Thrips are one of the biggest threats chrysanthemum growers face. But this hasn’t put off River Flowers in the central Dutch town of Zaltbommel: they grow one of the most sensitive chrysanthemum varieties using integrated pest management. “It works, but you need discipline to succeed.”
Chrysanthemum ‘Haydar’ grows here in all its glory. When you enter the 3.5 hectare greenhouse, you are confronted with a sea of plants graduating from green at the front of the greenhouse to purple with a white edge at the back. Not many growers are keen to grow this chrysanthemum variety because of the huge threat posed by thrips. In extreme cases these small, thin insects can even mean bankruptcy, but Peter van de Werken of River Flowers in Zaltbommel is up to the challenge. “We have the resources to deal with them and we have a person working full-time on crop protection. It is more labour-intensive, but we can charge more for this plant.”
At the beginning of each plant row is a yellow sticky trap hanging from a pin mounted on the truss. Some traps have more black dots on them than others. CEO assistant Rick van de Werken takes hold of one of the yellow cards and examines it with a magnifying glass. “Look, there’s a thrip.” He points to a dot that is barely visible to the naked eye. “Last week I found two different species.” He shows us a picture on his phone. “We need to be fully alert to keep the pressure as low as possible.”
His uncle agrees: “We base our cultivation decisions on the risk of thrips. Do we want to use the sprinklers? First we check whether that fits in with our pest management strategy. It is a huge threat and difficult to get to grips with, or at least it has been until recently.”
Until 2014, the growers fought the pest with chemicals and the predatory mite Cucumeris. “Every year we started off using Cucumeris in the spring, but things would often get out of hand between weeks 35 and 45, so we had to correct with chemicals,” says Teun de Leeuw, the company’s crop protection specialist. “That immediately killed off all the biological life, including the other natural predators. The biological balance got out of kilter and we had to rebuild the biology from scratch. We were always falling behind.”
So he decided to try something new: correcting with Nemasys nematode spray. “I had heard of this before and thought: this will enable the bio to continue to do its job,” he says. The nematodes did the trick and got the number of thrips back under control. Since then, the nursery has hardly ever needed to use chemicals to tackle an outbreak.
“This doesn’t mean we can sit back and relax, though,” Van de Werken adds. “Thrips get into your crop in three ways: through the windows, through the door or from the soil. To minimise the risk in the soil, we only grow one variety of chrysanthemum in each greenhouse. Luckily we don’t have any growers as neighbours, so there is less risk of contamination through the windows. We also carry out intensive chemical thrips control measures on the new cuttings for two weeks. And lastly, Teun is always on the case.”
Teun works on crop protection full-time. He checks the plants and the sticky traps daily. “We sit down with Alliance once a week. I keep an eye on the cost structure, they input their experience, and when the number of thrips rises and the biological balance gets out of kilter, we decide together whether we need to change the strategy.”
Focus on biological control
Nematodes are not the only thing they use to keep the numbers of thrips down. They also use tapes, fungi and turkey feed. According to Piet van Boven, bio-insecticides advisor at BASF, growers are increasingly looking for a biological solution. “Firstly because the list of legally permissible pesticides is getting shorter and shorter, and secondly because pesticides are detrimental to the biological life in the greenhouse. It takes time and energy to build that up again,” he says.
For the last four years he has been recommending Nemasys nematodes. Cage tests have revealed that the effectiveness of these nematodes is around 40-60% in the soil and 60-70% in the crop. Van Boven: “Of course, you can’t translate these results directly to the commercial setting, but they do show a continuous effect. We see nematodes as part of the package of control measures and we get good results in combination with predatory mites.”
Thrips determine cultivation choices
However, it is important to follow the nematode protocol. Among other things, that means leaving the leaves wet for two hours. “Sufficient moisture is vital,” van Boven adds. “We recommend spraying with a normal spray boom. This produces the best nematode distribution and avoids dripping.”
But the chemicals company is also on a learning curve. “We used to think that the best time to apply nematodes was in the dark, at around 4 am. But new insights have revealed that spraying them in the late afternoon can also be very effective.”
For de Leeuw, that means no more getting up at the crack of dawn, although at this time of the year the plants do start the night wet, which increases the risk of rust. “It’s always something to bear in mind, but as mentioned, the thrips determine our cultivation choices,” he says.
Chemicals as a back-up
Discipline is and remains the key to success. Van de Werken and de Leeuw discovered this in September last year, when disaster was narrowly averted. “When it came to harvest time, we suddenly noticed that the flowers in some of the bays ready for harvesting were damaged,” van de Werken says. “We went to look for the cause and found insect-damaged tapes. It turned out that we had had an infestation of mice, attracted by the bran in the tapes. Because the tapes had been eaten, the thrips had had a field day. The whole balance was out of kilter. We bought some cats and tackled the thrips with nematodes and a bit of chemistry. It took more than 15 weeks to restore the balance in the greenhouse. We have learned to be even more alert now. It was a really tense time, because if you don’t get an outbreak like that under control, you might as well shut down. You would simply go bankrupt.”
So it is extremely important to have some chemicals that can still be used, he believes. “As a grower I only feel confident in heading down the biological control path if we can continue to use chemicals as a back-up. If the government allows us some leeway in terms of chemicals, we will be happy to use biological control methods in return. Chemicals are a must when you are experimenting with biological pest management. You have to have something up your sleeve if things go wrong.”
Netherlands-based River Flowers grows chrysanthemum ‘Haydar’, one of the most sensitive chrysanthemum varieties to thrips, which they control using integrated pest management. They keep numbers down with nematode sprays, but discipline is a must. One employee specialises in crop protection full-time, and the nursery is constantly optimising its methods.
Text and images: Marjolein van Woerkom.