Determination of the critical stages of processing and tolerance limits for <i>Harmonia axyridis</i> for ‘ladybug taint’ in wine

Abstract

'Ladybug taint’ (LBT) has recently been reported in some wines from North America, and is associated with 2-isopropyl-3-methoxypyrazine (IPMP), produced by Harmonia axyridis Pallas (the Multicolored Asian Lady Beetle - MALB) when they are incorporated into the winemaking process. It is not known when IPMP is transferred from MALB (e.g. in the vineyard onto grapes or during must processing) nor what minimum MALB densities are required for production of LBT in the final wines. This study sought to clarify these issues through a series of three trials. In the first, MALB were added to 'Riesling' grapes or must at different stages of processing (harvest, crush/destem, pressing or directly to juice), and the resultant wines were analysed chemically and by paired-comparison sensory difference tests.
The presence of MALB during processing had minimal effect on the basic composition and spectral properties of the wine. Concentrations of IPMP were < 5 ng·l-1 for all wines except those produced after the direct addition of MALB to the juice (10.3 ng·l-1). Sensorially, control wines (no added MALB) could be differentiated from wines made after MALB were added at crushing/destemming (at 3 beetles per kg grapes), whole bunch pressing and when added directly into the juice, but not when MALB were added and subsequently removed from a simulated harvest treatment or when added during crushing/destemming at 0.3 beetles per kg grapes. In trials 2 and 3, sensory detection thresholds for LBT were determined for white and red wines produced with known densities of MALB. Estimates of ‘tolerance limits’ in the vineyard were then calculated using regression models, and correspond to 1530 and 1260 beetles per t grapes for white and red wines respectively. However, given the range of grape and wine processing options available to producers, many of which are not accounted for in this study, we recommend that a more conservative limit of 200-400 beetles per t grapes may be appropriate. These results should assist in directing appropriate interventions in the vineyard/winery, and provide baseline targets for reducing MALB density to avoid development of LBT.