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There is a lot of research focused on the effect of various fungal agents on the quality of finished wine; however, to date there has been very little examining the effect of insects on wine quality. While some insects are considered to be beneficial (i.e. ladybugs; since they are known to prey on pest insects), many other are considered to be pests as a result of their contamination of the grapes which result in undesirable aromatic characteristics in the finished wine.
One insect pest of grapevines in particular is the mealybug (Hemiptera: Pseudococcidae). Certain species tend to be more problematic than others, though in general they aren’t exactly wanted ever in the vineyard. In California and Argentina, the most common species of mealybug is Planococcus ficus(Signoret), whereas in Chilean vineyards, the most common species is Pseudococcus viburni (Signoret). As the grapes go through veraison and continue to ripen, the mealybug moves to the grape clusters where it continues to feed and reproduce. One problem with mealybugs in regards to grape health is that when they are going through the process of feeding, they excrete honeydew, which has been shown to act as a solid base for the development of sooty mold or other fungi. Also, it has been shown that mealybugs can carry and transmit grapevine viruses, including the leaf-roll virus.
Though it is generally understood that mealybugs (and other similar insects) are undesirable in the vineyard for the reasons mentioned above, there has been very little research done examining the effects of mealybug infestation on the quality of the wine produced from the exposed grapes. The goal of the study presented today was to test whether or not wine produced from different levels of mealybug infestation had different chemical and sensory characteristics for both red and white wines.
Grapes used were Chardonnay from the Casablanca Valley in Chile, and Carménère from the Colchagua Valley also in Chile. Mealybug damage on the grape clusters were based on a scale of 0-3; with 0 reflecting completely healthy grapes, 1 reflecting the presence of less than 5 mealybugs or a light bit of honeydew, 2 reflecting an infested cluster with only part of it useable, and 3 reflecting a completely infested cluster. After harvest, grapes were taken to the Enology lab at the Universidad Católica de Chile in Santiago, Chile.
180kg of grapes were harvested from the Chardonnay vineyard and another180kg from the Carménère vineyard, with approximately half the grapes being healthy, and the other half being infested with mealybugs.
Once grapes were brought to the laboratory, healthy and infested grapes were mixed together to create four different treatment types: 1) 100% healthy grapes; 2) 66% healthy and 33% infested grapes; 3) 33% healthy and 66% infested grapes; and 4) 100% infested grapes.
Wine was made using small-scale winemaking procedures. Chardonnay was not allowed to go through malolactic fermentation, while the Carménère was made as reds are made traditionally with malolactic fermentation. A total of 12 batches were made for each wine, representing the 4 treatments and 3 replicates each.
For the musts, the following were measured and analyzed: pH, sugar content, and titratable acidity.
For the finished wines, the following were measured and analyzed: free sulfites, total sulfites, pH, alcohol degree, titratable acidity, volatile acidity, residual sugar, and nitrogen content. For the Chardonnay, specifically, total polyphenols were also analyzed. For the Carménère, specifically; total polyphenols, hue, total anthocyanins, dimethyl amino cinnamaldehyde index, total tannins, and average degree of tannin polymerization were also analyzed.
Sensory analyses were performed by 20 people from the Pontifica Universidad Católica de Chile, including 2 enologists and 18 graduates of the ecology major program. Wines were randomly (and blindly) presented to the subjects in groups of 4, representing each of the 4 treatments. Tastings were done prior to the experimental tastings, in order to get a solid grasp of the taste descriptors that were being analyzed in this study. Flavors and aromas were scored on a 1-9 scale (1 being the worst and 9 being the best).
• The chemical compositions of the musts were not different between mealybug infestation treatments.
• After the completion of alcoholic fermentation, there were no differences in chemical composition between the different treatments in the Chardonnay. However, in Carménère, pH and alcohol content were lower in the 100% infested treatment than all other treatment than all other treatments.
o Also, Carménère showed greater levels of nitrogen in the 66% and 100% infestation treatments than the 33% and 0% infestation treatments (no difference in Chardonnay).
• In Chardonnay, total polyphenols decreased as mealybug infestation proportions increased, with the 100% infestation treatment being statistically significant.
• In Carménère, total polyphenols, anthocyanins, tannins, and dimethyl amino cinnamaldehyde index all decreased in wines made from mealybug infested grapes.
• In Carménère, as mealybug infestation increased, total anthocyanins, malvidin, acetylated anthocyanins, non-acetylated anthocyanins and cumarilated anthocyanins all decreased.
• In Carménère, as mealybug infestation increased, total tannin content and the proportion of galiolated tannin decreased, whereas the average degree of tannin polymerization increased.
• Principle Component Analysis on the sensory analysis results showed that Chardonnay made from the 100% mealybug infested grapes were associated with negative characteristics, including that of oxidation.
• Chardonnay wines from the 33% and 66% infestation treatments were associated with bitterness.
• Chardonnay wines from the 100% healthy treatment (0% infestation) were associated with positive flavor and aroma characteristics, as well as higher quality.
• Carménère wines from the 100% infestation treatment were associated with dry fruit and dry vegetable characteristics.
• Carménère wines from the 100% healthy treatment were associated with fresh fruit, body, and higher quality.
The results of this study indicate that mealybug infestation does, in fact, play a negative role in the overall quality of a wine created from infected grapes. The grape variety also seems to play a small role, as there were some differences between Chardonnay and Carménère when it came down to the changes in chemical composition and the sensory analysis of the wines. It was interesting to note that the musts of the wines actually did not differ in their chemical compositions, but throughout the alcoholic fermentation (and malolactic fermentation in the red) there appeared to be specific chemical composition changes related to the proportion of mealybug infested grapes used in the processing of the wines.
Specifically, the higher the proportion of mealybug infested grapes used to make the wine, the lower the phenolic content, which as the authors mention (and I
agree) could lower the overall quality of the wine. This lowering in quality was confirmed in the sensory analysis of the wines.
While it’s clear there is a negative effect of mealybug infestation on wine quality (at least with Chardonnay and Carménère), it is unclear exactly why. The authors mentioned the mechanism could be related to the insects themselves directly contaminating the grapes, or possibly indirectly through the honeydew left after feeding or through the fungus attracted to the grapes after mealybug feeding. Whatever the mechanism may be, it is clear that mealybug infestation should be controlled in order to avoid possible undesirable flavors and aromas in the finished wine, in addition to an overall decrease in the quality of the wine.
I would love to see more research focusing on determining the mechanism of this process, as knowing exactly what causes the lower quality wines would give vineyard managers a better idea of exactly how to target their protection and control defenses.
I would also like to see more research breaking down the influence of different species of mealybug and how infestations of one or the other (or both concurrently) affect the quality of finished wines. The authors made mention of differences between the two species in their introduction, however, these differences were not tested. I wonder if one species over another is more damaging to wine quality or if they are roughly equivalent in their harm.
What do you all think of this study? Do you have any experience with mealybug infestation? How have you been successful or unsuccessful in combating against these pests? Please leave these and any other general comments you have for discussion!
Source: Bordeu, E., Troncoso, D.O., and Zaviezo, T. 2012. Influence of mealybug (Pseudococcus spp.)-infested bunches on wine quality in Carménère and Chardonnay grapes. International Journal of Food Science and Technology 47: 232-239.