Controlling the stinkers

A brown marmorated stink bug trap. (Courtesy Tracy Leskey)

Some Eastern fruit growers worked with research entomologists last summer to try to recover their Integrated Pest Management system, which was effectively dismantled when brown marmorated stinkbugs invaded their orchards four years ago.

This year, using an effective new pheromone-baited trap that lures males, females, and nymphs, the researchers were able to evaluate a management scheme based on thresholds, a key concept in IPM.

Instead of “keeping covered” with insecticides, growers monitor traps and treat with insecticide when a certain number of adult brown marmorated stinkbugs are found in a trap. The magic number seems to be 10.

Dr. Tracy Leskey, the USDA entomologist who coordinates the nationwide Specialty Crop Research Initiative-funded project from the Agricultural Research Service Appalachian Fruit Research Station in Kearneysville, West Virginia, said things are slowly improving after the invasion of the stinkbug from Asia.

Recalling the fall of 2010 and a panicky meeting in Bob Black’s Maryland orchard where stinkbugs were visibly invading and nobody knew how to kill them, she thinks tremendous progress has been made since then. That year, the mid-Atlantic apple industry lost an estimated $37 million to the insect when apples emerged from storages with deep brown corky spots where the bugs had fed.

The fear and panic is gone now—growers can control the insect—but the next step for growers is to undo the damage caused by having to spray with insecticides that have disrupted the biological control they had achieved using IPM.

“We were able to mitigate the immediate threat by identifying the most effective insecticides,” Leskey said. “But there were significant consequences, such as increased cost of inputs—as much as fourfold greater—and labor required for more frequent spraying. There were also increased costs from frequent outbreaks of secondary pests. IPM programs were devastated.”

Several insecticides are now used in spray programs, but none of them is ideal. Even when lethality is high, residual activity is low and impact on beneficial insects is also high. No insecticides seem to have much residual activity against the stinkbugs, making frequent applications necessary.

In field trials in 2010, Leskey and her colleagues found that more stinkbugs were captured in baited black pyramid traps than in other traps, and that the traps were better placed on the ground than in the trees.

In 2011, a known attractant pheromone (methyl decatrieonate) was found to be effective only late in the season. Then in 2012, a new two-component aggregation pheromone was found. Not only did this male-produced pheromone attract all ages and sexes but, when mixed with the attractant, there was a synergistic effect. “Sometimes two plus two can equal eight,” Leskey said, “and that’s what happened.”

Armed with the new lure, the researchers last year set about matching numbers found in traps to damage found in orchards so they could determine threshold numbers that would trigger decisions to spray.

“We advised growers to apply two ARM (alternate row middle) sprays seven days apart when the cumulative threshold was reached,” Leskey said. “Growers are trying it this year, and so far it’s going pretty well.”

This year isn’t really a good test year because the cool, wet summer across the East, which triggered all kinds of rots, blights, and fungi, suppressed insect activity. By mid-September, however, all apple growers were reaching threshold levels of brown marmorated stinkbugs, she said.

Trapping

The entomologists are also working with four companies that have found the incentive to manufacture the lures, which are nearly ready for commercial sale. The brown marmorated stinkbug has invaded Europe as well as North America, so traps that can monitor the insect will find a large market, Leskey said.

The traps combine the visual stimulus of a black pyramid with the olfactory stimulus of the aggregation pheromone mixed with attractant. There is a capture mechanism that catches stinkbugs in an inverted funnel jar, where they are killed with a toxicant.

Traps are placed along the perimeter of an orchard near places where stinkbugs are often found on wild hosts, usually in or near woodlots.

In the test, the best threshold number was 10 adults per trap. Waiting for more to be trapped resulted in more injury, but treating too quickly—either when only one bug was found in a trap or when traps were ignored completely and growers just used a weekly spray—increased number of sprays and costs but didn’t reduce injury.

“A threshold of 10 adults per trap reduced insecticide applications by 40 percent with no significant difference in injury at harvest compared to weekly ARM,” Leskey said.

That conclusion was reached in 2013 in trials conducted in experimental orchard blocks, and this year growers worked with this provisional threshold to test it further.

One thing the researchers noticed right away was that injury levels in trees nearest to baited traps were higher. “This is an aggregation pheromone,” Leskey stressed.

That means the trees with traps become much more attractive to the insects, which come there quicker and stay longer, leaving other trees less infested.

“We found this combination lure not only attracted the bugs, it retained them longer—more than 20 hours,” she said. “This gives them more time to forage and pick up a lethal dose of insecticide.”

The researchers had found that the stinkbugs frequently recovered after being poisoned if they did not get a lethal dose. In the trap trees, “if they started to recover, they were attracted again and received another dose.” Leskey finds that “very exciting.”

The traps seem capable of doing double duty. Incorporated into a lure or trap, the bugs can be counted and the result used to trigger an insecticide spray.  Or they could be lured to target trees, where they can be killed in great numbers.

Widespread

The brown marmorated stinkbug is now considered a severe agricultural pest and a general nuisance to residents in the mid-Atlantic states of Pennsylvania, Virginia, West Virginia, Maryland, Delaware, and New Jersey. It is also a growing problem in Oregon and Washington on the West Coast and in North Carolina, Tennessee, Kentucky, Ohio, and New York. It is a minor nuisance in Michigan, Indiana, California, Ontario, and five New England states. It has been found in 41 states and three Canadian provinces. •