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Researchers have found three species of predatory bug in the Miridae family that can limit the establishment of the harmful predatory bug Nesidiocoris tenuis in tomato crops. “Omnivorous predatory bugs could potentially be a great help in biological pest control,” researcher Gerben Messelink says.

In recent years the bug Nesidiocoris tenuis has become a serious problem in tomato growing. Although it controls whitefly and tomato leaf miner, the insect causes so much damage to the crop that growers tend to view it as a pest rather than a predator. These Mediterranean plunderers puncture fruits and flowers and cause necrotic rings which lead to stems breaking off. They inhibit plant growth, resulting in deformed plants. Tips fall off and production stagnates.

“Tomato growers in southern Europe have been using Nesi to control Tuta absoluta for years,” says researcher Gerben Messelink of Wageningen University & Research’s greenhouse horticulture business unit in Bleiswijk, the Netherlands. “Various other predators such as nematodes and parasitic wasps have been tried as well, but Nesi appears to be the most effective. Southern Spanish growers therefore mainly see this creature as a useful predator.”

But things are different hundreds of kilometres to the north, where Nesi is a serious problem for Dutch tomato growers. Crop damage is severe and the predatory bug is difficult to control selectively. Chemicals also have an adverse effect on Macrolophus, a relative of Nesi, giving pests such as whitefly free rein and disrupting the whole biological control system in the greenhouse. What’s more, this creature develops faster than related species. “It is a thermophilic insect, so when the temperature in the greenhouse is 20°C the population is already growing strongly,” Messelink says.

Omnivores in a spectrum

To tackle this problem, in 2015 WUR teamed up with the growers’ organisation LTO Glaskracht Nederland and submitted a project proposal, “Pest control with omnivorous predatory bugs”. This public-private partnership launched in 2016, financed half by the Dutch Ministry of Agriculture and half by the private sector. The private funding for this project comes from the Dutch tomato, gerbera and rose growers’ cooperatives, Stichting Programmafonds Glastuinbouw (the Dutch greenhouse horticulture programme fund foundation) and Koppert Biological Systems.

“Nesi is a predatory bug in the Miridae family, just like Macrolophus,” Messelink explains. “Miridae are omnivores that not only use plants as food but also serve as predators. There are many different species of Miridae and they are all located in different places in this spectrum. One species eats more plant, the other more prey. Macrolophus is a predatory bug that is very pest-oriented and only causes limited damage to plants. Nesi, on the other hand, quickly causes a lot of damage but is also a good pest controller.”

Messelink and his colleague Ada Leman ran a greenhouse trial last year in which they investigated whether establishment of this insect in tomato is limited if a population of other bugs is already established in the crop. To determine whether Nesi also affects the densities of its relatives, controls with these bugs without the notorious predator were also set up.

Insect cages

The greenhouse trial was carried out in large insect cages with one tomato plant per cage: grafted Brioso plants with two stems. The effect was assessed using three types of new predatory bug, which the researchers brought in from southern Europe. “We investigated the effect these three species had on pest control and we looked at the development and establishment of these insects in tomato and their secondary effects on Nesi. The result was quite spectacular,” the researcher says. “Nesi was able to establish in all treatments, but what we saw was that where we had built up a population with the new species first before introducing Nesi, establishment was reduced by an average of 90%.”

The final population density of the predatory bug among its three southern European cousins averaged 85%, 92% and 95% lower than the controls respectively. There was no significant effect the other way round.

Whitefly and cotton whitefly

“We confirmed in the laboratory that the adults of the three new species feed on the young Nesi nymphs, but it is not yet clear what effect Macrolophus has on the insect,” Messelink says. “We have observed that Macrolophus is often squeezed out in the greenhouse, and in the laboratory too we have noticed that this bug doesn’t feed on young Nesi nymphs. So using these new species could offer an advantage over Macrolophus. But it’s important to find out whether they are just as effective in controlling the main pests.”

Now that the researchers have established that the three southern European bugs control tomato leafminer, this year they are looking into the effect these species have on greenhouse whitefly and tobacco whitefly.

Standing army

Messelink is enthusiastic about the initial result. “You have to view the predatory bugs as a standing army. If you can deploy the new species preventively, if they can control different pests, tackle Nesi and don’t damage the crops, then we will be taking a big step forward in biological control.”

At the end of the day, the researchers want to be able to offer growers a total package. “We are looking for a predatory bug that controls an infestation as effectively as possible and doesn’t damage the crop,” Messelink says. “I have high hopes. I think these insects will be a big help in biological control in the future.”

Experience in rose and gerbera

As part of the same study, the researchers also investigated the use of omnivorous predatory bugs in gerbera and rose. “We think that this insect can offer a solution in these crops too,” researcher Gerben Messelink says.

“Whitefly, caterpillar, Echinothrips – all these pests can be controlled with predatory bugs, so it is possible but we still need to do more research. We have observed that this insect has difficulty establishing in gerbera, mainly because of the mildew control technique that’s currently used. Growers spray the crop, and it is not so much the toxic value that eliminates the predator; research has shown that spraying under high pressure ‘blows away’ the predator. So if we can find another way of controlling mildew, we can also build up a population of predatory bugs in the crop and control pests that way.”

The researchers are also trialling building up a population of these predators in rose. “Rose is a woody plant, which makes it difficult for this insect to establish. Bugs prefer hairy, herbaceous plants. But we might be able to keep these predators in the crop by using a host plant, for example. We have already demonstrated that we can control Echinothrips in rose effectively this way.”


Three species of predatory bugs in the Miridae family can limit the establishment of their harmful cousin Nesidiocoris tenuis in tomato growing. Researchers looked at the effect they have on pest control. They investigated the development and establishment of these three southern European predators in tomato and their secondary effects on Nesi. The final population density of Nesi with the three predatory bugs tested was around 90% lower than in the control. It is likely that these omnivorous insects could ultimately also offer a solution in rose and gerbera.

Text and images: Marjolein van Woerkom.

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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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Crop protection products are sometimes unfairly labelled as losing their efficacy against pests or diseases. On closer investigation it becomes clear that these products don’t always end up in the right place or are not being taken up properly. In that case, adjuvants can be indispensable if you use them in the right combinations. Uptake can sometimes increase by as much as six to eight times.

Substances that support the use of crop protection products and enhance their effect are on the rise. A year ago, no fewer than 101 different ones were registered with the Dutch Board for the Authorisation of Plant Protection Products (CTGB). Thirty companies are involved in the development and distribution of these products.

One of these is the Dutch manufacturer SurfaPLUS, which is actively promoting their correct use in a series of events for consultants and users. Director Hans de Ruiter sees it as his mission to do this. That’s hardly surprising, since in his previous job he was project leader at Wageningen University & Research, where he was intensively involved in research into these substances. But this research rarely if ever takes place in the public domain these days. Instead, he has it carried out by private research institutions. After all, there’s no question that this extremely useful work must go on.

Variety of effects

“Adjuvants” is actually a collective term for products that work in a variety of ways. An important function that is particularly relevant to open-field cultivation is reducing spray drift. This keeps the active ingredients where they need to be to do their job. Another function is reducing volatilisation during spraying or after contact.

These products can also ensure that droplets of the solution stay on the plant or leaves and that the active ingredient is more evenly distributed. In other cases, the products can improve contact by “gluing” the active ingredients to the leaf surface. Lastly, they can boost uptake of active ingredients by making them soluble or making the plant’s waxy cuticle more permeable.

Every adjuvant therefore has its own characteristics, and some do several things simultaneously. “That’s important,” de Ruiter says. “because we know from the research how poor the uptake of active ingredients can sometimes be without these products.”

Resistance decreases

To start with a concrete example, de Ruiter cites the “Vertimec case”. This product, which is based on the active ingredient abamectin, is authorised for the control of spider mite, thrips and leaf miner in both ornamental and vegetable cultivation. In tomatoes, for example, it can be used three times per cultivation cycle, and more often in ornamentals. Vegetable growers tend to use it sparingly because it has an adverse effect on biological controls. But with the emergence of pests and diseases that are difficult to control, such as tomato russet mite, growers sometimes need to reach for the chemicals.

De Ruiter: “I hear complaints from growers that a product is becoming less effective at the correct dosage. The story goes that certain insects or mites have become resistant. But it doesn’t have to be that way. If you use the right adjuvant, a product generally does what it’s designed to do. In fact, an effective combination of the two products can actually reduce the risk of resistance.”

Abamectin is a product that is inadequately absorbed by the leaves when sprayed on its own. With the correct adjuvant, uptake can increase by six to eight times. It is poor uptake that increases the risk of resistance.

Combination works better

In 2014, research was carried out at the Westland Demo Nursery (Demokwekerij Westland) into the effect of abamectin and Hasten, an adjuvant based on an esterified canola oil, on an infestation of Californian thrips on sweet pepper. The treatments were as follows: untreated (water), 100% abamectin, 100% abamectin with Hasten, 50% abamectin and 50% abamectin with Hasten.

Thrips control with abamectin on its own was no higher than 15-25%. The combination with the adjuvant worked two to three times better. Previous research into spider mite control in cucumber yielded the same outcome.

Gentle products

In the summer of 2016, Botany BV carried out research into a combination of the adjuvant Elasto G5, a glycerol-based polymer, and XenTari, a biological agent based on Bacillus thuringiensis, against the golden twin-spot moth in sweet pepper. The treatments consisted of untreated (water), XenTari, XenTari with Elasto G5, and Elasto G5 on its own. Both pupae and adult moths were released into the crop and the researchers waited until various stages of caterpillars were present. A total of three treatments were carried out at weekly intervals.

The trial showed that the adjuvant improved the effect of the active ingredient. The product provided better coverage on the crop and improved distribution of the active ingredient (see graph).

These were mild substances that did not cause any damage and left no residues behind. Nonetheless, a warning would not be out of place, says De Ruiter. “Adjuvants can also boost the effect of products. They can make ‘hard’ products even harder.”

Fewer inhibitors

The use of Elasto G5 has also proven its worth in another way: in combination with inhibitors. In 2014, Delphy ran a trial with the adjuvant in combination with Alar (daminozide) in pelargonium which revealed that the use of inhibitors can be reduced by half. “That cuts costs for growers quite substantially, because inhibitors are expensive. It depends on the crop and the variety, but we have sometimes seen costs cut by as much as 44%.”

The days of pioneering with adjuvants are over, says de Ruiter. They have since found wide acceptance and the trial results are better than in the past, when some substances were too aggressive. The gentler products are gaining ground. “Of course, we have to keep on investigating new opportunities and we need to communicate the results we obtain with caution.”

Hence the events, which are held fairly regularly. Incidentally, SurfaPLUS is not the only company doing research into these substances. Crop protection product manufacturers such as Bayer Crop Science and Certis include them in their programmes, and the Dutch companies Modify and GreenA are also active players.


Adjuvants that enhance the effect of crop protection products are gaining ground. Not only do they get the active ingredients working better, they also help to avoid resistance. The right combination can halve the need to use inhibitors in some crops, delivering substantial savings. It’s important to know which combinations are the right ones because an adjuvant can also reinforce a product’s adverse effects.

Text: Pieternel van Velden.
Image: Botany.

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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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Erwinia is a bacterial disease that can cause damage in a range of crops. It often produces watery looking, rotten patches on the leaves, fruit, flower or stalk. The bacterium is quite common and can develop rapidly in the greenhouse, spreading from plant to plant through crop handling and splash dispersal. Hygiene measures are crucial for keeping an infection under control.

The rotten patch gives off a very specific odour similar to rotten eggs. The bacterium breaks down the cell walls of the plant with the aid of enzymes, enabling the infection to develop rapidly. Wilting and rotten patches cause the plant to die or damage the product to such an extent that it is unsaleable.

The general type is referred to as Erwinia spp. A crop can be affected by certain species that are particularly harmful to that particular plant. The species often found in pot plants include E. cypripedii, E. carotovora subsp. carotovora, E. carotovora subsp. atroseptica and E. chrysanthemi.
A new species which can cause a lot of damage in strawberries has recently been identified: Erwinia pyrifoliae. The symptoms are blackened fruits and bacterial slime.

Text and images: Groen Agro Control.

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Supplementary feeding of predatory mites with a pollen preparation has outgrown the trial phase. A large number of growers is successfully using this method. While pest numbers are low, predators that also eat pollen have the chance to build up a vigorous population. Dutch growers Wesley Klauwi from sweet pepper nursery Zuidgeest Growers and cucumber grower Bart de Groot explain how they do it.

“Snack peppers are different from standard pepper varieties, and you have to learn to respond to that.” Wesley Klauwi is responsible for crop protection at Zuidgeest Growers, a nursery famous for its Vitapep orange snack pepper. “Caterpillars and aphids are the biggest problem at this nursery. We have the other pests well under control, even though we have had a lot of pressure from thrips here over the last couple of years”, he explains.
The sweet pepper nursery gained experience this year with supplementary feeding of predatory mites with Nutrimite, a preparation based on cattail pollen. It is highly nutritious for predatory mites and unattractive to pests. What’s more, this pollen doesn’t produce allergic reactions.

Spreading pollen

In the first week of January, Amblyseius cucumeris was used to control thrips on the four hectare site in Maasdijk (south-west Netherlands). A bag was hung on every twelfth plant, and Amblyseius degenerans was added two weeks later. The rows in the greenhouse are 50 metres long. Klauwi opened the tubes halfway down the rows. He then provided three to four batches of supplementary pollen at 14-day intervals.
He disperses the product with a small leaf blower, first around the places where he has distributed the A. degenerans and then around the whole greenhouse. “We want to see plenty of predatory mites round about March, and we have achieved that at all our sites,” he says.

Good results

Supplementary feeding of natural predators is becoming more and more popular. Marcel Verbeek of Biobest says that about half of his customers now feed pollen. The major advantage of this method is that it allows natural predators to develop well at a time when there are not yet enough pests to sustain them. Predatory mites in a sweet pepper crop can often survive simply on the pollen from the flowers, but a cucumber crop produces no pollen at all. This makes it much harder to build up a good population of natural predators.
Several predatory mites eat the pollen preparation, namely A. degenerans, A. swirskii and Eurseius gallicus. A. cucumeris also responds to the preparation but less vigorously than swirskii and degenerans. “We are seeing good results. Proponents are finding that it makes the predatory mites more active and more vital. Others believe that the mites can get lazy or that they would prefer to eat pollen than catch pests.” Verbeek doesn’t agree.
Sometimes it is hard to make a sound choice. The advice is to disperse a total of 0.5 kg per ha each time. Supplementary feeding is not cheap but it delivers good returns because of the increased numbers of predatory mites in the crop.

Resilient population

“We are certainly seeing plenty of Amblyseius degenerans running around on the flowers,” says Klauwi. “Thrips are under control and this predatory mite also tackles spider mite. We hope that Orius will help with pest control in around week 25.” They prefer the combination of A. cucumeris, A. degenerans and supplementary feeding with pollen. “As far as we’re concerned, supplementary feeding is the future.”
Verbeek: “Our advice is to build a resilient population of natural predators before problems start arising in the crop. Sometimes this even starts with the breeder. Some growers are already releasing predatory mites at the propagation stage as well as providing supplementary feeding with the preparation.”

Gradual start keeps down costs

“Supplementary feeding doesn’t come cheap,” says Bart de Groot, Aad and Ruud Zwinkels’ partner from Kwintsheul in the Westland area. They have been using the pollen preparation at the 28,000 m2 cucumber nursery for a couple of years now.
The cucumber growers plant three times a year. To begin with they release A. swirskii at a rate of one bag per four plants to control thrips and whitefly. When the first spider mite appears, they add Phytoseiulus. Although the preparation is very effective for the predatory mite population, they decided last year to take a more gradual approach to keep costs down. In April 2015 the pressure from whitefly increased and there were not enough swirskii present to tackle the problem, so they decided to start supplementary feeding again.
This year, the course of events is the same as in 2014. De Groot is using 0.25 kg pollen per hectare per week on this fast growing crop. “But we still notice that this is saving us a round of predatory mites,” he explains. “Incidentally, we don’t have a lot of trouble from thrips. It’s mostly whitefly that we need to keep under control.”

Fan is faster

The leaf blower they used for dispersing the preparation in the first two years has now been replaced by a fan on the pipe rail trolley as dispersing the pollen with the leaf blower took too long. The grower can now get the job done in 45 minutes. He does it early in the morning before the vents are opened. He positions the trolley so that the fan is about half a metre above the wire. Then he rides down eight rows in the 300 metre-long greenhouse. The following week he changes the pattern and takes the eight rows in between.
The advantage of this method of dispersal is that the grower gets a good picture of the condition of his crop in a short space of time. “It means I can see straight away whether the plants are growing well and if there are patches of whitefly, spider mite or other pests remaining.”

Combining bio and pollen

This year, biological control is once again going according to plan. When the third crop starts, De Groot will be using a high dose of standard bags of swirskii, one for every two plants. He wants to keep this population vital until the end of the crop with supplementary feeding.
“We are noticing that supplementary feeding with the pollen preparation is adding a new dimension to our biological control. We’ve seen for ourselves how predatory mites can build up a good population when they get a varied diet, in other words a combination of pests and pollen. With more and more chemical pesticides being banned, I see this as a good strategy for the future,” Verbeek adds.


The number of sweet pepper and cucumber growers feeding predatory mites pollen preparations is on the rise. The varied diet of pests and pollen creates a good, resilient population of natural predators. Although this strategy means the money has to be spent before the benefits are felt, this trend looks set to continue.

Text and images: Pieternel van Velden.

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Several growing areas in the Netherlands and Belgium had to contend with them last year: Bemisia tabaci and tomato russet mite. These two pests are no strangers to the tomato sector but the intensity with which they appeared was unexpected and the damage they caused was considerable. Reason enough, therefore, to take a closer look at these insects. Advisors from crop protection suppliers Certis and the Delphy Knowledge Institute give their tips for preventing an outbreak.

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Hyperparasites in the crop can completely disrupt biological crop protection. Sweet pepper growers in the Netherlands know a thing or two about that. But where there’s hope, there’s life: in October 2015 a multi-year, fundamental research project was launched that aims to unravel the interactions between hyperparasites and their environment. Once these are sufficiently well-known, attractants may turn out to be an answer.

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Virtually nothing can be done about excessive root growth. However, this troublesome bacterial disease doesn’t always get the chance to develop. Other microorganisms that live in the root environment fight against its rapid spread. The search for underlying mechanisms is gradually progressing. It’s a brainteaser for the researchers.

Genetic changes have always occurred and they take place in nature all around us. Rhizobium rhizogenes for example, is a bacterium that very successfully infects plants with pieces of its own DNA. These pieces (T-DNA) merge with the plant’s DNA, which as a result behaves somewhat differently. For example, it can become more sensitive to the hormone auxin. This sets off explosive root growth. The roots then excrete substances that create a favourable environment for the bacteria to thrive, which perpetuates the situation further.
These ideal living conditions for the bacteria have large consequences for the host. Infected crops such as tomato, aubergine and cucumber show more vegetative growth and production lags behind. At least 400 plant types are susceptible to these bacteria and it can cause big problems for the greenhouse crops mentioned.

Difficult to grasp

Over the last decade a lot of research has been carried out into the mechanism that causes this abnormality and into methods (hygiene measures) to prevent the spread of the bacteria. Marta Streminska and Ineke Stijger, both researchers at Wageningen University & Research Greenhouse Horticulture, are researching a new strategy to prevent excessive root growth in collaboration with the Dutch growers’ association LTO Glaskracht Nederland, Rijk Zwaan and Koppert Biological Systems. This is within the project ‘Next Generation Plant Health’.
“It is still very difficult to get a grip on this disease,” says Streminska. “One year we see a lot of infection, another year hardly any, sometimes even on the same nursery and under exactly the same circumstances. We still don’t know why that happens.” There is no chemical cure. Once the plant is infected, the DNA changes forever. Since no solution can be found through chemical means, except the thorough disinfection of the watering system, a biological approach offers the best potential. In this respect the activity of microorganisms around the plant’s roots plays a central role.

Fathom out the system

Within the plant health project the two researchers are looking into all the fundamental principles that could set off or stop the excessive growth of roots. Until now this research has mostly been carried out within the four walls of their laboratory. The results and next steps are regularly discussed with the project’s committee of growers from the national committees for tomato and eggplant. “We are not looking for an antibiotic as this would increase the risk of resistance build-up. We are studying the entire microbial system,” explains Streminska.
Previous research showed that the symptoms of excessive root growth are not always observed in old slabs. These old slabs contain an established, stable microbial environment that it is not going to be pushed around by a wrongdoer.

Used substrate slabs

Therefore the researchers are now studying fungi and bacteria that were isolated from used substrate slabs to try and fathom out the microbial mechanisms involved. In addition they are testing a range of biological products that could offer solutions. In one tomato trial for example slabs were used that were treated with useful microorganisms. Here too excessive root growth reduced.
“The systematic assessment of all microorganisms is a huge undertaking,” says Stijger. “We want to know exactly what happens in the slabs. If there is a substance or microorganism that can slow down the explosive growth of this bacteria you could add this to new slabs.”

Disrupt communication

From research into human medicine we know that bacteria don’t strike without warning. Instead they wait until an ‘army’ of bacteria has built up and then launch a joint attack. Bacteria communicate with each other via the production of signal substances. Different bacterial types produce different signal substances. For example, Bacillus makes different signal substances than Dickeya or Rhizobium. Streminska and Stijger are looking at ways of disrupting this communication system to prevent a joint attack from occurring.
There are therefore several ways to approach the problem. The researchers are cautious about drawing conclusions too early. Stijger: “Even if we start to understand why excessive growth reduces in one crop, it does not automatically mean than it will be the same for another crop. Besides the root environment contains many organisms that work together.”
More than 90% of all the bacteria present live in symbiosis with the plant’s roots and actually stimulate growth. If you add organisms or substances that reduce the excessive root growth the rest still need to remain alive. If that doesn’t happen other undesirable changes might occur.

Disinfect and protect

In anticipation of the results the researchers sketch a scenario that could be used in greenhouse vegetable production. Firstly before the crop is planted the slabs and watering system need to be scrupulously clean. Opinions are divided on the cleaning effect of hydrogen peroxide and some people have preferences for certain types of brands or compositions. Stijger is not sure if this is a valid case. She does know that some strains of bacteria are more sensitive to peroxide than another. “The problem is that bacteria spread rapidly throughout the system if just a tiny little bit is left behind. The bacteria appear to protect themselves with biofilm that is very difficult to remove.”
Then, immediately after disinfection has taken place, the new substrate slabs have to be injected at the start of the cultivation with substances or microorganisms that will create a stable living environment for the plant’s roots to grow. As a result pathogenic bacteria have no chance to multiply and launch an attack.
Streminska and Stijger finished most of their laboratory research in 2016. They are now starting to test the substances on young plants.


Two years ago researchers in the Netherlands began a systematic approach to solving excessive root growth. Ineke Stijger and Marta Streminska virtuously studied the natural substances and organisms that could slow down the explosive development of Rhizobium bacteria. It is gradually becoming clear that a great number of different organisms are needed to develop a stable and resilient system.

Text and images: Pieternel van Velden