving growers worried that it might be worse the next year.
It wasn’t. In 2011, the damage was less, leaving growers unsure why that was and shifting their concern to the size of the next wave, coming this fall.
“So far this year, we have not seen significant problems in tree fruit,” said Tracy Leskey, leader of the USDA’s brown marmorated stinkbug project at the Appalachian Fruit Research Station in Kearneysville, West Virginia. “But it’s early yet,” she said in mid-July. “The summer generation is now reproducing, so it’ll be mid- to late August and October before we know how big the population will be this fall.”
It hasn’t been easy to sort it all out. The real world isn’t a good scientific laboratory.
Last year’s late-season tropical weather—hot and rainy—could have killed many of the bugs, drowning millions of them, reducing their damage and depleting the population going into overwintering sites, Leskey said.
Growers also learned a lot from that first wave of damage, and they were prepared last year with an intensive spray program and insecticides that would kill the bugs. Growers had to resort to older chemicals that they had abandoned because the newer insecticides, more targeted to other insects, wouldn’t kill the stinkbugs.
Nature snapping back?
There is a third possible explanation for what’s happening, and that’s one Dr. David Biddinger hopes is taking place. “Nature is starting to snap back,” he said. In a cute way of thinking about it, he said, “Our existing generalist predators are acquiring a taste for Chinese food.
“All of us are more hopeful that the natives will start taking a chunk out of the population,” he said. “Even a 25 percent reduction would really help.”
Biddinger is Pennsylvania State University’s biocontrol specialist at the Fruit Research and Extension Center in Biglerville. He’s part of the team of entomologists working to combat the brown marmorated stinkbug. He says, integrated pest management, in which predators and parasites are used along with pesticides to control insects and mites, has saved millions of dollars in spray costs.
The new stinkbug, accidentally introduced from Asia about 15 years ago and finally reaching damaging numbers in 2010, caused many growers to abandon the IPM approach, at least temporarily.
“We’re already seeing flare-ups of secondary pests that were being controlled by predators and parasites,” he said. These include woolly and rosy apple aphids, and tentiform leafminers.
One project under way is evaluating a parasitic wasp from Asia that is known to attack brown marmorated stinkbug eggs. It is being led by Kim Hoelmer, a research entomologist at the USDA’s Beneficial Insects Introduction Research Unit in Newark, Delaware. The wasp, in the genus Trissolcus, must pass from quarantine before it is set free in the general environment. It must first be shown not to attack beneficial native stinkbug species, of which about 25 species are predators themselves rather than plant-feeding pests.
While introducing natural enemies of a foreign pest from the region it came from can be a successful strategy, more often they fail to become established or end up eliminating nontarget native species, Biddinger said.
He’s hoping nature will provide a faster and surer solution. A number of our exotic fruit pests, such as codling moth and oriental fruit moth in the eastern United States, are mostly attacked by native biocontrol agents that adapted over time to a new food source.
“We found at least two wasp predators that were specialized on collecting native stinkbugs to feed their young in underground burrows have switched to feeding at the ‘Chinese buffet’ rather than feeding on the less numerous native bugs as they used to.”
Already in 2010, Biddinger and his biocontrol team began looking for indications that the invasive stinkbug was being attacked by native-born predators. There is some evidence that it is being significantly attacked by some of the same predators that control native stinkbugs, of which there are more than 300 species.
“We have lived with low levels of damage in Mid-Atlantic tree fruits from the native brown, dusky, and green stinkbugs for decades now, especially in peaches where they are one of a group of catfacing pests,” he said.
Their numbers have been kept low enough to live with because of perhaps 20 native predators, including predatory sand wasps, lacewings, lady beetles, spiders, ants, assassin bugs, and a dozen species of wasp parasitoids that attack native stinkbug eggs.
Biocontrol is essential
While the initial reaction to the rise of the brown marmorated stinkbug centered on finding pesticides that would kill them in the orchards, Biddinger—and most other scientists—don’t believe spraying is a viable strategy in the long term. There are too many of them ready to invade.
The stinkbugs feed on up to 300 different plants and move throughout the surrounding landscape to complete their development. They overwinter in natural areas or buildings outside orchards and spend only a small part of their lifecycle within orchards. For these reasons, Biddinger believes the only hope for long-term control of stinkbug pests is through natural regulation of populations by predators, parasites, and diseases outside of the various crops it attacks.
One key idea is, don’t rely on one or two predators or parasites.
A concept new to many growers is that biocontrol does not have to give 99 percent control of the pest, like a biopesticide such as Bt does, he said. A number of species, each providing some control of multiple stages of the stinkbug, can greatly decrease the overall population levels that develop outside of the orchards. This reduces the chances of them moving into orchards at levels high enough to cause economic damage.
Another effort is attempting to find pheromones that will act as lures—either of the stinkbug or of its predators—to make the insect easier to monitor.
The basic ecology of the stinkbug is also being studied. “We’re just starting to understand a little more about the host plants they’re using and where they spend their time when they’re not in orchards,” Leskey said.
A new Web site, www.stopbmsp.org, will become active soon, offering state-by-state information about the bug. •