Drosophila: a threat?

Apertura_portaleDrosophila suzukii species is native from South-Eastern Asia. It has been recently up in the news in Italy for the serious damages it caused to such valuable cultivations as cherries and small fruits. As for the actual danger of this insect for the vine, today the opinions are discordant. Some people came to believe that Drosophila suzukii is new phylloxera and someone, instead, states that there are no conditions for effective damages caused by this insect on grapes. So diversified opinions are primarily due to the extreme topicality of the problem as well as the few years of observation that have been carried out up to now: the first catches of this insect in Italy date back to 2008-2009.
Based on the literature concerning this subject, this note wants to try to shine a light on the characteristics of this insect and the main actions useful in order to prevent its proliferation and subsequent damages.

Behaviour and damages

As mentioned above, the damage caused by this insect is physical, due to the engravings produced by the females in order to lay eggs in healthy and ripe fruits thanks to their specific ovipositor apparatus. The larval development in ripe fruit is faster, while it slows down in unripe or too ripe fruits, and, if the fruit is very sour, the larvae fail to complete their development. In the case of fruit for fresh consumption, the larval development leads to the complete disintegration of the tissues, so making the fruit unfit for human consumption and thus depriving it of commercial value.

About Vitis vinifera

As for Vitis vinifera, its susceptibility to oviposition seems to be highly variable and no clear association with the different varieties has been verified. Behavioural experiments carried out in laboratory show lower preference to oviposition and more reduced percentage of development of Drosophila suzukii in grapes rather than other berries or cherries. On the other hand, both in the area of ​​origin and in Europe and the USA, Drosophila suzukii showed its ability to attack vine, preferring red variety, characterized by late harvest and thin epicarp. In 2011 some direct damages have been attributed to this insect in Trentino-Alto Adige, especially against Schiava gentile, grapevine variety widely cultivated in this region. Some reports have also been carried out as for some other noble varieties, but characterized by thin peel, such as Pinot nero.
Tests concerning oviposition on grapes have shown that this does not occur on intact bunches as long as the sugar content in the fruit is low. Damages are reported far beyond veraison, roughly after the middle of August, when the earliest varieties start to accumulate significant amounts of sugars. The grapes remain then in the field, even when more interesting cultivations for this insect are concluded. Therefore, late harvestings, because of either varietal / climatic reasons or especially valuable productions are especially at risk. Oviposition on unripe bunches previously damaged, too, has been recorded, but in this case the larvae cannot complete their development.
In the case of wine grape, the damage caused by Drosophila suzukii is rather treacherous. The low percentage of development of larvae obtained by eggs laid on grapes induces to avoid considering the direct damage it causes, due to the drilling and disintegration of the tissues of the berry, not particularly significant. Indeed, if the holes are made when the grapes are still endowed with a certain elasticity of their perimetrical tissues, a healing of the hole can occur. Therefore, the hole itself is made, in fact, harmless.

Relations with sour rot

The problems are related to an effect of Drosophila suzukii little known until now: the one of being a vector of significant microbial population. In practice, recent observations seem to support a correlation between the damage caused by this insect and the following, but massive, onset of sour rot on grapes. This adversity of bacterial origin is usually related to the presence of high concentrations of acetic bacteria and yeasts on the grapes, close to the wounds caused by other adverse conditions or atmospheric agents. Drosophila suzukii would act ˗ searches are still in progress ˗ as both causal agent, through physical damage exerted on the intact surface of grape, and infectious agent, carrying a high microbial load able to trigger sour rot in a few days. The main obstacle to this mechanism is given by the hardness of the grape peel.

Defence strategies

Drosophila suzukii is now considered as firmly established in almost all the Countries in Europe and North America and therefore it would be unrealistic to counteract its expansion. As now the number of authorized insecticides on vine is limited, it is necessary to lay down alternative strategies, which primarily rely on well designed catch plans. The research field concerning the most suitable tools for capturing Drosophila suzukii is giving interesting developments. By now, it seems clear that this insect, thanks to its very sensitive receptive apparatus, is attracted by different compounds secreted by both host fruit, and microorganisms present in the environment. In particular, as for microorganisms, it has been demonstrated that the fermentation products orient the insect, attracting it in an effective manner toward the traps.

Droskidrink trap

In Trentino region, Fondazione Edmund Mach has recently developed bait that has proved to be particularly effective in attracting adults of Drosophila suzukii. This bait, called Droskidrink, consists of a mix of apple vinegar and red wine. Its effectiveness is further enhanced if brown sugar is added. It is fermentation substrate for many microbial species, if the traps that contain it are dark red. In addition to the optimization of the bait, it is important to check the optimum arrangement of the traps (i.e. height from the soil and on plants, and location on rows and edges of the plant), as well as the intervention timing. The mass capture should be planned especially in spring and summer, in the absence of mature fruits, because it is unlikely that the bait can be more attractive of the fruit themselves, and because in late summer-autumn, the population of Drosophila suzukii has already reached high densities.

Agronomic measures

Among other agronomic measures, it is particularly important to remove any residue of fruit present in the cultivation in the ripening stage, taking into account the dangerousness of these residues as sources of attraction and re-infestation. For example, in the operations of thinning and selection of bunches before the harvest, residues cannot be left in the field, but should be quickly eliminated. The same caution should be given in maintaining a sparse grass cover, eliminating or promptly burying the remains of cuttings, so avoiding their fermentation in the open in the vineyard.

Organic control

Investigations concerning agents for organic control (fungi, bacteria, viruses, predators, and parasitoids) which could be used against Drosophila suzukii are also in progress. Recently, specific DNA viruses were isolated in some species of Drosophila and in the near future these viral agents may be evaluated for organic control of Drosophila suzukii. Similarly, researches are in progress as for the use of natural enemies for containing this species. Another approach for the biological control of Drosophila suzukii might consist in increasing the effectiveness of more generalist predators and parasitoids, of course already present in the areas of new invasion, which can already attack such similar species as Drosophila melanogaster. They have been occasionally described in different parts of Europe as predators of Drosophila suzukii. Some studies concerning the possibility of interfering with the populations of Drosophila suzukii through the manipulation of symbiotic microorganisms are promising, although they are still at an early stage of investigation.

Why should it be feared?

The real danger of Drosophila suzukii on vines is still subject of evaluation and discussion among researchers and technicians. There is a doubt that the observations made so far, and which reported a low level of attack, could be contradicted in vintages with particular climatic conditions, or in vine areas where the pressure of this pest would strongly increase in the coming years.
Certainly the biology of this species sums different formidable characteristics, among which the most significant are: (a) high prolificacy; (b) presence of a very large number of generations (up to 13-15) per year; (c) association with a large number of host plants; and (d) infestation of the fruits in the imminence of their maturation. It is therefore essential to remain vigilant as for the presence of Drosophila suzukii in Italian vineyards, even in the absence of any direct evident damage, indeed planning adequate researches and agronomic interventions aimed at preventing ideal conditions for the action of this insect in Italian vineyards.

 

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Eastern origins

The origin of Drosophila suzukii is to be found in South-Eastern Asia. From there, the insect has reached first of all the Hawaiian Islands, and then the United States and Europe. In Italy the first isolations of this insect date back to the 2000s. Extensive damages have been reported since 2010, in particular on small fruits. In that year, just in the Province of Trento, one of the best areas for this type of production, Drosophila suzukii has caused damages estimated at three million Euros. Today it is possible to consider this insect as widely present in Italy.

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Vitis labrusca is little appreciated!

A recent research concerning some varieties derived from hybridization between Vitis vinifera and Vitis labrusca showed a negative correlation between the intensity of the damage and percentage of the genome inherited from Vitis labrusca. The varieties with a high percentage of their genome in common with Vitis labrusca show a lower degree of attack, as it happens in the case of Early Campbell, Reliance, and Suffolk red varieties.

(article by Raffaele Guzzon, Gianfranco Anfora, Alberto Grassi, Claudio Ioriatti – Edmund Mach Foundation)

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