WINCHESTER — There are indications the local stink bug population may be making a resurgence.
Christopher Bergh, an entomologist at the Alson H. Smith Agricultural Research and Extension Center in Frederick County, said the number of brown marmorated stink bugs in the Northern Shenandoah Valley has gradually declined since peaking in 2012, but this year, it appears more bugs than expected are seeking shelter for the winter.
The 124-acre research facility southwest of Winchester conducts official stink bug counts each year from Sept. 15 to Oct. 15. This year’s count has not been completed, but Bergh said anecdotal evidence from area homeowners, apple growers and Extension Center staff makes him believe the numbers have “jumped big time.”
In recent years, brown marmorated stink bugs have been attacked from several fronts — fruit growers spray pesticides to protect their crops, a tiny wasp destroys their eggs, and a naturally occurring fungus has proven to be deadly to the insects.
Why, then, are people seeing more stink bugs this autumn?
The brown marmorated stink bug is native to Asia and first appeared in the United States in the 1990s after apparently hitching a ride on international shipping crates.
With few natural enemies in the U.S., stink bug populations increased dramatically as females laid as many as 400 eggs during their six- to eight-month lifespan.
Stink bugs feed on an array of crops, including apples, Asian pears, cherries, corn, grapes, peaches, peppers, tomatoes and soybeans. In 2010, the U.S. Apple Association estimated stink bugs caused an estimated $37 million in losses to Mid-Atlantic apple growers.
The tide began to turn in 2013 as pesticides started impacting stink bugs in local orchards and farms. Insect numbers further declined starting in 2014 when a tiny natural predator commonly known as a samurai wasp started destroying stink bug larvae. Now, research has found a fungal pathogen that, at least in laboratory experiments, is deadly to stink bugs.
Bergh said it will take years to determine if the fungus is responsible for depleting stink bug populations in the wild, but he suspects it is.
“That’s highly speculative,” he said. “That’s not the gospel by any means.”
Fungi thrive in cool, damp conditions, but this year has been hotter and drier than last year. Because of that, Bergh said he can’t be sure if the fungal pathogen is truly having an impact.
It is also possible that a percentage of brown marmorated stink bugs survived initial exposure to the fungus and went on to reproduce, resulting in a growing number of insects appearing late in the year, Bergh said.
Another potential explanation for the apparent resurgence of stink bugs is a declining use of pesticides in local farms and orchards. Samurai wasps caused significant damage to stink bug populations, so some growers started spraying less than they did five or six years ago.
“They’re backing off to some degree,” Bergh said. “We may need to be more vigilant next year.”
Even if stink bugs are bouncing back, Bergh said he doesn’t foresee their numbers rivaling those from 2012, when the insect population was at its peak.
“It looks like we’re beyond the worst period of the brown marmorated stink bug,” he said.