The brown marmorated stink bug is back for another season of damaging Pennsylvania fields, but it’s still too early to tell just how much of a menace the mottled, malodorous pest will pose to farmers this year.
“It seems like a very simple question, but to answer responsibly is very difficult,” Extension entomologist Greg Krawczyk says.
That’s partly because stink bugs do not go directly from houses and other winter shelters to fields, he says.
Most stink bugs are currently in the woods, where they live after leaving their winter shelters but before nearly ripe crops have caught their attention. Counting stink bugs in the woods is more difficult and less valuable than studying their behavior around crops.
Researchers are also still working on ways to measure spring populations of stink bugs, which could help predict how great of a problem the insects will pose during the growing season.
Krawczyk expects farmers will see more stink bugs within three weeks, when soybean pods and mature sweet corn will be more widely available.
stink bugs are showing up on apple, peach and cherry trees, and causing some isolated injuries to fruit, but Krawczyk says this damage seems to be from migrating adults at this point rather than populations infesting an orchard for weeks at a time.
He is also finding eggs and nymphs in orchards. When those stink bugs come to maturity, around mid-July, traps will start working much better, and population estimates will become “more reliable,” he says.
Krawczyk’s traps are capturing two to three times more stink bugs than they were at this point last year, but stink bug problems are often highly localized. Some growers have ongoing problems with stink bugs, while others see them only in September, when fruit is nearly ready for sale.
“That’s when (stink bugs) do their damage,” Krawczyk says. “If you’re not on the lookout all season long, you may be very surprised.”
The severity of the stink bug problem is difficult to compare with previous years because, although stink bugs were first noted in Allentown in 1998, they didn’t explode as an agricultural pest until 2010, after which research efforts were increased.
Compared with other fruit-damaging insects, such as the Oriental fruit moth, which Krawczyk notes researchers have been compiling data on for almost two decades, the stink bug’s statistical record is still thin.
Still, researchers know much more about stink bug habits, such as where to look for them, than they did in 2010.
Spraying is still an important tool for controlling stink bugs, killing 90 to 95 percent of the insects in the field at the time of spraying. Lingering pesticides do not affect stink bugs, however, which means they can enter a field after the initial pesticide application and be fine.
Krawczyk says that finding alternatives to spraying is key to controlling stink bugs as agricultural pests.
He and others are experimenting with ways to deter or attract stink bugs — “anything that can modify the behavior of the insect,” he says. “If we use (them) the right way, we can manage the whole movement of the stink bug.”
For example, an orchard owner might plant some rows of wheat around his trees and spray the wheat with a chemical unsavory to stink bugs, keeping them out of the orchard.
Some growers are also trying mechanical plastic nets that go over their orchards. The nets are sprayed with substances that lure stink bugs — but also with insecticides. The technique is inspired by mosquito netting used in Africa that is sprayed with human-safe insecticides.
Attractant netting would save horticulturists from having to spray their entire orchards.
This year Krawczyk is also testing light traps, in which a lamp of 1,000 watts or more is used to attract and then collect the pests.
Krawczyk says he captured a “huge number” of stink bugs with light traps last year.
He cautions that light traps are not a proven pest-control method, and he is too early in his research to know whether he can recommend their use.
Steve Jenkins, a Grove City College entomologist, explains that light traps typically work by luring stink bugs into a chamber (in do-it-yourself assemblies this might be as simple as the smooth inside of a soda bottle) that frustrates the insect instinct to walk upward when faced with a barrier. The insects become trapped and die.
Jenkins also is not sure how effective commercial light traps and homemade contraptions are.
Despite the challenges posed by stink bugs, Jenkins says, humans have been able to control invasive species in the past.
Pesticides can reduce insect numbers, but he says that introducing a natural enemy, called a biological control, can be more effective.
He notes that a decapitating fly has been used to combat the spread of fire ants, and several species of beetle have been introduced on his campus to eat the invasive purple loosestrife plant.
While he has not seen scientific evidence, Jenkins has received anecdotal proof that these biological controls are working.
The USDA is currently experimenting in a Delaware laboratory with a nonstinging parasitic wasp that preys on brown marmorated stink bugs in their native China.
Jenkins says such pre-release testing is necessary because “you don’t want the (species) you release to become a pest as well.”
Krawczyk says a contact in Maryland recently showed him a bluebird feeding a brown marmorated stink bug to its hatchlings. While the photograph does not prove much scientifically, it does offer some hope that predators native to North America may find a taste for the increasingly familiar bug.
Generalist predators such as spiders, lacewings and preying mantises might also help control stink bug populations, Krawczyk says.
It’s only a “matter of time” until other animals work stink bugs into the American natural system more completely, he says.
Of course, controlling stink bugs is different from eradicating them from North America. No matter the effort humans put into killing stink bugs, Jenkins thinks the critters are “here to stay” and that we will “just have to put up with them.”
While he says, “Don’t feel bad about flushing (stink bugs) down the toilet,” Jenkins cautions that “pest does not mean just an insect that you don’t want around.”
Jenkins says the desire to ruthlessly destroy stink bugs should be tempered by the recognition that, just like insects that have beneficial effects on crops and that make less obnoxious houseguests, the stink bug is “a creature created by a marvelous God.”
“It certainly makes it more meaningful to study,” Jenkins says.