Mar 5, 2012
Tiny wasp could be stink bug’s downfall

Starker Wright shared a horror story with New York state growers in January.

A scientist with USDA’s Agricultural Research Service, Wright was a speaker at The 2012 Empire State Fruit & Vegetable Expo in Syracuse, N.Y.

The villain of Wright’s horror story is the Brown Marmorated Stink Bug (BMSB), a voracious killer from a foreign land who will eat just about any crop and appears nearly impossible to kill.

Wright works in the insect behavior and ecology program at ARS’ Appalachian Fruit Research Station in West Virginia. He said BMSB has done so much damage to fruit, vegetable and other crops in the mid-Atlantic region that about 90 percent of the program’s focus these days is on finding ways to fight the bug.

BMSB is an invasive species native to China, Japan, Korea and Taiwan. The bug came to the United States (probably from northeast China) around 1996, via Allentown, Pa. ARS researchers first detected the bug in Hagerstown, Md., in 2003, and confirmed it in West Virginia in 2004. By 2008, they started getting reports of weird, late-season injuries in apples, but they didn’t realize at first that BMSB was the cause, Wright said.

In 2009 and 2010, BMSB damage “exploded” in an “extraordinarily broad” range of host crops in mid-Atlantic states. Grower losses in Pennsylvania, Virginia, West Virginia, Maryland, Delaware and New Jersey have been “profound,” he said. The bug has spread to other parts of the country – 35 states and counting – but its population still centers around its introduction point, Wright said.

Tracy Leskey, research entomologist at the Appalachian station, said BMSB feeds on more than 300 host plants including tree fruit, small fruit, grapes, vegetables, ornamentals and row crops.

“If we can grow it, they can eat it,” Wright said.

Wright laid out other reasons BMSB is such a tough, destructive pest in the mid-Atlantic region. The bug breeds two generations per year there, making it a season-long threat. Every life stage of the bug after first instar nymph feeds on the host plants. BMSB can survive winter in great numbers, whether in pole barns, garages and woodpiles or more natural shelters like dead trees and fallen logs. They often reside in the tops of tree canopies, where it’s impossible to spray.

Pesticides labeled for native stink bugs are mostly ineffective against BMSB, Leskey said, and the invasive bug exhibits avoidance behavior that shows it has the capacity to escape sprays and other treatments.

To top it all off, BMSB has no natural enemies in the United States, Wright said.

Killing the killer

ARS’ BMSB research is part of a national effort to push back at the invasive pest. Scientists across the country are working as rapidly as possible to develop management programs. Right now, BMSB management ideas are based mostly on chemical control. As researchers learn more, however, they expect to integrate cultural, biological and attract-and-kill strategies, according to Leskey.

Wright put it this way: The short-term goal is to figure out how to kill BMSB. The medium-term goal is to figure out where and when to kill it. The long-term solution will probably involve biological control. Meanwhile, a project is underway to identify a predator or parasite that can contain BMSB.

At the ARS quarantine laboratory in Newark, Del., research entomologist Kim Hoelmer is evaluating a tiny wasp, in the genus Trissolcus, from Asia. The Trissolcus wasp is a parasitoid that attacks BMSB’s eggs.

Hoelmer’s team found the wasp on a trip to China, South Korea and Japan. They collected live egg masses of BMSB and brought them to the ARS lab in Delaware (USDA permits allowed them to import the specimens). Trissolcus wasps emerged from most of the egg masses, and the team used those to establish research cultures, according to Hoelmer.

He’s discovered that female Trissolcus wasps search the surfaces of host plant leaves looking for BMSB eggs. When the wasps find egg masses, they deposit their own parasitoid eggs inside each BMSB egg. After the Trissolcus egg hatches, it grows into an adult wasp by completely consuming the BMSB egg. Since BMSB eggs are usually laid in clusters of 28, each cluster can produce as many as 28 new wasps, according to Hoelmer.

The research has been underway for a year or two, and Hoelmer hopes to release Trissolcus into the field in the next one to two years.

Permission to introduce exotic species, however, is closely regulated by USDA’s Animal and Plant Health Inspection Service. Hoelmer must demonstrate that the wasp will only attack stink bugs and will not significantly affect other species.

According to Hoelmer, Trissolcus wasps are specific enemies of stink bugs and do not attack other kinds of insects, animals or plants. The only potential risk he sees is the threat posed to other stink bug species. Some native stink bugs are pests, but others are beneficial because they prey on other insect pests.

“A common concern we often hear expressed is that newly introduced parasitic wasps might move to other hosts once they are here and once BMSB populations have been reduced to low levels, or will sting people and animals,” Hoelmer wrote in an email. “This will not happen because Trissolcus are ecologically, behaviorally and physiologically tightly linked to their host species, and they are incapable of surviving in other types of hosts.”

In general, parasitic wasps are incapable of stinging a person or animal due to their tiny size (Trissolcus adults are one-twentieth of an inch in length), he wrote.

BMSB in New York state

The Brown Marmorated Stink Bug hasn’t hit New York like a “tsunami” as it did in the mid-Atlantic region.

However, the invasive bug has brought its own challenges to the state, according to Peter Jentsch, a senior Extension associate with Cornell University who spoke during The 2012 Empire State Fruit & Vegetable Expo.

Once BMSB nymphs reach the second instar stage, they start feeding on crops – not the case with the native green and brown stink bugs growers have contended with in the past, Jentsch said.

BMSB adults aggregate in overwintering sites, as well as on commodities. At this point, researchers believe there’s only a single generation of BMSB per year in New York. However, there could be a partial second generation in some areas, depending on the season, he said.

In spring, BMSB migrates to forest hosts or agricultural commodities. Last year, they moved directly to deciduous host trees, but there wasn’t much movement off those host trees into agricultural commodities. Jentsch wasn’t sure why the ag crops were spared. Maybe the BMSB population wasn’t high enough, or maybe the seasonal conditions weren’t right, he said.

The first BMSB specimens were found in the Hudson Valley in 2008. They gradually increased in the southern part of the valley, and by 2010, they were moving across much of the state. Cornell personnel have created a working group to find ways to monitor and manage the pest, Jentsch said.

By Matt Milkovich, Managing Editor

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