Parasitoids provide effective biocontrol of stinkbugs

While moving from the East Coast to California and shipping their belongings, a family inadvertently shipped a stowaway they later found in their barbecue grill—a brown marmorated stinkbug. This agricultural pest also managed to hitchhike across the country inside a box containing a new electric drill that a customer bought from a local store.

Storage containers, potted plants, packages, cars, airplane cargo—the stinkbug is not very particular about its mode of transportation in its quest to be a world-traveling pest.

"It's an invasive species that first arrived on the East Coast in 1998. It's of Chinese origin and likely came from around Beijing, via air cargo or a container," said Mark Hoddle, entomologist and director of the Center for Invasive Species Research at the University of California, Riverside.

"It's part of global trade. We're not just victims, we also end up exporting bugs accidentally," he said.

The stinkbug, so named because it releases a foul odor when it's crushed, is a major agricultural pest in Virginia, where there are a lot of apple orchards. It's also infected plums, pears, peaches, cherries, berry crops and grapes in many Eastern states, including Pennsylvania, Georgia and upstate New York. It took about seven years to migrate from the east to the west, with Oregon also reporting recent sightings.

In California, Hoddle's team has found big populations in urban areas like Pasadena, Hollywood, Thousand Oaks and Simi Valley. Homeowners called Hoddle earlier this year when they found large numbers of the bugs clustered in concrete cracks and inside buildings.

He said this is how pests typically spread, from urban to agricultural areas, which is why entomologists want to stop its spread before its population explodes and it migrates to agricultural zones, causing large-scale damage.

The stinkbug feeds on fruits with piercing mouth parts that suck the juice, leaving markings that make an apple look like a cat's face. On the inside, it leaves brownish stains on the fruit, making it look badly bruised with numerous spots the size of a nickel or quarter.

By 2010, the stinkbug had become the primary pest for apples on the East Coast, replacing the codling moth, and damages resulted in losses of about $37 million to the agricultural sector there.

In fruits like berries, clusters of stinkbugs can make them shrivel up and turn brown or have feeding wounds that allow other pathogens like beetles to get into the fruit and feed on it. Young berries that are infected die before they mature, or whole bunches of them will drop from the vine.

"If the stinkbug is crushed inside grapes during winemaking, it releases a foul odor that taints the wine too," Hoddle said.

During winters, the stinkbugs form overwintering aggregations, when large populations cluster together and often invade homes, where they have been found in attics. They resist removal by releasing their foul odor.

Hoddle warned that this habit of theirs is significant for California, because they could target produce-packing areas during winter and get into packing crates, possibly getting shipped to other countries alongside fruit exports.

The stinkbug has a shield-like body with mottled brown coloring and antennae with alternating light and dark bands, very similar to California's native stinkbug species.

The California Department of Food and Agriculture is funding a study that Hoddle leads, with a team of researchers drawn from UC Riverside, UC Cooperative Extension and CDFA, focused on finding how widespread it is in the state.

The research is also part of the United States Department of Agriculture biological control program for the stinkbug. The USDA found that the stinkbug population could be controlled by certain parasitoids that lay their eggs inside the stinkbug—when the eggs hatch, the larvae of the parasitoids eat the stinkbug's own eggs, then emerge outside as tiny wasps looking for more stinkbug eggs.

The parasitoids act as biological control agents, even though they don't target adult stinkbugs, only their eggs. In China, Hoddle estimated that about 50 percent of stinkbugs are killed by the parasitoids, but he said in the U.S. less than 5 percent of the bugs are parasitized, so the vast majority of eggs end up becoming adult stinkbugs.

So Hoddle's team has sourced parasites from China, and the USDA has quarantined them in a facility in Delaware. Once released, the team will test them in California to make sure they don't damage the state's native stinkbugs—since the native bugs are helpful as predators of other crop pests.

Once the safety tests are completed, in about 18 months, the researchers will present an environmental assessment report that will be reviewed by the USDA animal and plant inspection service, which will then recommend whether the parasites can be released.

On the East Coast, scientists have studied the biology of the bug at length, so Hoddle's team is focused on quarantine studies and surveys to see how widespread it is in California.

The surveys have mainly been conducted in Southern California, but Hoddle said he thinks it's possible the bug has spread to many more areas of the state, given how it travels.

"This last fall, there were reports of over-wintering aggregations in Sacramento. So we know it's there, just how widely spread it is we don't know," he said.

So far, there have been no reports of damage to commercial crops in the state, and Hoddle wants to keep it that way for as long as possible.

Aside from berries and grapes, he is concerned about citrus fruits, since the stinkbug is a citrus pest in China. And he has personally observed the bugs targeting kumquats, which looked like shriveled raisins when he went back to check on them 10 days later.

The stinkbug also seems to like attacking ornamental plants in urban areas, which may be why it multiples so rapidly, since urban areas have more ornamentals.

Hoddle thinks the pest could attack table grapes, raisin grapes and winegrapes, pistachios and almonds, and vegetables like sweet corn, peppers and tomatoes, based on reports from the East Coast.

"One thing we've learned is that if it's a pest for one crop in another area, it's bound to be a pest for that same crop in other areas too. It's consistent that way," he said.

(Padma Nagappan is a reporter in San Diego. She may be contacted at padma.nagappan@gmail.com.)