Crop tour Onions - Week 22
Pest pressure on plot increases: 'everything sticks'
The Crop Tour this spring clearly shows that the differences in crop development are great, depending on the location and the availability of water. Hubert Linders from Nederweert (Limburg) has seen his onions grow to the fourth pipe after one irrigation. Hail damage occurred in Groningen.
In Zeeland, the pressure from pests is actually increasing. Jacky Dieleman from the Philippines reports that there are already lice present: "Everything is sticking." Reinder Hogenhout responds: "The pressure is exceptionally high, especially considering the cold." He keeps a close eye on his beets, potatoes and tulips. In Groningen, René Haaijer is dealing with a different challenge: there it did rain, but in the form of hail, which caused considerable damage. "You are desperate for rain and then you get a hailstorm like this," he is annoyed. Several plots of land have been damaged, including those with (red) beets and onions, both in his own area and in his surroundings.
Fortunately, the damage to the plant onions was limited, but there is definitely damage to the seed onions. The onions are now extra sensitive to bacteria, fungi and spraying. "You have to pay close attention now, it really requires extra attention," says René. The main question is how these onions will behave in storage. Fortunately, the onions are not yet bulbing. "As a result, the damage is still limited for the time being," he explains. In this phase, the plants are still producing foliage. If that is damaged, it has more consequences than when the bulb formation has already started. Then the onion stops producing foliage and is less vulnerable.
Klaasjan Boer from Kortgene (Zeeland) cannot irrigate his onions, but is still satisfied with how they are doing. Since sowing, a total of around 30 mm of rain has fallen, including a shower of 11 mm last weekend. "That was a nice shower, but it blows away again quickly," he says. Despite the limited precipitation, there are surprisingly good plants to be found. "Some onions only have one tube, while others already have four or five. It varies enormously."
Increasing pressure from onion fly
Peter Holster from Marknesse (Flevoland) also reports four pipes, but expresses his concerns: "Some onions already have filling and there is onion fly in them. That is not as acceptable as is sometimes said." Although there are no reports of catches according to monitoring, Holster remains sceptical: "I still have my doubts about releasing flies. I find it strange that we are seeing them now."
The presence of the onion fly is recognizable, confirms Petra Rijkens of De Groene Vlieg. According to her, the pressure of this pest in areas such as the Noordoostpolder and Flevoland has been increasing for years. "In regions where onions are grown intensively, especially with first- and second-year crops, the population can grow quickly. Nationally, we see that the population in areas with high pressure is decreasing, and areas with low pressure the pressure is increasing."
The increasing pest pressure has various causes. According to Rijkens, the disappearance of certain insecticides since 2019 plays an important role: "In the past, we still had agents with a systemic effect that were also effective against the onion fly, but now the range of agents is much more limited." Climate change also contributes, because warmer conditions allow insects to develop faster and become active earlier. The damage caused by the onion fly is serious. "We hear from growers in areas with high pressure that they are already happy if the decline remains below 30 to 40 percent," says Rijkens. She notes that growers are now combining many agents and techniques to limit the damage. However, effective control remains difficult: "The fly often stays at the edges of plots, for example near ditches and shrubs, and is difficult to hit directly with agents."
A method that De Groene Vlieg has been using for years is the Sterile Insect Technique (SIT). In this method, sterile male onion flies are released throughout the growing season, which mate with the natural population but do not produce offspring. "You tackle the plague at the source, in a natural way. A good coverage rate is crucial for the effect," Rijkens emphasizes.
Not only Flevoland is noticing the increasing pressure. "In Groningen, for example, we see relatively many bean flies, and last year a lot of stem nematodes were suddenly reported. Each area has its own problems, depending on soil, crop, pest pressure and weather conditions. The sooner you know what is going on, the more targeted you can act,".
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