Good to Know

Brown marmorated stinkbugs feed on the foliage and fruit of grapevines.

Grape growers have been closely watching the spread of the brown marmorated stinkbug (Halyomorpha halys Stål). It was first identified in 1996 in Allentown, Pennsylvania, and has spread to 36 states. In 2010, a warm winter and spring caused an early population surge of BMSB that caught fruit and vegetable growers off guard and caused severe economic losses. It is currently a significant economic problem in five Mid-Atlantic states, causing millions of dollars of damage to over 300 susceptible crops, including grapes.

Armed with a multistate grant from the U.S. Department of Agriculture, university researchers and growers started a research project in 2011 to discern details of the stinkbug’s lifecycle, monitoring techniques, and economically and environmentally sustainable management practices. Meanwhile, orchardists have had to depend on broad-spectrum pesticides to keep damage at a tolerable level. No native natural enemies of the BMSB have been determined to date, and scientists are exploring native habitats for natural predators and parasitoids in China.

BMSB in vineyards

Adult BMSB emerge in early spring from overwintering sites, such as dead trees in the woods, under rocks, and in homes, to mate and migrate to the vineyard. All stages (five nymphal stages and adults) readily move in and out of vineyards from adjacent fields and hedgerows from spring through fall. As the season progresses, harvest of other crops surrounding the vineyard, like soybeans, may induce migration into the vineyard, causing dramatic increases in populations.

All BMSB life stages cause direct damage to foliage and berries. The stinkbug feed by piercing the fruit with its tubelike mouthparts, injecting saliva to predigest plant tissue, and then sucking up the plant fluids. Feeding damage on fruit continues from fruit set through harvest, although damage is less problematic on developing fruit, as the berries are more resistant to rot.

Monitoring and control

Growers should monitor foliage and fruit regularly for presence of eggs, instars, and adults. Egg masses (27 eggs per mass) should be physically destroyed when observed. Vigilant scouting for the damage and subsequent fate of the fruit (healing, rots, etc.) is necessary to assess actual and potential damage at various developmental stages. There has been a significant amount of research to find pesticides that will help control the stinkbugs.  Please see the following site for pesticide recommendations: www.grapesandfruit.umd.edu/TimelyVit2/TimelyVit BMSB2%20rev.pdf

To add insult to injury, the stinkbug is capable of “drop and recovery,” which means the insecticide appears to kill them, as evidenced by a significant population on the ground, but after 24 to 48 hours, they recover and can get back into the clusters. To minimize stinkbugs at harvest, some growers are spraying “knockdown” type insecticides with short reentry and preharvest intervals and harvesting the fruit as soon as legally possible after spraying—and before the insect can reinfest.

As nights become cooler during harvest, the brown marmarated stinkbug tend to hide inside clusters ­(seeking sugar and warmth) and may consequently be harvested and transported to the winery along with the grapes. Insects need to be hand sorted out of the clusters in the vineyard.

BMSB in wineries

Insects that escape detection can foul the juice once infested clusters are crushed and pressed. All BMSB immature instars have been found to emit a distinct odor which taints items they contact. The distinct, and to some, repulsive odor of the brown marmorated stinkbug is similar to the smell of fresh cilantro. The primary aromatic compounds are trans-2 and cis-2 decanal. Other descriptors used for the taint are “skunky,” “citrusy” and “piney.”

The bad experience with the multi-colored Asian lady beetle (Harmonia) caused winemakers to fear that crushing stinkbugs with the fruit could result in tainted wine, and, due to the newness of this pest, scientists were unaware of the actual impact of the BMSB on finished wine.

Research was therefore conducted by the author to determine any effect and possibly develop a threshold for noticeable brown marmorated stinkbug taint in juice and wine. In 2010, from 0 to 20 BMSB were added directly to the crusher/destemmer with each 25-pound lug of Vidal Blanc and Cabernet Sauvignon grapes.

The inoculated white musts were pressed, cold settled, and fermented. The inoculated red musts were cold soaked for four days, and then fermented. Brown marmorated stinkbug were very visible during punch downs. Unfermented juice samples were taken for testing. In 2011, the treatment was increased to 50 BMSB adults per lug, again compared to an uninoculated control.

The juice samples were evaluated by many professionals (including research entomologists), certified wine judges, and lay volunteer “judges” who were familiar with the BMSB aroma. A few could detect the taint in the juice at one insect per lug but a comparable number also thought they detected it in the control, therefore that was considered just background noise and not correct identification. However, a majority could detect the taint in the juice at five and ten per lug relative to the clean juice. Testing samples over time has shown that the taint in the juice was perceptible for about four months.

Good news

And now the good news. The wines resulting from fermentation of the inoculated juice/must was similarly compared to controls, and, overall, there was no detectable taint in the white or red wines. In various tests, including the typical triangle comparison, there was no consistent recognition of wine made from the inoculated juices versus the control. Even in the second year, there was no clear taint perceived in wine directly inoculated with 50 BMSB per lug.

The conclusion from these early studies is that perceptible taint in the juice has not resulted in a perceptible sensory taint in the wine after fermentation. Currently, the wines are being tested for the presence of the compound to determine whether it can be in a latent or undetectable form that could later volatilize. However, the chemists I have queried have concurred that based on the chemistry of decanal, it should not be stable through ­fermentation or over time in a wine (alcohol) environment.

Summary

The future outlook for the brown marmorated stinkbug is uncertain, but it is reasonable to assume that populations and damage will increase over time and it will be a serious economic pest in agriculture. It can cause yield loss via direct damage to grape berries and induce fruit rots, depending on the population. Thresholds for economic damage are a significant part of the current research.

But the good news is that inadvertent inclusion of some brown marmorated stinkbug during crush does not seem to result in taint of the wine. So, relax and enjoy a good glass of wine—preferably the uninoculated type!

Joseph Fiola is viticulture and small fruit specialist with ­University of Maryland Extension, Keedysville, Maryland.