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.



CropObserver offers growers more insight into a plant’s growth process

CropObserver offers growers more insight into a plant’s growth process

Growers are, of course, keen to ensure that their crops grow as optimally and quickly as possible. An efficient – or more efficient – growing method will, of course, automatically produce a high – or higher – yield. To help growers gain greater insight into the growth process PhenoVation developed a device that makes photosynthesis visible.

The device that makes photosynthesis visible is called the CropObserver. The measuring device is suspended from the greenhouse ceiling and measures the values of the crops growing several metres beneath it.


PhenoVation’s device measures parameters that correlate with the maximum efficiency and effective efficiency of the crop. Based on this data, growers can gain insight into the effect of specific light and growth strategies on a crop’s growth processes, thus allowing them to determine the optimum growth conditions for their crops. Additionally, growers can optimise their Leaf Area Index (LAI) using the input provided by the CropObserver.


Chrysanthemum grower and chairman of LTO Glaskracht’s National Chrysanthemum Committee David van Tuijl is currently testing the Crop Observer in the greenhouse in Brakel where he grows his flowers. Van Tuijl is the first grower to use energy-efficient LED lights throughout an entire greenhouse, making it a prime example of what a conventional greenhouse would look like in the future.

Initial assessment

Van Tuijl’s experiment receives assistance from various experts at Wageningen University & Research, Philips Lighting, the Delphy knowledge centre and the Glastuinbouwpact greenhouse horticulture association. The pilot project will last one year, in principle, but an initial assessment will be take place after six months. “This assessment will decide if another six months will be worthwhile”, says Van Tuijl. “We have only been operating for one cycle, so it is too early to draw any definite conclusions about the CropObserver.”

A grower request

A wish communicated by growers for a different way to measure photosynthesis prompted the development of the CropObserver. “Before the CropObserver, measurements were taken with a system that recoded the values of only one leaf”, says Vincent Jalink, who developed the device. “Besides, this system was not wireless, which was rather inconvenient for some growers. Growers indicated wanting to measure the values of multiple leaves via a wireless system.”

Compatible with climate system

“Depending on how high it is suspended, the CropObserver can measure 4 to 6 square metres of crops growing beneath it, fully wireless”, explains Jalink. “What’s more, it is compatible with LetsGrow and Hogendoorn climate systems, which allows us to link certain actions to specific values. One of our customers is already doing this. As soon as the CropObserver measures a specific value the climate computer will open the screens. This has enabled the grower to shorten his crop cycle, thus allowing him to fit more cycles into a single year.”


According to Jalink the CropObserver can be used to measure photosynthesis and growth in all crops. “Of course, not all plants lend themselves equally well to using the CropObserver for climate control purposes”, adds PhenoVation’s developer. “Tomatoes, for example, flourish when exposed a lot of light and heat and will therefore not respond as strongly to changes in light ingress and temperature in the greenhouse. Therefore having your climate computer controlled on the basis of photosynthesis makes less sense when growing plants like these. However, with crops like tomatoes the system can be used to measure production by gaining insight into the ETR (Electron Transport Rate), considering that the ETR value correlates very well with the amount of carbon dioxide absorbed into the crop.”

Perfecting the growth strategy

According to Jalink sun-sensitive crops and potted plants perform much better under those conditions where a CropObserver is linked to a climate computer. “These crops respond more strongly to a change in light ingress or temperature, which allows growers to perceive the effect of their actions within a day, or even within a few hours – and to perfect their growth strategy accordingly. Various tests have shown that this will increase production yield by at least five per cent for the same surface area.”

For rent

Because Jalink does not yet know which crops respond well to the CropObserver-climate computer combination, he is also making the measuring equipment available for rent. “This will help growers independently decide if the CropObserver is an interesting device for them, without having to purchase it immediately.” Growers who are interested in this system can contact Jalink through the contact details on the PhenoVation website.

Text: Leo Hoekstra. Photo: Marleen Arkesteijn.


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.


Too little insight about factors that effect flowering

Too little insight about factors that effect flowering

We know much more about photosynthesis than about the flowering of the plant. This sometimes leads to surprises, especially with new crops. The grower has to take into account the juvenile phase, effect of temperature, light, size of the plant, day length, and the interaction between hormones, sugars and other compounds in the plant.

Before a plant can flower it first has to become an adult. Many plants have a juvenile phase. Even under optimal conditions they are unable to flower during this stage.
This is logical because a plant flowers in order to reproduce. Therefore the flowers must be of sufficient quality to actually achieve this. They have to be developed to the extent that they can be pollinated, for example by insects. And after pollination all kinds of processes need to start for the fertilisation and development of seeds and fruits. All of this costs a lot of energy. So from the plant’s point of view it’s considered ‘wise’ to postpone these processes until sufficient assimilates are available in the plant.

From juvenile to adult phase

The duration of the juvenile period varies enormously, from a few days to a few decades in trees. Of course, for the grower this can be very unprofitable if you have to wait a very long time for it to become productive. That’s why it’s good that a cutting or graft taken from a plant that is already in the adult phase also remains an adult.
The switch from juvenile to adult phase happens quite abruptly. The moment at which this occurs can depend on the size of the plant, age, number of leaves and growth factors.

Hormonal factor

From a wide range of research it’s clear that a hormonal factor plays a part in the transition from vegetative to generative. Suddenly the apical bud changes in shape as a forerunner to flowering. For a long time researchers looked for a hormone that stimulated flowering. The unknown flowering hormone was even given a name, namely florigen. But, it is now clear that florigen does not exist.
Although gibberellins play a role in many plants – this group of hormones was for a long time the leading candidate for the role of florigen – the situation is still ambiguous. In some plants gibberellins actually slow down flowering. Bearing this in mind, it’s also remarkable that growth inhibitors, such as daminozide, that slow the activity of gibberellin, do prevent the long and thin development of flowering plants, but not the flowering itself.
Another hormone group, the cytokinins, plays an important role in the induction of flowering. But again no general rules apply.
It seems that an interaction between hormones, such as gibberellins, cytokinins and ethylene, as well as sugars and other substances, such as polyamines, causes the induction of flowering. It’s different for every crop. The limited knowledge about the mechanism of flowering makes it difficult to effectively influence flowering. This is especially the case for new ornamental crops. Usually, the practical research focuses on achieving the most appropriate cultivation measures, without knowing exactly what happens inside the plant.

Leaves under first truss

Fortunately, a lot of research has already been done on the major horticultural crops. One of the many crops examined is tomato. A grower would like the plant to start producing quickly, and in terms of the tomato this means: the number of leaves under the first truss has to be limited.
In theory, a certain amount of assimilates must first be present in the tomato plant before it can start to flower. Indeed, research shows that any procedure taken to increase the amount of assimilates speeds up flowering. More light means fewer leaves under the first truss. A higher temperature at a low light intensity also leads to more leaves under the truss because the plant consumes more energy at a higher temperature.
As well as having a minimum quantity of assimilates, distribution is also important. At a lower temperature the top of the plant – the apex – has an advantage as it competes with the leaves.
This knowledge is difficult to translate into other crops. In fact the influence on flowering should be examined separately for each crop.

Short day = long night

A special phenomenon is the sensitivity of a flower to day length. In this respect, the origin of the plant makes a big difference. At the equator the length of day and night are the same and tropical plants are not day length sensitive. Plants from higher altitudes, that flower in the spring or even in the autumn, do tend to be sensitive to day length.
Sensitivity to day length exists as a result of natural selection. Therefore it’s also possible to remove this sensitivity by selection. By consistently selecting and further propagating the most insensitive plants it’s possible to solve this inconvenience. This doesn’t work sufficiently well with all crops, so we do encounter short day plants such as poinsettia, chrysanthemum and kalanchoe and long day plants such as gypsophilia, trachelium and carnation.
The naming is actually wrong. A short day plant is actually a long night plant because it’s all about the length of the dark period. And if this is broken – even for just a very short period – the whole effect of the dark period is lost.

Length of dark period

The plant registers the length of the dark period in its leaf but flowering takes place elsewhere. Therefore there has to be some communication between the leaf and the point where flowering occurs. This is carried out by a hormone that is produced in the leaf and then travels to the point of flowering.
How does the plant measure the length of the dark period? Previously researchers thought that the pigment phytochrome slowly broke down during the night into another form and that this was a signal to the plant to start flowering. But it’s more complicated than that. There is an interaction between the endogenous rhythms in the plant (‘the biological clock’). As a result the same length of darkness can sometimes produce different effects, whereby temperature can also play a role.
Some short day plants need just one long night. One of the most well known short day plants in horticulture is the chrysanthemum and it actually needs several weeks of long nights. If a grower stops the dark period prematurely, abnormalities occur. Just after a few short days the growth point becomes generative and stops producing leaves. Yet a grower has to continue with the long night regime for several weeks. It’s likely that multiple genes are involved in the flowering of chrysanthemum and it’s not simply a transition from vegetative to generative based on one gene that can be turned ‘on’ or ‘off’.

Mutual competition

Once the plant has switched from vegetative to generative and then flower buds have actually formed, many things can still go wrong. The buds can dry out or fall off and the flower may not open properly. This is mostly a question of how well the flower bud and the flower have been supplied with water, minerals and assimilates.
The flower has to compete with other parts of the plant and sometimes loses the fight. Optimal climate conditions, providing enough light and water, reducing the competition with the young leaves (by picking leaves) are all ways to ensure that flowering is successfully achieved.


A plant can only flower when it is mature. In horticulture, we bypass the juvenile phase by using cuttings and grafting. The transition from vegetative to generative appears to be controlled by a hormones. Flowering is the result of an interaction between several substances, for example, gibberellins. We still know too little about flowering which is sometimes difficult when working with new crops. A lot of research has been carried out on the major horticultural crops such as tomato and chrysanthemum. The latter is the best-known short day plant, although we should call it a long night plant.

Text: Ep Heuvelink (Wageningen University) and Tijs Kierkels. Images: Theo Blom (University of Guelph, Ontario, Canada).