The onion fly and its control remains one of the major challenges in onion production. The resource package is shrinking visibly, especially with the discontinuation of the pesticide Vydate last year. The question then arises: is successful management and control of onion fly still possible? Onion magazine takes stock with Nemacontrol, De Groene Vlieg, BASF and Koppert.
“Actually, we see the onion fly increasing in all areas”, says Sebastiaan ten Napel of Nemacontrol. "Mainly in the existing areas where high pressure has been known for some time, but also in the outlying areas around these 'hotspots' and in the newer areas. The newer areas, the greatest danger lurks in the long run’, he says. "Only you don't see this immediately translated into proportional damage, because it takes years for the fly to become well established. But it is certainly a harbinger of what is to come."
Frank Druyff of Koppert agrees: "Besides the well-known problems in the Northeast Polder (NOP) in the Netherlands, we are also seeing the onion fly expanding strongly, especially in the northeast of the country. On lighter soils, the physical barrier for larvae to migrate is less compared to heavier soils. Moreover, the area of onions in these areas is expanding so much that there is an onion plot everywhere."
According to Sanne Graafstra of De Groene Vlieg, this is therefore one of the main reasons for the expansion of the onion fly problem. "Moreover, there are areas where, meanwhile, winter onions, spring onion sets and seed onions are almost year-round. This cultivation intensity greatly increases the pressure."
Shrinking resource base
With the discontinuation of the granulate Vydate last season, the chemical pesticide package has been slimmed down some more. We have noticed in recent years that the removal of the more effective agents and, more recently, agents with a lesser but still solid effect have a lot of effect on onion fly pressure in particular and soil insects in general, says BASF's Bert Westhoff. "If you control soil insects in large crops, you often succeed in smaller crops too. But if the toolbox runs empty, you can't recover everything."
Monitoring half work
Both De Groene Vlieg and Nemacontrol monitor onion fly presence and pressure using a signalling and observation system. Both companies stress the importance of this. Graafstra: "Onion fly pupae awaken at a certain soil temperature. We follow that closely to know when to start monitoring the onion fly and to determine the right time to release the first sterile flies to control the onion fly."
For Ten Napel, good monitoring is also a crucial step towards good control. "If a threshold is crossed, it is possible to calculate when egg deposition will take place and when the larvae become active. At that time, the product has to be there to work." Also, bean fly is a growing problem. It occurs early in the growing season, so monitoring and control at that time has a dual purpose/effect.
Correct soil temperature
Koppert and BASF have two agents based on insect parasitic nematodes, infected with a bacteria, that can control onion flies. Because they work with living organisms, weather conditions play a major role in the success of their action. “Temperature is an important factor here”, says Westhoff. "Below 7-8 degrees the nematodes are not active, while between 8-12 degrees they are also only partially active. The soil must therefore be sufficiently warmed up, which may sometimes require the use of other agents early in the season."
In addition, humidity plays a big role, according to Druyff. "Nematodes are swimmers and not crawlers. So, the soil has to be well moist. That is why it is best to spray these agents in the rain or after. If nematodes cannot penetrate the soil properly, they dry out. You should not spray these agents in full sun."
Prevent population build-up
The main focus is often on controlling the first flight, because it causes the most damage, Ten Napel says. But the second flight can also be very dangerous and even the third flight needs to be handled properly to prevent population build-up, he warns. Westhoff also agrees. "When controlling and combating the onion fly, in addition to avoiding direct damage from runaway, preventing population build-up during the season is certainly as important. This sometimes seems like an unprofitable extra cost for growers, because the second and certainly the third flight cause less damage. But everyone has every interest in ensuring that the initial population for the following season is as low as possible."
Pull out all the stops
Although the resource package has shrunk significantly and biologicals are listening more closely to the right application conditions, the companies do not see onion fly control as bleak. Druyff: "We clearly see that with applications at the right time and under the right conditions, we can give the first fly a serious blow. This keeps the onion fly manageable. But it doesn't stop after the first flight and with one pesticide application. We have to pull out all the stops to successfully control the fly."
In areas where the profitability of onion cultivation is under pressure, you sometimes see growers dropping out because of the cost price, Graafstra continued. "As a result, the SIT coverage ratio is unfortunately no longer guaranteed. While it is precisely the coverage ratio that is so incredibly important for optimal results. Where yields are good, it is easier to get everyone on board. New growers also join or are encouraged by their colleagues to join. So, you can completely cover an area and make joint efforts."
Onion fly is not a one-off problem of an individual grower, but of the whole sector, concludes Westhoff. "If we want to get and stay on top of onion fly, joint efforts are definitely needed."
