Chrysanthemum growers finally get thrips control back on track

Chrysanthemum growers finally get thrips control back on track

Dutch chrysanthemum growers are feeling quite optimistic about thrips control. After years of high pest pressure, growers are getting more adept at integrated pest control, thanks to meticulous scouting, a good predatory mite, supplementary feeding and biological crop protection products. Of course, there isn’t a one-size-fits-all approach that works for every nursery. Chemicals are still an indispensable part of the mix as a backup. Growers and suppliers outline the latest developments.
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Even finding a thrips to take a picture of for this article isn’t easy. It’s the middle of May and we’re looking for a specimen on the sticky traps at Kamuro Flowers, Frank, Rick and Arthur van Zeijl’s 38,000 m2 year-round chrysanthemum nursery in the Hook of Holland.

That’s extremely good news, especially when you consider that the chrysanthemum sector has some difficult years behind it. With the use of chemical controls declining, thrips were popping up everywhere, posing a real threat to the crop. Frank van Zeijl: “About this time last year there would be at least thirty on each sticky trap. This year is different. Partly because of the weather, because by April we hadn’t had an extended period of warm weather in the Netherlands.”

More thorough scouting

But that’s not the whole story. The van Zeijl brothers have also changed their crop protection strategy, including introducing more thorough scouting. In 2016, their company, which grows disbud chrysanthemums like Anastasia, Astroid and Siberia, joined Zentoo, a quality mark that guarantees 100% pest-free flowers. To make this a reality, members have to follow a strict protocol for scouting and thrips control. Since last year every member has had one person responsible for monitoring the protocol.

“Scouting is more important now than ever before,” van Zeijl says. “Our supplier Van Iperen does that for us. They not only check the sticky traps, they walk through the crop as well. Because the sooner you discover a hotspot, the better. Once a week we also spray with NeemAzal-T/S or Azatin, which are both products of natural origin. They work well with Phytoseiulus persimilis, which we use against spider mite. We don’t use the predatory mite Transeius montdorensis because we regularly get bugs and the biological products we use are also effective against them.”

Production scaled up

Unlike the van Zeijls, however, many other growers are opting to use this “new” predatory mite against thrips this year. Two or three years ago, Royal Brinkman (with its partner Agrobio) was the first supplier to achieve good results in thrips control with montdorensis, at chrysanthemum nurseries in Bommelerwaard, the Netherlands. Koppert and Van Iperen (with its partner Bioline) subsequently also stepped up production of this natural agent.

Grower Michel Grootscholten of Monster, the Netherlands, who grows chrysanthemums on 11 hectares, has been using this predatory mite to control thrips since 1 January. “It’s been around for a while but I used to find it too expensive. A few other growers got through the summer very well with montdorensis. After that, all three suppliers scaled up their production, which brought the price down, fortunately. I don’t yet know whether it will turn out to be a good thing for us. We have got thrips under control now, but other pests have been grabbing their chance, like caterpillars and snails. So in mid-May I started using NeemAzal-T/S and the bacterial preparation Turex, two biological agents that fit in well with our biological system. But the system isn’t watertight yet. It’s important for us growers – and the suppliers – to share knowledge.”

Collecting data

Knowledge-sharing is already happening, but mainly between groups of growers. Supplier Van Iperen coordinates crop protection for all Zentoo growers, for example, but it does more than that. Throughout the year, adviser Richard van Spronsen collects data from all the affiliated nurseries – including from washing and counting, for example – and compares the results obtained.

Van Spronsen: “Of course, every nursery and every season is different, but we are building up a database of information this way. As yet there is no such thing as one ideal strategy. Some Zentoo growers are controlling thrips with Transeius montdorensis, some are using our recently revamped bugline with Amblyseius cucumeris, and the van Zeijl brothers only use biological products. Whether the predatory mites are supplementary fed with prey mites differs from one grower to the next. There are a lot of unanswered questions around that, but my impression is that it works. We are continuing to work with the growers to see which integrated solution is the most effective. What is certain is that pesticides – biological or chemical – are still indispensable.”

Products of biological origin

Daniel Cornelisse, adviser at crop protection product manufacturer Nufarm, can confirm that. “Growers usually use chemicals to enable them to get off to a clean start or to supply insect-free plants. Broad-spectrum chemicals are becoming less and less readily available. We will continue to lobby for selective chemicals because it’s important for growers to be able to firefight. For instance, Nocturn (pyridalyl), a selective pesticide for controlling thrips, was approved for Dutch chrysanthemum cultivation last year.”

The focus at Nufarm is on plant protection products of natural origin, Cornelisse says. “We invest in products that fit in well with integrated pest control, like NeemAzal-T/S, which is made from neem tree extract, and Dipel DF, a bacterial preparation. We want to help create a robust integrated system in chrysanthemum cultivation.”

Lianne van Wijk, technical advisor at Certis, also has the same message. “Combining natural predators with biological products makes the system strong. Not least because the products that growers cite – Azatin and Turex, but also BotaniGard – also control other insects apart from thrips. We are constantly looking for products that fit into this system. For instance, NEMguard, a biological agent against nematodes, has just been approved. Of course, chemical pesticides such as Winner are still indispensable.”


Thrips has been a serious problem for chrysanthemum growers in recent years. But Dutch growers are increasingly winning the battle against the pest: mainly because they are scouting much more thoroughly, but also because many nurseries are successfully using predatory mites to control attacks. Biological products are an indispensable link in this integrated approach. It is important for suppliers and growers to continue to develop and share knowledge.

Text: Karin van Hoogstraten.
Images: Studio G.J. Vlekke.



“It works, but you need discipline to succeed”

“It works, but you need discipline to succeed”

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.”

Fully alert

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.”

Nematode sprays

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.

Biological balance

“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.

Researchers make breakthrough in thrips control in chrysanthemum

Researchers make breakthrough in thrips control in chrysanthemum

The predatory bug Orius has been used to control thrips in sweet pepper for many years with great success, but the results have so far been disappointing in ornamentals. Researchers Marjolein Kruidhof and Gerben Messelink now think they have found a solution. With a new method of using the bugs that involves supplementary feeding, thrips can now be successfully controlled in chrysanthemums.

Thrips are the biggest threat to ornamental growers’ crops. Research into biological predators for this pest has been going on for many years. Good results have been achieved with predatory mites, but this has often failed to eliminate the problem because the predatory mites only attack the young larvae. The predatory bug Orius is a very effective weapon against thrips in both the larval and adult stages but it has trouble establishing in ornamental crops. Numerous ways of overcoming this problem have been investigated, ranging from banker plants to feeding stations, but there has been no real breakthrough. Until now, that is.
In the spring of 2017 the Wageningen University & Research Greenhouse Horticulture business unit in the Netherlands started experimenting with a new approach to thrips control in chrysanthemum cultivation. Instead of starting off with chemical crop protection products, the researchers are now introducing biological agents in the cuttings phase. The predators are given high-quality supplementary food so that they can form a strong population or a “standing army” to nip the outbreak in the bud.
“The results that have been achieved this time are due to good coordination between two projects: the PPS Thrips project, in which we are looking for a good alternative supplementary food source, and the Green Challenges project, in which we are optimising the role of biodiversity in crop protection and achieving paradigm shifts,” says researcher Marjolein Kruidhof.

Biological start

In chrysanthemum cultivation, there is usually only a short time window in which you can start using biological control, according to Kruidhof. “Also, the presence of chemical residues delays the growth of populations of natural predators,” she says.
The researchers experimented with a biological start using the predatory bug Orius. They ordered cuttings that were almost pesticide-free, rooted the cuttings themselves and added the bugs a few days before the plants went into the greenhouse. “A biological start is a real change in thinking,” says Kruidhof’s colleague Gerben Messelink. An important part of this strategy is the supplementary feeding, he stresses. “After a series of trials in which we compared different types of food, we ultimately went with Artemia, the cysts of the brine shrimp. This is a potentially good food source and has a long shelf life.”
Trials using Artemia as a feed supplement for predatory bugs had been carried out before but with only moderate results, he says. “The quality of the Artemia that is available on the market at present is good enough for feeding predators like Macrolophus in tomato but not for Orius.”

Significant effect

The researchers therefore got together with the University of Ghent to come up with a good quality food source. Meanwhile, the Israeli company Biobee had also started producing high-quality Artemia which the researchers were able to use in subsequent experiments.
The results exceeded expectations. The number of Orius rose substantially as a result of the supplementary feeding. Having started with fewer than one bug per cutting, by the end of the production phase the researchers were counting 40 bugs per plant. What’s more, the natural predator seemed to respond very well to the availability of food. “It turns out that they are highly mobile,” says Kruidhof. “This has potential because it allows you to manage your biological control better. Plus it means you will very likely be able to reuse the bugs. If you end up with 40 bugs per plant, it would be a shame to spray them dead. That’s destruction of capital. You might be able to lure the adult specimens to new cuttings with targeted supplementary feeding.”

More effective than predatory mites

The impact on thrips damage was significant. “In the control section, in which no Orius or Artemia were used, half the younger leaves were damaged by thrips,” says Kruidhof. “The figure for the plants with the bugs was less than two percent.” The predatory mites did less well than the predatory bugs in terms of thrips control, despite the fact that they had built up a good population with the chosen food source. Researchers still found about 20 to 25% thrips damage on plants following the use of these biological predators. “So Orius really are more effective than predatory mites because they also attack adult thrips,” says Messelink.
“We have proved that the system works,” says Kruidhof. “We can build up the population of bugs by using biological controls and good quality nutrition right from the start, and this population provides good thrips control even in the presence of another food source.” However. that doesn’t mean that this method can simply be replicated in the commercial greenhouse setting. “We still need to optimise certain aspects,” she says. “For example: when is the best time to introduce the bugs? Should they be used in the rooting phase or can they be brought in later? How many bugs should you use? What will your feeding strategy be? How much food should you provide?”

Excellent development

This method of control is based on one generalist. What do you do as a grower if you also have to deal with leaf miner or aphids? “Growers will have to control leaf miner with additional biological measures or selective chemicals. Aphid control can become a problem, but the expectation is that high densities of this predatory bug will also keep aphids under control. Other possibilities for controlling aphids are parasitic wasps, gall midges or perhaps other predatory bugs. We therefore want to investigate whether other types of bugs can be combined with Orius to deal with aphids.”
Crop protection specialist Helma Verberkt of the Dutch growers’ organisation LTO Glaskracht sees this as an excellent development. “It is a good addition to developments in the commercial greenhouse setting, where good results have been obtained in recent years using predatory mites,” she says. “For use in practice, there will need to be enough affordable, good quality Artemia available and it is important to ensure that Orius is compatible with other biological agents and pesticides used.”

Pesticide-free cuttings

The question is also whether cutting suppliers and producers will be willing to come on board. Cuttings with few or no crop protection product residues are currently hard to find. “It’s a bit of a chicken-and-egg situation, but I think we will manage,” says Messelink. “There’s also a real change in thinking going on among cutting suppliers. More and more growers want to start biological control earlier and are asking for cuttings with fewer or no chemical residues. Cutting suppliers are also looking for alternative options. I think biological control is the solution.”
“We have shown that it works now, and that is quite a breakthrough,” Kruidhof adds. “We plan to carry out another greenhouse trial this year and we expect growers themselves to start developing the strategy further as well. As a result, the market for pesticide-free cuttings will only get bigger and more demand-driven. So producers and suppliers will have to meet that demand.”
Both projects are funded through the Top Sector Horticulture & Propagating Materials and are being implemented within this sector with funding from the government, various crop cooperatives and Koppert. The projects are coordinated by LTO Glaskracht Nederland.


Researchers in the Netherlands have made a breakthrough in controlling thrips in chrysanthemums. By starting biological control early on and providing good quality nutrition, it is possible to build up a good population of the predatory bug Orius. This population controls infestations well, even in the presence of food.

Text and images: Marjolein van Woerkom.