Home Posts Tagged "thrips"


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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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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.

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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.

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More and more research is revealing that endophytes – microorganisms that live in the plant without harming it – can significantly boost a plant’s resistance to pests. These findings prompted researchers to investigate the potential of endophytes in pest control in greenhouse horticulture. Experiments in tomato, sweet pepper, bedding plants and chrysanthemum have shown that while the fungal isolates tested thus far don’t deliver complete pest control, they do significantly inhibit population growth.

Endophytic fungi are therefore of particular interest alongside other control measures, but there are still many unanswered questions about how they work and how to use them. A recently launched fundamental project on endophytes is expected to provide more basic knowledge to support applied research into endophytes.

Entomopathogenic fungi

The study of microbial ecosystems in humans, animals and plants has been the focus of much research in recent years. Just as gut flora is essential for good health in humans, microorganisms in plants also play an important role in resilience to pests and diseases. Precisely how this works is not always clear. Fungi can have a direct effect on pests by secreting certain metabolites, but it has also been shown that fungi and bacteria in plants can accelerate resilience in the plant (known as priming).
Some of these fungi are also known as entomopathogenic fungi, such as those of the Beauveria, Metarhizium, Lecanicillium and Isaria genera. Many of these fungi are used as a product in crops to control insects such as whitefly and thrips. We are increasingly discovering that most of these types of fungi can also grow in plants without harming the plant. Over the past two years, Wageningen University & Research in the Netherlands has been running a number of studies looking at the effects of endophytes on pests that are relevant to greenhouse horticulture.

Inhibiting aphid growth

Aphid control in greenhouse horticulture is still often based on the use of chemical crop protection products, but new regulations and customer requirements are making chemical control increasingly difficult. The alternative – biological control with natural predators – is not always as effective. Aphids are known for their ability to grow explosively on plants, making it difficult for natural predators to get the infestation under control fast enough. Measures that could inhibit the explosive growth of aphids would therefore be a welcome addition.
A study funded by the Dutch Horticulture Product Board was set up to investigate whether endophytic fungi could inhibit the growth of aphids in sweet pepper and the bedding plant Calibrachoa. Many isolates produced no effect at all, but the fungus Lecanicillium muscarium was able to inhibit the increase in aphids by 25-30% in both Calibrachoa and sweet pepper. This may not seem very much, but on a rapidly growing aphid population and in combination with other pest control methods, it can make a big difference. In further research, it is being investigated whether these effects actually result in better control when combined with natural predators.


Another major problem in greenhouse horticulture is the western flower thrips, or Californian thrips, Frankliniella occidentalis. This pest is particularly difficult to control in ornamental crops, partly due to the ever diminishing package of chemicals available and growing resistance to pesticides. In a large public-private project (Masterplan Thrips), various endophytic fungal isolates are now being screened for their effectiveness against these thrips in ornamental crops.
In the fight against thrips, increased plant resilience would be a welcome addition alongside other measures. More than 40 fungal isolates have already been screened, some of which show significant inhibition of 40-50% in the population growth of thrips. Here too, therefore, what we are seeing is not complete control but inhibition of growth. However, the results are promising and the expectation is that they will provide a very useful addition to the various measures for keeping thrips in check. In research carried out in other countries, good results have also been achieved on the tobacco thrips in onion, with endophytes reducing thrips damage by 80%.

Further research

The next step is to take a look at potential applications in greenhouse horticulture. As with aphids, the question in terms of thrips is how to combine endophytes with biological pest control. A delayed development time could be beneficial for predatory mites, which would then have longer to work on the susceptible stages (predominantly L1), but there may also be effects that impact negatively on predatory mites. These and other predators could be affected by certain substances that enter their prey (the pest) via plants containing endophytes. So it is important to test the effects in the overall system of natural predators.
The initial results with endophytes in greenhouse horticulture indicate that there is definitely potential for using these fungi to increase resilience to pests. However, there are still a lot of questions to be answered in the research: for example, what spectrum of pests they are effective against and whether they have any adverse effects on some pests and plant pathogens. Other very practical questions concern the methods of inoculation and longevity during cultivation. It has been found that there are several types of microorganisms in plants (bacteria and fungi) that interact with each other but also with the host plant. These interactions are complex and as yet not fully understood, but they will certainly affect the behaviour of introduced endophytes.

Endophyte project

The basic questions surrounding the plant microbiome are being investigated in a new fundamental endophyte project led by Leo van Overbeek of Wageningen Plant Research. In this project, he initially wants to determine how endophytes can best be introduced into the plant. He is looking at methods of application in practice, such as via treatment of seed or young plant material, possibly in combination with irrigation water treatment.
The researchers are also taking a close look at how endophytes behave in a plant, for example which parts of the plant they spread to which also contain the various pests and plant pathogens.

Measurable effects

Using modern DNA sequencing and other so-called ‘omics’ technologies, including metabolomics, the research is also determining how and to what extent the microbiome of the plant is influenced, based on a) the presence of the causative agent, b) the presence of the endophyte applied, and c) the combination of both. This should shed light on how pests and endophytes respond to each other and what factors play an important role in that.
The hypothesis is that endophytes will affect the composition of the microbiome, which would be expected to have an effect on the metabolism of the plant and, in turn, on pests and diseases. The researchers hope to be able to correlate measurable effects with parameters that provide an indication of a plant’s resilience. This fundamental project is expected to culminate in an integrative model that indicates how to boost resilience in plants by applying endophytes. The model will then be used to develop new technologies to make plants more resilient to pests and diseases.


Fungi and bacteria that live in plants can help make the plants more resilient to pests and diseases. It has been shown in a range of crops that population growth in aphids and thrips can be inhibited by certain endophytic fungi. Further research is looking at developing practical inoculation methods for different crops, integration with natural predators, endophytic behaviour in plants and the underlying mechanisms that increase plant resilience with endophytes.

Text and images: Gerben Messelink and Leo Overbeek, Wageningen University & Research.

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As every grower knows, thrips are a huge problem in ornamentals grown under glass. The usual suspect is the Californian thrips, a species with a strong preference for flowers. But in recent years another polyphagous thrips has been increasingly raising its head: Echinothrips americanus. Without timely intervention, this typical leaf-dwelling thrips can cause considerable damage to ornamentals such as gerbera and rose. Scientists have been taking another close look at how to control this thrips with a range of species of predatory mites and bugs.

In this study, striking differences were found between the four species of predatory mite used and the controlling effect was boosted in some cases by providing pollen. Predatory bugs in the Miridae family have been found to be very effective predators of Echinothrips but the options for using them very much depend on the type of crop.

Switching methods

Unlike the Californian thrips, which is a typical flower thrips, Echinothrips americanus prefers leaves. It is easily identified by its black body with two distinct white spots on the wings. But because it often hides away low down in the crop, its presence in the greenhouse can be something of a surprise. Although the species has been present in the Dutch greenhouse horticulture sector for about 20 years (it was imported from North America, as its name suggests), we still know very little about this creature.
In recent years there has been a clear increase in this species at nurseries switching to integrated pest control. The reduction in the use of broad-spectrum insecticides has given this thrips more chance to establish itself in crops. To avoid disturbing biological control of other pests, it makes sense to also tackle this insect with natural predators. Predatory mites and bugs are good candidates for this.

Control with predatory mites

In the lab, the researchers investigated how susceptible the various stages are to predatory mites and how many individuals of each stage are eaten per day. This was tested with the predatory mites Amblyseius swirskii, Amblydromalus limonicus, Euseius ovalis and Euseius gallicus. The latter two Euseius species establish readily in rose and are therefore interesting candidates for this crop.
Striking differences were found between the species. A. limonicus ate the most Echinothrips larvae, followed by E. ovalis and A. swirskii (see figure). A. limonicus also laid the largest number of eggs out of these mites. Surprisingly, the thrips were more or less left untouched by E. gallicus. The exact reason for this is unclear.
Interestingly, the pupae were also susceptible to the mites. This immobile stage, which is also found on the leaf, cannot defend itself and is therefore suitable prey.
With the exception of E. gallicus, all species of predatory mite ate the pupal stage. However, as the pupae are much larger than the larvae, the number of individuals eaten per day was a lot lower than in the larval stage. None of the predatory mites ate adult Echinothrips. In addition to the number of thrips being eaten by each individual predator, an important indicator in pest control in a crop is how well a predatory mite establishes itself and in what densities: after all, a high density can easily make up for lower predation rates per individual.

Results in the crop

The predatory mites A. swirskii and A. limonicus were compared in a greenhouse gerbera crop, with the results matching those in the lab quite closely. The thrips were controlled better with A. limonicus than with A. swirskii. This difference has also been observed in other studies with roses and sweet peppers.
Curiously, though, Echinothrips often doesn’t disappear completely: it is somehow able to survive the pressure from the predatory mite. So in a subsequent study the scientists looked into whether control could be improved by offering pollen as an alternative food source. Pollen can massively boost the density of predatory mites, which could lead to better pest control. The downside is that the Californian thrips also feeds on pollen, potentially causing scenarios that are detrimental to thrips control.

Effects of pollen

An important question, therefore, is whether this also affects Echinothrips. With bulrush pollen, the answer was a resounding no. Unlike with the Californian thrips, there was no impact on development time, egg laying or population development whatsoever. This is helpful in that it means supplementary food can be provided for predatory mites selectively, as long as there are no Californian thrips present.
The effects of pollen on thrips control were tested on non-flowering pepper plants using the predatory mites A. swirskii, E. gallicus and E. ovalis. Providing pollen increased the densities of all these mites. Control of Echinothrips using this method was significantly better with A. swirskii but despite the higher densities there was no control effect with E. gallicus. E. ovalis responded extremely well to pollen but the control effect was just as good on plants without pollen.

Predatory bugs

It has been known for a long time that the bug Orius majusculus is an effective predator of Echinothrips on sweet peppers. Orius is used to control thrips in this crop with great success. But the bug doesn’t establish in many ornamental crops. In recent years, scientists have been looking at the effect of omnivorous predatory bugs in the Miridae family on gerbera. In addition to Macrolophus pygmaeus, various Dicyphus species such as D. maroccanus, D. tamaninii and D. errans have also been trialled. All these species controlled thrips extremely well. Plants were made completely clean and remained so.
The bugs seem to be excellent candidates for preventive use against a range of pests and they can maintain themselves well because they eat a variety of prey and plant sap. This can also be a disadvantage, however, because feeding on plants can cause flower damage. Further research will be carried out in the future to see whether and when that happens and whether it can be avoided. The use of these bugs in gerbera could be a breakthrough in biological control of Echinothrips as well as other pests. Future research in other crops, such as roses and pot plants, is planned to ascertain whether the use of predators with banker plants can be supported.


Echinothrips is appearing more and more frequently in ornamentals grown under glass. In trials with a number of species of predatory mite, A. limonicus provided the best control while E. gallicus had little if any effect. Control with A. swirskii was improved by offering pollen. Besides predatory mites, predatory bugs of the Miridae family were found to be excellent predators but further research is needed to prevent potential flower damage and to improve establishment in different crops.

Text and image: Gerben Messelink, Somayyeh Gasemzadeh and Ada Leman (Wageningen University & Research).

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Controlling thrips in cut flowers biologically may be difficult but it’s not impossible. Supplementary feeding with pollen can help to build up and maintain a vigorous predatory mite population. But this strategy isn’t without risk, since pollen is also a food source for thrips. This was the starting point for a research project on roses which also looked at how other predatory mites react to supplementary feeding.

Supplementary feeding of biological predators with pollen preparations is a strategy that is becoming increasingly popular. In vegetable production, particularly in cucumbers and sweet peppers, a lot of growers are using this method to build up a healthy population of predatory mites. But is it also suitable for ornamentals, an area in which pest tolerance is low?
Entomologists Gerben Messelink and Ada Leman of Wageningen University & Research in the Netherlands are studying the development of Californian thrips in roses and their control with a range of predatory mites. Part of this research involves supplementary feeding with pollen, a method that has raised questions in the past.

Taking no chances

“Biological control in ornamental production is no easy option,” Messelink says. “Pest tolerance is so low that it is difficult to build up a good biological balance. A few thrips on a glue trap are enough to trigger panic.” In other words, growers don’t want to take any chances if there is a risk of infestation, especially with Californian thrips. But there are not enough chemicals available to control or correct an infestation.
The entomologists’ research is focused primarily on getting biological agents, in this case predatory mites, better established in the greenhouse and the crop. They call this the “standing army principle” – an army that is on standby ready for deployment.
If biological agents are difficult to establish in a crop, banker plants can sometimes help. These are plants on which useful predators establish readily and then spread throughout the crop. But this method doesn’t work for predatory mites because they don’t fly and only move around slowly.

Pollen a firm favourite

Amblyseius swirskii has so far proved one of the most successful predatory mites for controlling thrips in ornamentals. But to do its job, this creature must be in tip-top condition. As thrips pressure is kept as low as possible, there isn’t very much food around. Fortunately these mites are omnivores and are happy to tuck into a good meal of pollen. So providing pollen to help build up the predatory mite population would seem an obvious choice.
And yet there’s a danger lurking in this method. Thrips also like to feed on the same pollen. So the question is whether providing pollen actually does more harm than good. There is also the risk that the predatory mites could become lazy and leave the thrips in peace.
So the researchers first wanted to get a clear idea of how both thrips and predatory mites would react to supplementary feeding. They studied a thrips population fed with cattail pollen, maize pollen, Ephestia (Mediterranean flour moth) eggs and cysts of Artemia (brine shrimp). The thrips population remained lowest in the control treatment without supplementary feeding and increased most in the treatments with pollen and Ephestia. “We noticed that the number of eggs laid by the thrips tripled on average when pollen was given,” says Messelink. This confirms that thrips love pollen and thrive on it.
The predatory mite A. swirskii was studied separately and was given the same food sources. This study also showed that pollen and Ephestia had a considerable impact on egg laying.

Supplementary feeding needed

With these results in mind, the researchers devised a strategy for the rose, a crop in which a predatory mite population can be difficult to maintain. Using pollen, they quickly built up a vigorous population, and stopped supplementary feeding when the thrips started flying in. But that didn’t work: the number of predatory mites dropped to such an extent that the thrips gained the upper hand.
Messelink: “We have changed our strategy now. Supplementary feeding is still needed once the thrips have arrived. A vigorous predatory mite population can easily tackle a nascent thrips infestation despite the presence of pollen.”
The question then arises as to how much pollen is needed to maintain the population. It’s difficult to say, and it also depends on the conditions on site. The equivalent of 5 kg of pollen per hectare per week was administered in the study, but in practice it has been found that good results can also be achieved with smaller quantities.
Research in 2015 with a combination of A. swirskii supplemented with the predatory mite Macrocheles robustulus also delivered quite promising results. A. swirskii attacks the adult thrips and M. robustulus goes for the pupae that fall on the ground. The two predatory mites make a good team.

Other good natural predators

In many tests, Amblyseius swirskii has proven to be one of the better predatory mites for controlling thrips, but this has always been tested without the use of pollen. It may well be that other species are even more effective when given a supplementary food source. So in greenhouse trials with roses, A. swirskii was compared with Iphiseius degenerans, Euseius gallicus and Euseius ovalis. They were all given pollen.
The test confirmed that both I. degenerans and E. ovalis responded very well to supplementary pollen feeding – even better than A. swirskii, in fact, proving that these predatory mites are an interesting option for use in roses. Many predatory mites don’t do well in a short period of heat with low humidity, but I. degenerans appears to be reasonably resistant to that, making it a useful addition to the overall package.
If Echinothrips also occurs alongside Californian thrips, E. ovalis in combination with pollen is a good species to use. In short, various species of predatory mite are suitable for controlling thrips in roses in conjunction with pollen.

Total approach

Messelink predicts that good thrips control will consist of a total approach involving a range of biological agents at different times. Close monitoring and providing supplementary pollen feeding at the right moment are also part of this mix.
In their follow-up research, the entomologists are not only looking at control with predators but also at the “push-pull” method, which is based around repelling and attracting pests. For example, the crop can be made unattractive to insects with certain smells, or the pests can be lured away with plants or smells that are more attractive to them. Increasing plant resistance also plays a role in this strategy. Research is currently being carried out in wind tunnels, in particular using smells that should scare off thrips.


Controlling thrips in rose cultivation biologically is difficult: due to the low pest tolerance, the predatory mite population has trouble establishing properly and staying vigorous. Supplementary feeding with pollen goes a long way towards solving this problem. This strategy also puts other predatory mites back in the picture. A total approach with a range of biological agents is likely to be the most effective one.

Text and image: Pieternel van Velden and Wageningen University & Research.

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Chrysanthemum grower Maurice van Os, like many Dutch colleagues, has a problem with thrips in the nursery. He solves it mainly by using Amblyseius swirskii, supplemented with Amblyseius cucumeris in the cold season. He steams to control primarily Fusarium, nematodes and Rhizoctonia. Good hygiene practice during steaming and the stimulation of soil life after steaming are also necessary, he says.

Maurice van Os has a nursery of 2.5 ha in the southwest of the Netherlands and grows the varieties ‘Feeling Green Dark’ and ‘Alana’. The grower comes from a true chrysanthemum family. He started in 1997 as the third generation. His grandfather, father and uncle also grew chrysanthemums at a different location. When the family built a new greenhouse in 1998 his uncle left the company and his father followed later.
The location of the company was deliberately chosen for its 'good' ground: a sufficiently airy soil, with a sandy subsoil containing humus. The good soil structure makes watering easy.
He steams the nursery annually just like his father, uncle and grandfather. They used to hire a steam boiler but in 1998 Van Os purchased a high-pressure steam boiler. In those days heating and steaming with one boiler was the trend but now he wouldn’t purchase such a high-pressure boiler. “The mandatory annual inspection is an extra cost. The advantage is that the soil becomes less wet and the sheet cover over the soil gains a sphere shape faster.”

Steam after the summer

Opinions about the best time to steam vary. The chrysanthemum grower steams annually between week 30 and 39. “Some of my colleagues steam before the holiday. I chose afterwards. After steaming the plants grow better.”
Combined with the greenhouse temperature, which during the summer can rise to more than 32ºC, it can lead to too many buds in the bunch; ten instead of six. “We call that a ‘wild’ stem. In addition, the varieties that I grow are already quite heavy. If we steamed before the summer they would become too heavy. Then you also have more chance of yellow leaves. If the autumn is disappointing then you are very pleased with the heavier stems.”

Steam via the drainage

The chrysanthemum grower laid a steam drainage system 55 cm deep. Before steaming, Van Os loosens the ground to a depth of 50 cm using pens. Then the ground is nice and airy and the steam can spread optimally through the ground. He rolls out the steam sheet with an automatic roller and anchors it at the front and rear with steam chains and the sides with heat proof water pipes. Then the mesh is lowered down and the steam boiler and fan are turned on. The small pipes belonging to the steam drainage system stick out above the cover. The fan sucks the steam through the pipes, pulling it from under the cover deep into the ground.
“I steam for about 6.5 hours. Usually I start steaming as soon as the staff go home at the end of the day. I leave the cover in place and keep the fan running until the next morning.” He uses 3.5 m3 gas per m2 for steaming. Contrary to advice he does not use an extra energy cloth over the steam sheet. “As far as I am concerned, the mesh and supports can also get hot. The steam scorches everything it touches."


Only steaming is not enough according to the grower. Steaming is carried out in one area while the rest of the nursery is in full production. Therefore it is important to ensure that the clean ground is not immediately contaminated again. “Before steaming we thoroughly sweep the path and we ensure that the tilling machine and tractor are clean.”
He purposefully doesn’t partition the greenhouse by lowering the internal walls around the steamed area because he doesn’t believe this helps combat the spread of thrips.

Thrips the biggest problem

Thrips is the biggest problem at the moment. “No one really has the answer.” Every week since the spring he has released Amblyseius swirskii: 200 units per m2. “Every two weeks I take samples to see if there are still plenty of predatory mites in the crop. Many colleagues release Amblyseius cucumeris. I only do that in the colder period from week 48 to week 3. The idea behind the early release of swirskii is that you create a biological balance early in the crop. That tends to work better with swirskii than with cucumeris. So far it doesn't help enough but I don’t need to correct too much in between.”
Before harvesting he sprays the crop clean with Vertimec and Actara.

Quickly bring soil in balance

According to the grower healthy soil leads to more resistant plants, which are essential for reducing the risk of pests and diseases. “After steaming the soil is sterile and the plants, which grow faster have a softer leaf that is extra attractive to thrips. That is not an ideal situation,” says the grower. Therefore once or twice in the winter, between week 40 and week 10, he scatters poultry fertiliser and lime granules to redress the soil balance as quickly as possible. “The fertiliser also helps create an airy soil at the bottom. That’s good because as autumn approaches the ground is wetter.”


Chrysanthemum grower Maurice van Os uses a high-pressure steam boiler and special steam drainage at a depth of 50 cm to combat Fusarium. He uses Amblyseius swirskii, supplemented with Amblyseius cucumeris in the cold months, to combat thrips. In addition he maintains a healthy soil by ensuring an airy soil structure and applying poultry fertiliser and lime granules after steaming.

Points of attention for steaming

René Corsten of the Delphy chrysanthemum team has some points to remember when steaming the greenhouse:
- Make sure that the ground to be steamed is as dry as possible. Trying to steam wet ground is a waste of energy so stop the watering early enough.
- Use insulation material over the steam sheet. Insulation saves up to 0.5 m3/m2 and there is less condensation under the cover so the ground shuts itself off less quickly especially at the outer edges.
- With a little effort the steam transportation pipes can also be insulated.
- Preferably use rainwater.
- Flushing on time and good water treatment are important for effective steaming.
- Measure the temperature to check what you are achieving. These days good thermometers are available.
- If there is a problem with nematodes or soil fungi aim for 60-60-60: 60 centimetre deep, 60ºC and 60 minutes. The aim is not to use as little gas as possible but to get the best result. Vacuum (suction) steaming is a must. After steaming allow the fan to run for around 12 hours. Then there should be a continuous heating effect to a greater depth.

Text and images: Marleen Arkesteijn

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Combating aphids with biological crop protection agents is everything but easy. This is why Biobest is involved in relentless research into new ways to get rid of aphids. The result? The hoverfly, Sphaerophoria rueppelli, is being deployed in the battle.

Many commercial crops are plagued by a plethora of aphid varieties that can create a tremendous amount of damage. A lot of preventive pest control strategies have already been explored. Many of these focus on the use of parasitoids. Although these are quite effective, they still aren’t effective enough to be truly reliable. According to Biobest, the biggest problem is that every parasitoid attacks only a limited number of hosts.

The latest weapon in the war against aphids, Sphaerophoria rueppelli, is a native species that is very common throughout Europe and in numerous Mediterranean countries. Its larvae are highly efficient predators of various ahipd vaieties, including Macrosiphum euphorbiae. Not only do hoverflies feast on aphids, they also consider whiteflies, thrips, and spider mites to be a delicacy. ‘Sphaerophoria rueppellii will go actively in search and fly long distances to detect even the first signs of an aphid colony,’ explains Yann Jacques, Product Manager for Macrobials at Biobest. ‘The Sphaerophoria System is therefore also a preventative system. It is a perfect complement to our existing range of parasitoids and predators.’


Adult hoverflies are virtually harmless, as they feed only on pollen and nectar. It’s the larvae, however, that are the leading players in the war against aphids. Adult females prefer to lay their oval-shaped greyish white eggs in large colonies of aphids, to ensure that their offspring will have plenty of nourishment. Each female can lay up to 20 eggs a day, and up to 400 eggs in all. The green larvae that are hatched can consume an average of 200 aphids during their larval stage, which lasts nine days at a temperature of 25°C. They will also feed on other pests, such as whiteflies, thrips, and spider mites. ‘What’s so interesting about this system is that the hoverflies can be deployed concurrently with aphid parasitoids. Hoverfly larvae will devour only the non-parasitised aphids. By deploying both, the number of aphids will be reduced even more dramatically,’ concludes Jacques. S. rueppellii is highly efficient when deployed in sweet pepper crops, but it looks as if this system will also promote aphid control substantially in other vegetable, fruit and decorative plant crops.

Source/photo: Biobest.