Why Cherry Growers Have Such A Persistent Problem With Fruit Flies

The House of Representatives wants Dutch cherry growers to have access to the pesticides Exirel and Tracer to effectively combat the Suzuki fruit fly. LNV Minister Adema, who previously no longer allowed these resources, is now looking for a solution for cherry growers.

WUR researcher Herman Helsen has spent more than ten years researching the Suzuki fruit fly, Drosophila suzukii . He is not going to get involved in the political debate, but does say: 'Cherry growers have a serious problem without these resources.'

The Suzuki fruit fly is an invasive pest. The fruit fly hitchhiked in fresh fruit from Asia to Europe and the United States in 2008 and has been present in the Netherlands since 2012. The females lay their eggs in soft fruits such as cherries, strawberries, raspberries and berries. They do this not only with Dutch fruit growers, but also with soft fruit in (allotment) gardens and nature. The humid Dutch climate, with its mild winters and mild summers, is ideal for the Suzuki, says Helsen.

Biology
After the invasive fruit fly arrived in the Netherlands, Helsen investigated the biology of the animal together with Wageningen and Groningen entomologists. The great strength of the Suzuki is that it can go without fruit for a very long time in winter and early spring. When the cherries ripen in the spring, they appear en masse. During the rest of the year they manage to reproduce on a long list of host plants: almost all plants with soft fruits in horticulture and nature. That is why the Netherlands is now flooded with Suzukis. “They are always there and everywhere,” Helsen summarizes.

With this knowledge in hand, the researchers started looking for control methods. They examined native parasitic wasps for biological control, but they did not know what to do with the Asian fruit fly and were therefore eliminated.

Insect netting
Helsen also tested fine-mesh insect mesh. This turned out to be a huge success: most Dutch cherry growers now use mesh to keep the Suzukis out of the orchard. But there are always a few fruit flies that come in anyway. And those few flies are enough to cause major damage, says Helsen. Organic cherry growers who tried mesh without additional additives had an infestation of fruit flies in the cherry orchard within a month and were able to throw away their harvest.

Bait sprays
That is why the research broadened to the question: how can growers combat fruit flies as effectively as possible with minimal amounts of resources? For example, Helsen started testing bait sprays ; these are spraying with a bait. 'You take a mixture of sugars and proteins that the fruit flies like to eat and combine it with an active substance that kills the flies. You can spray this on the plants and then you can combat the flies with a very low dosage (5-10% of a standard spray) of such an insecticide.'

Biological agent
And until recently, those insecticides were used by cherry growers, but are now no longer permitted without the exemption. This concerns the Exirel and Tracer resources. Tracer is a biological product and is regularly approved for use in onion cultivation, among other things, to combat thrips, but is not regularly authorized for cherry cultivation. This partly depends on the admission procedure. 'Such a procedure is long and expensive for the manufacturer. Such a procedure is often not worthwhile for small crops with a limited area, such as cherry cultivation," Helsen explains.

Controls
To accommodate cherry growers, the Ministry of Agriculture, Nature and Food Quality announced an exemption in recent years, allowing cherry growers to use these resources in the summer months, but that exemption has not now been granted. The ministry indicates that NVWA checks showed that cherry growers have not used the products as prescribed. Helsen: 'In order to be allowed to use the products, strict requirements are imposed in advance on the emissions of these products to the environment of the orchard. Research by my WUR colleagues has shown that growers can meet these requirements with the right set of techniques and measures.'

Long ripening time
Why do cherry growers specifically have a problem and other fruit growers less so? This is due to the relatively long ripening time of cherries, Helsen answers. The Suzuki fly lays eggs in the cherries when they start to color and those eggs have grown into a pupa before the cherries are harvested, resulting in a next generation of hundreds of offspring from 1 clutch making the orchard unsafe. The ripening time is shorter for crops such as raspberries and strawberries. By harvesting very frequently in these crops, working hygienically and quickly removing all rotten fruit, you can prevent the Suzuki populations from building up. But problems can also be serious in those crops, and in the cultivation of blackberries and blueberries, especially later in the summer.

Clay preparation
What biological alternatives are there for the two insecticides mentioned? None at the moment, says Helsen. He has done tests with clay preparations – you spray a thin layer of clay on the cherries, which makes the fruit unattractive to the fruit fly. But you can no longer remove such a layer of clay from the fruit, making the cherries unattractive to the consumer. All kinds of other biological agents also provide insufficient protection.

Sterile males
The sterile male technique is more promising: you breed large numbers of sterile males of the Suzuki that displace the virile males, so that the eggs are not fertilized and the population shrinks. 'Technically this could work, according to research in England and France. But it is still stuck on large-scale production. The cherry growers are quite willing to pay for an effective technique, but you need a lot of creatures and it is not yet possible to produce and sterilize them efficiently.'

Natural enemies
What also helps: introduce natural enemies of the Suzuki from Asia. "There are a few Asian parasitic wasps that specifically eat the Suzuki fruit fly and they are now being studied in some European laboratories," says Helsen. These parasitic wasps will not be able to prevent Suzuki fruit flies from appearing in fruit crops, but they will probably lead to lower pest pressure from the environment.' The big issue here was: can you introduce them safely? You don't want this exotic parasitic wasp to become a pest as well. It now appears that some species are highly specialized on the Suzuki fruit fly. 'That is very positive in my view, because the whole of the Netherlands is now full of Suzuki fruit flies and they also have a huge impact on nature. I used to be able to pick healthy blackberries and elderberries in nature, but now almost all of them are damaged.'

Introduction of natural enemies requires cooperation with the government and with companies that want to breed such parasitic wasps. 'You will then have to investigate whether the parasitic wasps can survive and what their effect is. This is something that we see as a perspective and a sustainable alternative and that we would like to work on in the coming years.'