The South Tyrol perspective

Spotted wing drosophila has forced European berry and stone-fruit growers to rethink their crop-protection strategies. By Anna Mouton.

The first trap catches of spotted wing drosophila in Europe occurred in 2008 in Spain and Italy. The first economically significant damage was reported from Italy the following year. Spread continued, and spotted wing drosophila was considered present in all European countries and the United Kingdom within a decade.

Dr Urban Spitaler first worked with spotted wing drosophila in 2014 during his internship at Laimburg Research Centre in South Tyrol. He started his doctoral research on this pest in 2017 and has been studying it ever since.

Spitaler currently leads the plant-protection-product testing group in the Institute for Plant Health at Laimburg Research Centre. He shared his observations of spotted wing drosophila with Fresh Quarterly.

Q. What was the initial response to spotted wing drosophila, and how has that evolved?

A. At the beginning, many farmers may have thought that Drosophila suzukii will disappear without any control measures. It was also not clear that there will be damage in each orchard because some farmers hoped they are too high — over 1 000 metres above sea level — and that they will not have problems.

Then, in the first years farmers had significant damage. Now we know we have a problem with Drosophila suzukii in all areas. The farmers prepare themselves, and they pay more attention to this.

So the impact overall is that we had to change the plant-protection strategies completely. For example, before Drosophila suzukii, it was not necessary to apply any insecticide against insect pests for most berries at harvest because no important insects were present.

After Drosophila suzukii, we have had to include treatments against it and change the way berries, stone fruit, and grapes are cultivated in our area. We started to use excluding nets — insect nets that prevent Drosophila suzukii from migrating into the orchards.

In other regions of Europe there are also almost no commercial cherry orchards without excluding nets.

Q. Could you talk more about excluding nets?

A. The mesh size should not be larger than 1 mm. You have to cover the orchard so that you enter just once with the tractor without opening the complete side. So, where you go from one row to the next, you stay within your insect cage without going out, and when you go in or out, you immediately close it behind you.

It is also important that there is no space between the ground and the net. After each storm, you must check that there are no holes or other gaps where the flies can go in. Monitoring under the net is very important because you can never be sure that there is really no gap where they can go in.

In some large orchards, farmers sometimes place the insect nets only on the borders without covering the whole crop. This has an effect, but the effect is not as high as when you cover the whole orchard.

The insect nets are expensive, and not all small farmers could apply them. They had to change to producing something else.

Excluding nets are not always applicable. For example, for berries like strawberries, if you have a rain cover as well, it’s too hot overall. But on cherries, excluding nets are now the standard here.

Q. Does monitoring play a role in controlling spotted wing drosophila?

A. The advisory services have a large monitoring programme. They offer a service for the farmers to bring their trap catches and fruit samples for validation because we only apply treatments after an infestation is observed. So, it’s not a standard application based on time, but it’s based on the presence of Drosophila suzukii in the orchard.

We use standard commercial Drosophila suzukii traps with a liquid bait. They are normally red and have fine meshes to prevent too many other insects from entering. The good thing is that it’s easy to distinguish Drosophila suzukii from other Drosophila species.

The population can really grow from one day to the next, so you have to check the traps for presence or absence every second or third day — not just once a week — but the harvesting season is not too long. For example, each cherry variety is mainly harvested within one to two weeks, and monitoring is not necessary over the whole season.

You have to consider infestation pressure and safety intervals — whether it’s better to harvest earlier or spray and wait until later to harvest. So, before doing the application, you should wait until you have the infestation and then see if there is enough time left to do an application.

In really large orchards, it is possible just to apply pesticides to the rows on the borders because the pressure in large orchards normally comes from the borders. However, in South Tyrol we don’t have such large orchards, so this is more applicable to other European countries.

Q. What else should growers consider when controlling spotted wing drosophila?

A. It’s not possible to produce cherries without a professional plant-protection strategy anymore because the impact of Drosophila suzukii is too high. The fruit quality is not good enough for the market if you are not applying insecticides or excluding nets.

If you have different varieties, you should avoid having the earlier ones close to the later ones because then the infestation will spread from one to the next. Here in our region, Drosophila suzukii lives in the forest where we have wild cherries and other host plants. These wild host plants are mainly early, and when there are no fruits such as wild cherries left, the whole fly population will move to the next host.

One thing that contributes to successful control is cutting cover crops before harvest to lower the humidity overall. Drosophila suzukii prefers high humidity and lower temperatures.

During summer, when we also have high temperatures here in northern Italy, they can go down to the ground and find some humid spaces in the vegetation below the trees. By cutting that vegetation before harvest the environment in the orchard is not so good for Drosophila suzukii.

If you are cultivating fruit for processing, it’s possible to treat the fruit with kaolin to change the colour and as a barrier for oviposition. This is used here for wine grapes.

Q. Can growers look forward to new technologies to help manage spotted wing drosophila?

A. During my master’s thesis, we tried to develop a sustainable approach to controlling Drosophila suzukii. We checked if different yeast strains impact different life stages.

My PhD was a follow-up project in which we combined these yeasts with spinosad to develop a formulation for field application that is not only toxic but also attractive to Drosophila suzukii. It is an attract-and-kill system.

We would like to have this on the market. At the moment, it’s being validated by external institutions selected by a commercial company.

There is research on smart traps so that the farmers don’t have to validate the trap catches by themselves. Those are mainly based on sticky traps so that there’s only one surface, and the camera can select the Drosophila suzukii flies to tell the farmer directly if it’s present.

The smart traps also switch the sticky cards automatically. This is something in development. It’s not used yet.

In the past, there were also mass-trapping experiments, but they were unsuccessful. When Drosophila suzukii was introduced, many farms tested mass trapping, but the catches were never high enough so that you were able to see a reduction in infestation.

Companies in different regions are producing sterile Drosophila suzukii for research. I don’t know if they are already selling them to farmers. Based on the research, it’s hard to say whether this will be successful.

We need additional results. It’s been more than 10 years since Drosophila suzukii was introduced, and sterile insects aren’t used to control Drosophila suzukii in Europe at the moment.