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Scientists, Farmers Continue Battle Against Stinkbugs

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There are between 250 and 300 species of stink bug in the U.S., and some of them are very helpful as predators of agricultural pests.
Sabri Ben-Achour
There are between 250 and 300 species of stink bug in the U.S., and some of them are very helpful as predators of agricultural pests.

A year and a half ago, Bob Black was not in a good place. Standing one misty April morning in his budding fruit orchard, he took a deep breath.

"This thing is gonna put a big chapter in my book of life," he says. "I mean I've never had anything affect me like this."

Black runs Catoctin Mountain Orchards in Thurmont, Md., and he was contemplating another year of assault by Brown Marmorated Stinkbugs. Many homeowners will recognize the little brown — and smelly — shield shaped insects. They are invasive insects from Asia, and they disfigure crops by piercing the flesh of fruits and vegetables with their long needle like mouthpieces.

They've been spreading from Allentown, Penn. where they were introduced in the 1990s, and in 2010 they really hit Virginia and Maryland hard.

Black says he sustained a 50 percent damaged to the harvest of one of his popular apple varieties, Pink Ladies. "I can handle a few percent, but [when it] gets up to 25 percent, 50 percent, that's pretty devastating to me."

Fast forward to the present day, and things have settled down from an arthropod Pearl Harbor to a war of attrition. This year has been better than normal, but Black says the stinkbugs are still around.

"We do have some damage again, but nothing like 2010, which I never want to go through that again," he says. "But we had an exceptionally large apple crop here in the east, we were very lucky, so you could say we were able to make up some of the loss."

Black has found some pesticides that work, but they require regular spraying and constant vigilance. Stinkbug attacks are impossible to predict because they can come out of nowhere, lying in wait in a neighbor's field or some woods up the mountain side. Black says one time he was saved just by luck after his neighbor cleared a wheat field where swarms were hiding.

"One of the field folks called me immediately and says, 'Bob I'm not sure what happened here, but your count went up to 300 from 20.' Luckily I happened to be spraying over that weekend."

They come from anywhere

That's really one of the big problems with these bugs — they can do massive damage. The population is enormous and they can come out of nowhere because they can survive everywhere. It's not like, say, Japanese woolly aphids that rely on just one or two plants. Stinkbugs like wheat, they like corn, they like tomatoes, they like woods, they like weeds.

"The fact that it has so many hosts adjacent to commercial plantings means that there's this potential for constant immigration," says Chris Bergh, an entomologist at Virginia Tech.

In 2010, growers in Virginia lost $37 million in insect damage to apples alone. And while 2012 was horrible for a few farmers, overall it hasn't been so bad. But that's little comfort because nobody is entirely sure why, says Bergh.

"For some reason we don't understand there was high nymphal mortality in the fall of 2011 and that translated fewer adult bugs in spring 2012."

Basically a bunch of bugs mysteriously died in 2011. The early spring helped too, so fruit trees had an early start against the bugs.

If all that meant 2012 was an off year, 2013 looks like it'll be a big one.

"What was interesting in 2012 is those populations have essentially recovered, and we're seeing populations that are six times larger than they were the previous year," says Tracy Leskey, an entomologist with the U.S. Agricultural Research Service.

Leskey has been working on developing pheromone traps so that the bugs can at least be counted properly. Researchers have made some other key discoveries, too. The stinkbugs love to attack fruit trees right after they're pollinated, and at least one pesticide has been approved by the EPA on an emergency basis to treat against stinkbugs.

But one of the most anticipated lines of research has been going on in an agricultural research service lab in Newark, Dela.

Reunion with nemesis is nigh

In spring 2011, Kim Hoelmer was trying to figure out if it was safe to introduce an Asian parasitoid wasp into the U.S. to attack Brown Marmorated Stinkbugs — basically to reunite the stinkbug with its ancient evolutionary nemesis.

"These small wasps will deposit their eggs inside the stinkbug eggs," says Hoelmer. "Then the parasite egg hatches, and the immature feeds on the inside of the stinkbug egg."

The tiny non-stinging Trissolcus wasps had evolved in Asia to like Brown Marmorated Stinkbug eggs and only Brown Marmorated Stinkbug, in fact to depend on them for reproduction.

"If they can't find stinkbugs or stinkbug eggs to lay their own eggs in, they'll die," says Hoelmer. "They can't survive on anything else.

That was how things looked in 2011. There are some fuzzy signs that, under some circumstances, the wasps may enjoy parasitizing some of the 300 species of native stinkbug eggs now and then.

"We've been through about 20 species of non-target stinkbugs, 30 actually, and half are unambiguous — the Chinese Trissolcus will not attack the native stinkbugs in those cases. So we know there will be no impact. In the others we see some sign of attack."

It's not a good sign, but Hoelmer is quick to note that it doesn't mean the wasps risk environmental havoc, because the level of attack is low, and he believes wouldn't occur in the wild. But he's still testing.

The testing goes like this: lock the wasps in a box with the native stink bug eggs. If the wasp dies without attacking the eggs, then there is no way the wasps present a threat. But sometimes they will parasitize one or two eggs out of 30. In those cases, the wasps that emerge from those eggs are stunted and die quickly.

So Hoelmer moves on to "choice testing:" lock the wasps in a box with both native stinkbug eggs and invasive stinkbug eggs and see which one the wasp prefers. In those circumstances, the wasps usually ignore the native stinkbug eggs.

Finally, Hoelmer says, the wasps may behave differently when they are locked in a box with something they'd rather not attack, versus when they are in the wild and could fly away from the unappetizing bait. In the box, he says, the wasps may just eventually resign themselves to attacking because there is nothing else available.

So testing continues.

He and his colleagues have 14 new species of Chinese wasps they're going to look at in search of the ones that might be a perfect match for the invasive stinkbug.

At the earliest, Hoelmer might get clearance to start releasing approved wasps next fall. Meanwhile, farmers across the mid-Atlantic are gearing up for a big year of war with airborne armies of stinkbugs in their fields.

[Music: "Opening Titles (Z's Theme)" by Mark Mothersbaugh from ANTZ Soundtrack]


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