The harlequin ladybird Harmonia axyridis – Sensory thresholds of the “ladybird taint” in wine under different processing conditions

Abstract

Introduced as a biological control agent in North America and middle Europe the Harlequin Ladybird Harmonia axyridis has spread all over the countries and are capable to become an increasing problem for winegrowers and also households. Due to its preference to overwinter indoors H. axyridis enters private homes and people may have allergic reactions to its odor or bites. During harvests, the beetles feed on grapes and may get crushed with them during processing causing release of hemolymph into the must. The hemolymph of ladybirds contains specific substances which consist mainly of pyrazines like the 3-isopropyl-2-methoxypyrazine. These chemical compounds are responsible for a specific alteration of the smell and taste of wine called “ladybird taint”.According to the results of a survey (n = 50), 80 % of winegrowers in the Palatinate region, 65 % in Frankia and 55 % in the Mosel region have already spotted H. axyridis in their vineyards. More than 50 % of the Palatinate winegrowers, 40 % in Frankia and 30 % at the Mosel, respectively, have even found the beetles in wine presses and harvest crop. It did not matter whether the beetles were harvested by hand or with a harvester as H. axyridis could be detected in the press in both cases.Sensory trials on Riesling and Pinot Noir wines produced in 2009 with added live beetles of H. axyridis showed that the ladybird taint is detectable at a threshold of 4 beetles per kilogram of grapes. But the sensory detection depends on the winemaking practices and the cultivar. In red wine the sensory threshold of pyrazines is lower (1 ng/l) than in white wine (2 ng/l). In addition fermentation technique affects pyrazine contents in wine: Fermentation after must heating also increased the detection limit of the ladybird taint compared to crushed must fermentation.Further aspects of winemaking are represented by the power and duration of pressing and if the grapes were de-stemmed before. The mortality rate of the beetles in the mash and therefore the amount of pyrazine in the must rises clearly with higher pressing power (3 bar), meaning that a lower presspower should be applied at high beetle loads. In addition, pressing of grapes which are not de-stemmed can lower the mortality rate as the beetles can find more holes in which they can survive the pressing. The duration of pressing can be seen as another factor for the reduction of ladybug taint in wine. The longer the grapes are pressed, the more pyrazines can be dissolved in the wine. A risk assessment should be carried out in order to deliberate about the surplus of must and the contamination of the wine by beetles.In the nineteen seventies and eighties the wine quality in German wine growing regions suffered from a contamination with chemical compounds of Coccinella septempunctata (7-point-ladybeetle) which at this time was not yet considered a possible hazard. Cudjoe et al. (2005) found that H. axyridis has a hundred-fold higher quantity of pyrazine in their hemolymph than C. septempunctata. Up to now it is unknown whether there is a difference in the quantity of beetles of the two species necessary to deteriorate wine quality. However further studies are in progress.