AgBioResearch

In 1998, residents of Allentown, Pennsylvania, began to notice an unfamiliar insect lurking in the shadows of their homes and gardens.

Though it bore a passing resemblance to a number of stink bugs – with brown, mottled coloration and triangular, shield-shaped carapace – this one had distinctive dark bands on the rear of its wings and lighter colored bands on its antennae.

Entomologists now recognize this as the brown marmorated stink bug – an invasive pest native to China, Japan, Taiwan and the Korean peninsula – which poses major threats to crops and infests homes.

Insects have ranked as one of the greatest challenges for farmers worldwide since the beginning of agriculture. Management methods have become increasingly sophisticated, including strategic insecticide sprays, natural predators and pest-resistant cultivars.

At the same time, the pests have changed. In today’s globalized economy, the threat of invasive pests, which local ecosystems have little natural ability to hold in check, is greater than ever.

Tracking the invasion

The brown marmorated stink bug probably arrived in the United States inadvertently as a stowaway in a shipping container that made landfall on the Atlantic coast.

Entomologists estimate that this occurred several years prior to its first detection in 1998, and it has been slowly spreading west and north ever since. It’s now been found in 43 states and four Canadian provinces. In 2010, the stink bug was discovered in southwest Michigan’s Berrien County, and as of last summer, it had spread to 59 counties spanning both peninsulas of the state.

Julianna Wilson, tree fruit integrated pest management outreach specialist with the Michigan State University (MSU) Department of Entomology, has watched the spread of the stink bug in Michigan from the beginning.

“It’s been a slow invader, creeping its way across the country since we first found it,” said Wilson, co-author of the first brown marmorated stink bug management guide for Michigan growers. “They’re good hitchhikers, so we have to keep an eye on where they pop up.”

In 2015, Wilson sent a notice through MSU Extension asking that sightings of the invader in houses and offices be reported online. Sightings were entered at the Midwest Invasive Species Information Network (MISIN) website, a tool that collects user-submitted data on a wide variety of invasive species and displays it visually on a map.

Since then, reports of the marmorated stink bug in the MISIN system have grown from just eight submissions in 2015 to more than 9,000. The brown marmorated stink bug’s coloration makes it difficult to identify, both because it renders it relatively similar to native stink bug species and it helps it blend into its surroundings.

“It’s a really cryptic insect,” Wilson said. “They’re just a mottled brown that blends in almost perfectly with tree bark, and unless you get a close look at them, they look a lot like the kind of stink bugs that are supposed to be here.”

To help apple and peach growers properly identify the pest, MSU Extension held a workshop in Grand Rapids, Michigan, early this summer. Experts were on hand to teach attendees about the distinctive characteristics of the bug.

With a grant from the U.S. Department of Agriculture (USDA) Specialty Crops Research Initiative (SCRI), Wilson and MSU AgBioResearch entomologist Larry Gut will use a statewide monitoring network to develop a biological model of the insect to better predict where and when it will become a problem for growers.

The SCRI project uses brown marmorated stink bug traps laced with pheromones, which attract the bugs to a sticky panel where they are collected. The traps are spread across orchards, vineyards and vegetable plots at over 70 sites around Michigan.

After making observations, the team will add data to that of similar teams across the nation to improve understanding of its behavior and distribution.

“In the short term, our monitoring work will help growers make decisions driven by scientific data,” Wilson said. “In the long term, it will help us better understand how the insect operates and spreads around the country, and come up with more focused, specific solutions for managing it.”

Defending farms

The greatest threat of the brown marmorated stink bug is damage to crops. Michigan, with 300 potential host plants for it to choose from, including approximately 100 crop and ornamental plants, is a significant attraction.

In 2010 alone, the brown marmorated stink bug is estimated to have caused over $37 million in damage to the apple industry across the mid- Atlantic, with some growers losing up to 60 percent of their crop. Through a long, strawlike proboscis, the stink bug can penetrate the protective epidermal layer of even tough fruits such as apples and pears to feed on the juices inside.

Though this causes nominal damage to the fruit, it creates entry points for bacteria, fungi and other insects, which cause the bulk of the damage much later.

“The damage from this pest often doesn’t appear until three or four weeks after it feeds,” Wilson said. “It can come and go and you wouldn’t even know it was there until it was too late.”

MSU AgBioResearch insect ecologist Ernest Delfosse, professor emeritus in the MSU Department of Entomology, has spent the past five years developing methods to help Michigan farmers combat the brown marmorated stink bug.

“Since it targets such a wide range of crops, the brown marmorated stink bug affects a huge segment of agriculture in Michigan,” Delfosse said. “Growers need to have tools to manage it, especially sustainable, long-term, low-input, low-cost methods.”

Two management strategies are being developed to control brown marmorated stink bugs and protect crops. The first employs a series of traps with pheromone lures called ghost traps, which draw the pests away from crops and ensnare them in a net laced with insecticide.

This effectively kills the bugs and has the potential to lower costs for farmers by reducing the need to spray insecticides directly on crops. The shortcoming of this trap-and-kill strategy is that it fails to take proactive measures to reduce the overall population of the pest.

Brown marmorated stink bugs venture into farm fields only during the day to feed – they retreat at night to the shelter of nearby tree canopies, where they also lay their eggs. To strike the pest at its source, Delfosse and his lab have been exploring a second management option: biological control through natural predators.

“The population of brown marmorated stink bug is growing in Michigan, and we’re on track for some major infestations,” Delfosse said. “You can’t spray all the trees in the forest, so you need something that attacks the eggs to really reduce their numbers.”

To gather a better understanding of both the pest and its potential weaknesses, scientists from the USDA traveled to eastern Asia, where they studied its life cycle to determine when it is most vulnerable and which natural predators attack at each stage.

They discovered that a number of wasp species prey on the eggs of the insect. One of them, Trissolcus japonicus or the Samurai wasp, found throughout the brown marmorated stink bug’s native range, showed particular promise.

The Samurai wasp, a small, black insect measuring no more than 2 millimeters in length, is a parasitoid that reproduces exclusively by implanting its eggs in the eggs of brown marmorated stink bugs. Upon hatching, the wasp destroys its host.

A female Samurai wasp carries 42 eggs, making her capable of wiping out entire stink bug egg masses. Introducing new species always bring risks, so, to determine if the diminutive wasp would be effective at controlling brown marmorated stink bugs without causing additional damage to native ecosystems, Delfosse’s lab, through a grant from USDA’s Animal and Plant Health Inspection Service (APHIS), conducted host specificity testing.

The researchers exposed the eggs of brown marmorated stink bugs and 28 native stink bug species to the Samurai wasp under quarantine conditions to see whether the new wasps posed a threat to the native species.

“In classical biocontrol, like this, you want to reunite the pest with its key natural enemies,” Delfosse explained. “But you need to make sure you’re not introducing something that could become a new invasive pest in its own right.”

Ultimately, Delfosse’s team determined that the Samurai wasp posed too great a risk for a large-scale introduction into North America. They found that the wasp fed on 10 to 15 types of stink bugs, including the brown marmorated stink bug.

The team prepared to move on to the next likeliest natural predator for testing. Nature, however, apparently had other plans. As the team and their APHIS colleagues were readying a new round of tests, reports came in from the USDA that Samurai wasps had appeared in fields in both the western and eastern United States.

After genetic testing, USDA scientists determined they had not escaped the lab quarantines and had likely arrived in the same way their prey had years before, in shipping containers.

Delfosse’s lab has continued studying potential impacts in the wild through sentinel egg testing. This involves scientists depositing masses of stink bug eggs, both native and invasive, waiting until the wasps implant the eggs, then taking them back to the lab to see which species of wasps emerge.

This will allow the team to track whether Samurai wasps have entered Michigan and whether native wasp species are now adapting to the presence of brown marmorated stink bugs.

“Since Trissolcus japonicus is already out there and moving, the only ethical thing to do is to evaluate it through more study,” Delfosse said. “We know from 150 years of classical biocontrol on everything from mites to weeds to mammals that sometimes environmental factors mediate the spread of natural predators, which can therefore mitigate the harm they might cause. We need to determine if that’s going to be the case. If it is, this has a very good chance of being a successful program.”

Trouble for homeowners

Though the most concerning issue for Michigan is the danger to crops, the pest has also become a nuisance for homeowners around the state. In fact, homes are where the pest tends to appear first.

“A lot of the calls we get here at MSU are from homeowners,” Delfosse said. “Back east, where it’s been around a lot longer, we’ve seen as many as 20,000 individual bugs overwinter in a single house. People used snow shovels to get them out.”

Most infestations fall well short of that mark, however. In the Lansing area, a house might have between a few dozen to a hundred stink bugs attempting to outlast the winter, but that number is expected to rise as the insect spreads.

Howard Russell, entomologist with MSU Diagnostic Services, points out that besieged homeowners already have multiple tactics at their disposal to expel these invaders.

“Brown marmorated stink bugs generally seek shelter inside houses and other buildings in the fall,” Russell said. “While they aren’t known to bite people or pets or spread disease, they do produce a pungent odor that’s readily transferred.”

Though the insects can be removed by manual means, such as brooms or vacuums, Russell advocates taking more proactive measures to ensure long-term prevention. Caulking or otherwise sealing cracks and other openings in the exterior of a building can be effective at reducing the number of entry avenues.

This is significantly less effective in homes with vinyl siding, which is loosely attached to the walls so that it can expand and contract with temperature changes.

Applying insecticide sprays around the outside walls of homes in September and October, when the insects begin their inward migration, is another effective approach. And if all else fails, caulking indoors – around electrical outlets, ducts and ceiling lights – can at least confine the insects to the inside of the walls and keep them away from living spaces.

“It will probably still be a few years before we see the numbers they get back east,” Russell said. “But it’s better to be prepared now.”