Gardeners’ Friend Causes a Stink
Certain insects offer gardeners and commercial growers a green alternative to synthetic pesticides. High on the list of these beneficial agents are black-spotted bugs, known alternately as ladybugs, ladybirds, and lady beetles. Whatever they’re called, the insects’ appetite for aphids makes them a gardener’s friend.
Among vintners, however, an Asian member of the ladybug family now rapidly spreading throughout the United States has acquired quite a different reputation: They’re stink bugs.
When crushed or even just stressed, the insects release blood from their leg sockets. Several volatile compounds in that blood are highly aromatic, with distinctly vegetal scents, notes Iowa State University analytical chemist Lingshuang Cai. The barest whiff can bring to mind a mix of green bell peppers, roasted peanuts, raw potatoes, and musty earth. It may not sound too bad, she says, but “it’s actually quite noxious” and can definitely spoil a batch of wine. Indeed, her team notes, “entire harvests and some vineyards have failed because of multicolored Asian ladybug contamination.”
Why wine? Because these pretty little ladybugs have a penchant for noshing on damaged grapes and so can easily be harvested along with the fruit and crushed during winemaking.
At the American Chemical Society’s spring meeting in Chicago last month, Cai reported that her team had confirmed the role of several specific chemicals in the Asian ladybugs’ distasteful odor, one of which was a surprise. Her team’s research also showed that among the tested bugs, orange Asian ladybugs produced dramatically more of the smelly compounds than their yellow-coated kin—even though both varieties belong to the same species (Harmonia axyridis).
Like Sauvignons . . . but more so
Since 1916, U.S. farmers have periodically tried introducing this beetle, in the hope it might vanquish the aphids that attack plants such as soybeans. Early efforts failed, and the beetles—sometimes called harlequin ladybugs for their variable coloring—disappeared. However, efforts in the 1970s and ’80s appeared to take, and by 1988, the insect was firmly ensconced in Louisiana. The harlequin is now the most widespread ladybug species in North America. The bug also recently spread—not by invitation—to Europe.
Noxious ladybug smells trace to compounds known as methoxypyrazines. However, Australian researchers 15 years ago identified some members of this chemical family as contributing to the distinctive aroma of Sauvignon blanc wines. Scents emitted by Cabernet Sauvignon and Cabernet franc wines—at least those made from grapes grown in the United States—also owe some of their vegetal notes to methoxypyrazines.
The bugs, however, can easily imbue wines with a broader mix and higher concentrations of these chemicals, Cai says.
Although the Iowa State team’s interest in studying the compounds was motivated by the trouble they can cause vintners, the researchers didn’t study tainted wine. Instead, they extracted methoxypyrazines from the beetles.
Cai’s group put highly absorbent fibers along with five ladybugs in each of several vials. A day later, they removed the insects and analyzed chemicals among the fibers. They identified 38 distinct compounds including four methoxypyrazines: ones known as IPMP, SBMP, IBMP, and DMMP.
The first three chemicals on that list had been linked to wines before, especially those tainted by ladybugs. However, Cai says, previous tests had been unable to trace the methoxypyrazines’ source or determine the extent to which bugs contributed to a wine’s off aroma or flavor. Her team’s new focus on the chemicals in ladybugs themselves “provides conclusive evidence” of the insects’ release of methoxypyrazines, she says. The last one on the list, DMMP, was never before associated with wines or the beetles.
To be sure that the four compounds are the ones responsible for a ladybug’s foul smell, the researchers trained colleagues to recognize the insects’ scent. Later, these individuals sniffed each of the purified methoxypyrazines extracted from the absorbent fibers. The tests confirmed that all four chemicals contributed to the noxious smell of ladybugs and presumably ladybug-contaminated wines. Further, the testers agreed that IPMP, SBMP, and IBMP smell like bell peppers, whereas DMMP had a largely musty smell.
Cai’s group also quantified the release of each compound by the bugs, which she surmises to be a defensive action. Concentrations of IPMP turned out to be highest, and those of IBMP were lowest.
The harlequin
The harlequin species that the Iowa researchers studied exhibits wide individual variation in shell hue. In the United States, the species ranges from tomato red to yellow. In Asia, some H. axyridis specimens even have black shells with red spots.
Cai told Science News Online that the biggest surprise in her research was stumbling over a relationship between a bug’s color and the output of three of its methoxypyrazines. An average orange bug released 0.31 nanogram of IPMP, 0.11 ng SBMP, and 0.60 picogram of IBMP. In contrast, yellow beetles produced less than a tenth as much of these methoxypyrazines, the researchers report in the April 13 Journal of Chromatography A. The scientists didn’t report such numbers for DMMP.
If color proves a solid determinant of the species’ methoxypyrazine output, the Iowa State researchers say, this could explain the discrepancy between their new data and reports of far smaller methoxypyrazine concentrations associated with the beetles. The color of a harlequin ladybug is determined in part by genetics, but also to some extent by larval diet.
Paying attention to the color of ladybugs in vineyards might also give growers a clue to how seriously their wines might be tainted with methoxypyrazines, Cai says.
Gary Pickering and his colleagues at Brock University in Canada have been investigating ways to remove the taint of methoxypyrazines in wines. Last year, they reported some success by incubating red and white wines with oak chips. The treatment “reduced the intensity” of the taint, they reported in the January 2006 International Journal of Food Science & Technology. The scientists tried a host of other techniques, including irradiation of wine with ultraviolet light and mixing in absorbent clay or activated charcoal, but none had more than a “limited effect” on red wine and all had no effect on white wine.
For now, the only cure for ladybug-tainted wines is a quick trip down the drain or perhaps nose plugs.
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