by Sally Colby
When the brown marmorated stink bug (BMSB) was first discovered in the United States in early 2000, entomologists scrambled to figure out what to do about the exotic pest that seemed to have a taste for nearly every fruit in the orchard.
Entomologists’ early efforts were aimed at learning about the reproductive habits and feeding preferences of the BMSB. Dr. Tracy Leskey, USDA entomologist, began working on the BMSB issue fairly soon after it was first discovered. She credits Robert Black of Catoctin Mountain Orchard in Thurmont, MD, for raising the profile of the new invasive. “We were driving around the orchard looking at peaches that had just been sprayed with Lannate®, and 60 stink bugs flew out when someone bumped a tree,” she said. “At that point, we didn’t know much about the bug – we didn’t know all the plants it was feeding on or how to kill it. At the beginning, the effort was to figure out what killed the bug because we had growers dealing with knockdown and recovery – bugs were recovering and climbing back up into trees.”
Leskey, who is based at the Appalachian Fruit Research Lab in Kearneysville, WV, says that over the past six years, a lot has been learned through research and grower observations. “It’s a border driven pest,” she said. “The most damage is at the borders of the orchard. Bugs are coming in from woodland trees; particularly wild hosts like tree of heaven, paulownia, locust, black walnut and some row crops like soybeans, which are also a good host.”
According to Leskey, recent research is based on the idea that growers could move away from the calendar based sprays that were necessary when the stinkbug first hit. “We really didn’t have a good sense of the phenology of the bug and what was going on in our orchards,” said Leskey as she described the initial BMSB management. “When we needed to start spraying and when we could stop spraying. And do we need to spray the entire orchard, or can we get away with border sprays or attract and kill sites?”
Although growers soon learned what would kill the BMSB, there was no good information on how to effectively decrease insecticide use. Two key projects helped uncover more effective management strategies: A Specialty Crop Research Initiative (SCRI) project and a Sustainable Agriculture Research and Education (SARE) project.
Dr. Rob Morrison, researcher at the Appalachian Fruit Research Lab in Kearneysville, WV, focused on the behavioral aspect of the BMSB for an attract-and-kill strategy. Morrison explained the attract-and-kill requires four components: Attract bugs to the site, retain them there for a long period of time, kill them and have little spillover to neighboring trees.
Morrison said prior to 2012, there was only one attractant, MDT, a pheromone produced by another Asian stinkbug that is cross-attractive to the BMSB. Researchers recently identified the BMSB aggregation pheromone, which attracts males and females of all stages of BMSB.
“We found that when we combine these stimuli, we get a synergistic effect and greater attraction,” said Morrison. “This has been pivotal for the attract and kill system.”
Designated attract-and-kill trees are hung with numerous cups containing pheromone that attracts adults and immature stages of the BMSB. The goal is for all of the bugs to come to the tree, then the tree is sprayed regularly with an insecticide to remove those bugs from the foraging population.
In preliminary trials in 2013, four trees were baited in late season, then sprayed. Adults that fell onto tarps below treated trees were counted to determine the kill rate. “Over a six-day period, we killed 30,000 adults and over 5,000 nymphs,” said Morrison. “Those are high numbers. We found also found that in using more pheromone, we got higher kill.” But how can growers be sure that stinkbugs aren’t moving into the interior portion of the orchard? In studies conducted in 2014, tarps were placed under baited and nearby un-baited trees to determine the level of spillover. “We found that 90 percent of adults and nymphs will be at the baited tree, with very little spillover,” said Morrison. “Damage was concentrated on one tree with no elevated damage on neighboring trees. We can get good attraction, good retention and reduced damage in the interior of the orchard.”
Brent Short, USDA support scientist based at the Appalachian Fruit Research Lab in Kearneysville, WV, says that during the early days of BMSB management, sprays were only holding a certain percentage of the population at bay. “We knew the bugs were everywhere,” he said. “What did we do about that? Used broad spectrum pesticides – pyrethroids, neonicotinoids, organophosphates – at unknown intervals; as frequently as possible because we saw bugs when we were out thinning or picking.” However, that approach is expensive and not sustainable. Ideally, treatment intervals would be regulated so that each treatment would be as effective as possible. The first step toward that goal was to develop a trap and a lure that would bring bugs to the trap. Various ground-based and tree traps were tested in multiple trials.
“The most sensitive trap for the BMSB is a tall, black pyramid,” said Short. “The black color is the most visually effective and is designed to act as a visual stimulus for the bug. It’s consistent with both nymph and adult behavior, and that behavior is to move upward. They hit the surface of the trap, climb upward into a funnel and into a collection jar.”
The other critical part of the trap is the olfactory attractant. One of the first attractants included the MDT pheromone. However, MDT only worked in late season. Once the actual BMSB pheromone was identified, USDA chemists formulated the pheromones, and caught bugs throughout the season during testing. When the BMSB pheromone was blended with the MDT pheromone, the synergistic effect dramatically increased attraction. The next challenge was to keep the bugs in the trap long enough to count them, since the main point of the traps was to monitor counts to determine optimum treatment time. Further research showed that an impregnated vapor tape inside the jar would gas and kill the bugs.
“Now we had a trap that would actually capture the BMSB,” said Short. “What do we do with that information? All we know is that we have a bug in the trap. Can we relate the information in that trap to a management decision?”
Rather than designating a certain day of the week as the management trigger, researchers wanted to base treatment on a threshold. “That was the goal of developing our treatment threshold,” said Short. “With that lure system, we treated plots based on numbers of bugs captured in traps. There were several treatments: Unsprayed plot, or spray every seven days regardless of how many bugs were present; then spray other plots based on how many bugs were in the traps. There were plots we sprayed if there was one bug in a trap, and plots we sprayed if there were 10 bugs or 20 bugs. It was based on a cumulative number, and didn’t have to be all in one week.”
When researchers looked at fruit injury in the interior of the plot, they found significant damage. “If you wait to spray until there are 20 bugs in the trap, there’s a lot of damage (overall),” said Short. “But if you spray every week, which is a lot of unnecessary sprays, or when there’s one bug or 10 in a trap, then you have substantially lower injury. So in waiting until there were 10 bugs in a trap or spraying every week, there was no difference in fruit infestation, and this established the threshold: 10 bugs.”
Short says that if growers use a threshold-based trap in a high-pressure year, the number of sprays can be decreased by at least 40 percent without sacrificing fruit quality. “You’re timing sprays based on need rather than on guessing,” said Short. “In a low population year, the trap system will tell you whether you need to spray at all versus spraying every week or spraying just because it’s August. That means not wasting money on spray or killing beneficials that are present.” However, in that system, the entire plot is still sprayed. Short says growers can do a complete plot spray or perhaps two alternate row middle sprays spaced seven days apart.
With effective trap and kill system, growers can adopt a perimeter-based management strategy. “Stink bugs move from outside of the block, find something tasty and stop,” said Short. “If we can use perimeter-based management against them, we can increase efficacy without having to treat the entire block with chemicals. Attract and kill is one technique for accomplishing this, and we can use very high dose of pheromones. The concept is that the bugs come to the tree in high numbers, then the grower sprays the tree that is baited and usually the adjacent trees. That is done weekly.” Short noted that while bait trees become damaged, the interior of the block is protected.
Leskey says the bottom line is that long-term control (for BMSB) has to be biological. “We need biocontrol agents to deal with these bugs on the borders of orchards and reduce populations,” she said. “We’re trying to facilitate this as much as possible.”
Researchers refine stinkbug management
by Sally Colby