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.”
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.
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.”
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.
As a cucumber grower you have to have iron discipline and nerves of steel. An apparently invisible enemy – CGMMV – is always lurking ready to attack. It’s a troublesome disease that can have profound consequences. If an infection strikes early in the year, the costs in terms of loss of production, early clearing, cleaning up and starting again quickly mount up. Shortfalls of €10 per square metre are no exception.
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Reducing energy consumption in greenhouses is associated with higher humidity levels. Many growers are concerned that this could make the crop less active, impacting on transpiration from the top in particular. Last year, a Dutch research project was launched with the specific aim of measuring that. The conclusion: rather than a reason for concern, intensive screening is actually a way of improving the crop.
The “Transpiration from the Top” study took place at the Wageningen University & Research greenhouses in Bleiswijk, the Netherlands. Over two months, greenhouse climate and energy researcher Feije de Zwart tested a measurement method in a mature tomato crop and evaluated the quality of the technique. “We looked at the extent to which we could specifically measure transpiration at the top of a crop using a thermal imaging camera. And it worked. But this method requires an extremely accurate camera and a lot of attention. So for the time being, although this camera is excellent in the lab, it is not really suitable for use in the commercial setting.”
Low-energy cultivation not only requires better greenhouse insulation, but it also means the crop has to be grown in higher humidity levels. Growers tend to use their screens more often and don’t often open gaps in them. This increases the humidity in the greenhouse. “One of the reasons why growers are reluctant to accept this situation is that they are concerned the crop may not transpire enough,” de Zwart says. “After all, the more humid the air in the greenhouse, the lower the difference in vapour pressure between the crop and the greenhouse air, and the less the crop will transpire. They are most concerned about the top of the plant.”
These concerns stem from the fact that inadequate transpiration can affect nutrient transport to growing tips. He adds: “To measure is to know, so we developed a measurement system that can accurately determine transpiration at the top. A sensor of this kind could put growers’ minds at ease and could result in wider acceptance of higher humidity levels, particularly in vegetable cultivation.”
Camera plus artificial leaf
To test the system, the researchers installed a thermal camera above the crop. Besides the leaves on the crop itself, there were always two artificial leaves in the camera’s line of sight. One of these artificial leaves was fitted with a PT-100 temperature sensor to check the temperature registered by the camera. With this setup they were able to compare the temperature of a leaf at the top of the plant with that of an artificial leaf that was not transpiring, in the same conditions. The lower the temperature of the real leaf compared with the non-transpiring one, the greater the transpiration.
De Zwart again: “In fact, we noticed that the temperature of the real leaf fell quite a bit below that of the artificial leaf at night and that the temperature difference increased as the humidity level dropped. The best thing was that the behaviour of the real leaves was very much in line with our expectations based on our calculations. When we took another close look at the calculations, we noticed that the reduction in radiated heat loss brought about by screening really does increase the temperature at the top of the crop quite significantly. And this in turn leads to higher levels of transpiration at the top.”
De Zwart’s conclusion is therefore that transpiration at the top of the plant simply continues when screens are used, even if humidity is higher. “Intensive screening can limit transpiration from the crop as a whole but, conversely, stimulates it from the top of the crop.”
Vertical differences in the crop
These results could perhaps explain why good yields were achieved in all those practical trials with Next Generation Growing, despite the expectation that the high humidity would cause problems. De Zwart again: “If you look at water uptake, for example by comparing the amount irrigated and the drain, or by using a weighing gutter, you can barely see the effect of screening at all. But if you look at the increase in temperature in the crop, then you can see that closing the screen increases the temperature at the top, while often lowering it slightly further down in the canopy. This is because the use of the screen means less heating is needed further down.”
In any case, the temperature gradient across the crop drops, making transpiration more even throughout the crop. “That last factor, the evenness, had never really occurred to me,” he adds. “So rather than being a reason for concern, intensive screening is actually a way of improving the crop. This realisation is essentially the most important outcome of our research. It is such interesting information that it has been added to the Radiation Monitor.”
Growers and other interested parties who attended the Next Generation Growing course in the Netherlands will already be familiar with the Radiation Monitor. This online simulation model calculates the effects of screening and greenhouse covering materials on energy consumption and vertical temperature distribution.
De Zwart used the same model to establish the expected difference in temperature between a transpiring and a non-transpiring leaf. However, the data obtained from the project mentioned above demonstrated that the original calculation method was too inaccurate. Following improvements, the program now calculates transpiration at each layer of the crop.
The basis for this is that the difference in vapour pressure between the greenhouse air and the leaves plays a bigger role in driving transpiration than the local leaf temperature. The program can be used via the greenhouse horticulture models website.
De Zwart is slightly less enthusiastic about the results of the original project setup. “The measuring equipment was quite tricky to set up. We didn’t have a problem collecting images with a thermal camera, but focusing the lens was difficult. The plant was growing, so we had to constantly refocus the lens. Also, you have to use artificial leaves. We now know that a tomato leaf transpiring at the normal rate at the top of the plant is around 0.4°C cooler at night than a non-transpiring leaf in the same place.”
If this measurement method is used to distinguish normally transpiring leaves from leaves transpiring at a lower rate, the temperature differences measured should be in the magnitude of 0.2°C. These are such small differences that you would need to know exactly how warm a non-transpiring leaf would be in that position. That is why you need artificial leaves and a very accurate camera. He adds: “Actually, you do wonder whether the information you get is really worthwhile. After all, we now know that transpiration at the top of the plant simply continues when screens are used intensively, even if the air humidity is higher.”
Low-energy cultivation means a lot of screening hours and higher humidity in the greenhouse. Research shows that although higher humidity causes transpiration to decrease, the use of screens does not affect transpiration from the top of the plant. The project used a thermal camera and artificial leaves. The setup worked and, besides providing figures for the top of the crop, it also highlighted the vertical temperature distribution in the crop. The data was integrated into the online Radiation Monitor.
Text: Jojanneke Rodenburg.
Images: Wageningen University & Research and Jan van Staalduinen.
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.
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.”
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?”
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.”
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.
A tomato that has optimised its defence mechanism against insects will not only suffer less damage from caterpillars, but will also incite caterpillars to eat one another. This is a defence tactic that, until it was brought to light by a recent study conducted at the University & Wisconsin, has been hitherto unknown.
When tomato plants are attacked by insects, they produce antibodies in order to protect themselves. They actually start doing this as soon as other plants in their direct vicinity are attacked. This mechanism is activated when they detect volatile substances emitted by plants that are being eaten by predators. Researchers from the University of Wisconsin, led by John Orrock, sprayed tomatoes with volatile substances like these with a view to activating what is referred to as the jasmonic acid route. This defence route is activated to keep insects at bay.
The American scientists wanted to investigate the details of this defence mechanism. They wanted to discover if the plants only ward off the insects (because they are unappetizing or toxic) or produce another detrimental effect on them (e.g. by inhibiting reproduction). The study was conducted with caterpillars of the small mottled willow moth (Spodoptera exigua). The results were remarkable: not only did the plants sprayed with these substances produce five times as much biomass, because they had barely been eaten by insects at all, but the caterpillars began to feed on one another As a result, their population shrunk notably.
Finding the right balance
It is known that caterpillars have a tendency towards cannibalism when faced with food scarcity. The researchers believe that the low nutrient value of plants in full combat mode triggered the increased instances of cannibalism. The effect of the altered composition of the plants has, until now, never been investigated. These findings have been published in Nature Ecology & Evolution. In an accompanying interview, John Orrock points out that the cost to the plant of activating its defences is very high: the plant has to invest heavily at the expense of its growth. He believes that plants will always strike a balance between maximum defence and accepting minor damage from predators.
Text: Tijs Kierkels.
The strawberry is a vulnerable crop: the plants and fruits are susceptible to all kinds of diseases and insects. Crop protection is therefore a high priority. And yet the use of chemicals is being ever more tightly controlled. What other ways are there to produce healthy, resilient crops? Van den Elzen Plants is hosting a trial with a substrate adapted specifically for strawberries combined with biological supplements.
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For more than ten years the application of Trichoderma fungi has been common practise in horticulture. They help cultivated crops grow better and become more resistance to diseases. How this symbiosis works is a complex area of research. Nevertheless the developments keep coming. This year a new strain is being launched on the market.
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Researchers and companies are continually developing new techniques for greenhouse production. Therefore, in principle crop protection can reach a much higher level. But to achieve this, all these techniques need to be integrated. A system has been developed within the Gezonde Kas (Healthy Greenhouse) program that links them all together.
Gezonde Kas is a large Dutch-German project comprising 10 research institutes and 22 commercial companies. It was completed in the middle of 2015 and achieved its goal: The development of an innovative integrated crop protection system. The different parts of the system alert the grower to the presence of pests and diseases, even before any symptoms are visible. The system then helps the grower to make decisions. Furthermore, the system suggests measures that are based on the use of as few chemicals as possible.
Many of the techniques were known already but during the project were adapted for use in practise, geared to fit one another and integrated into one system. This has, to a large extent, been set up in a trial greenhouse in the research centre, Versuchszentrum Gartenbau, Straelen, Germany.
Ready for translation
Project leader Carolien Zijlstra, of Plant Research International (part of Wageningen UR, the Netherlands), is a molecular biologist by origin. “At PRI we have the capability to identify crop predators based on DNA or proteins. We need just one fungal spore to detect the problem at an early stage. But the method needs to be converted for use in practise. That was one of the reasons for starting this project. And, in fact, many more methods are ready for translation,” she explains.
The Interreg IV-A project is financed by the EU (EFRO), the Dutch and German Governments as well as businesses. The new system consists of four process steps. For each of these, suitable methods have been developed for use in practise. “They have been developed for large innovative nurseries but parts of them can be used by every grower,” she says. “Growers can purchase all these products or choose from the possibilities.”
Step one is called Gezonde (Healthy) Start. “Just before the start of the crop you check the surroundings and materials to make sure they are free of pests and diseases and take preventative measures to avoid contamination,” says Zijlstra. The Luminex-test shows exactly if seed, plant material, water or substrate contain pathogens. “The system works with very tiny balls that are coated with antibodies or DNA. If a pathogen is present the coating binds with its protein or DNA. If that happens the tiny balls light up under a laser beam and you know which pathogen is present. You can test for a hundred pathogens at the same time.”
It's impractical to test all the plants in the greenhouse, so pre-screening with another technique is useful to highlight the potentially infested specimens. This is made possible with a chlorophyll-fluorescence camera (see below).
This first step also contains two disease-suppressing measures: Firstly a top layer on the substrate ensures that predatory mites become better established and therefore can better attack thrips. Secondly, disease-suppressing substrate reduces the risk of contamination by Sciaridae by 80%.
The second step consists of advanced monitoring. This can be on a macro-scale; the whole greenhouse, or a micro scale; plant- or leaf level. Several techniques are available for both levels. The chlorophyll-fluorescence camera (CF-camera), for example, passes over all plants and assesses whether photosynthesis is being carried out properly. If not, it can be an indication that plants are contaminated. The multi-spectral image sensor is another method for detecting suspicious spots in the crop but uses a different technique. It sees better than the human eye.
Other techniques that detect on a macro level are the electronic nose and the spore collector. The latter collects air-borne fungal spores which are later analysed. “If you discover contaminated areas with the macro screening you can analyse them further using the micro scale techniques,” says the project leader. For example, the previously mentioned Luminex-test is very suitable, but there are other methods that can detect diseases present in the plant, such as TaqMan- or a LAMP-test. The Nsure plant vigour test indicates whether specific genes in the plant are turned ‘on’ or ‘off’. This indicates if the plant is diseased or not.
In the meantime the wireless sensor network keeps a check on climatic differences, in case something needs to be altered here.
“With this combination of steps you highlight diseases which you can’t yet see with the naked eye. The CF-camera notices if the plant is in trouble while, visually, nothing appears to be wrong. Then by implementing the other tests you can figure out what it going on. Also the vase life of flowers can be predicted with such a camera,” she says, describing the advantages of the new technology.
The disadvantages are the costs: the CF-camera currently costs around Euro 100,000, and it’s the same for the electronic nose.
After the problem is detected a decision has to be made. The third step in the process involves interpretation of the data and giving advice. Various decision support systems are available for this. The Climate Vitalizer gives advice on achieving a more homogenous greenhouse climate. Notify Me predicts crop development and a further three systems can analyse the risks of Botrytis and pest insects as well as propose control measures.
These measures fall within process step four and relate to better climate control and pest management. The preferred method is a non-chemical approach. The project has developed a new strategy using endophytes, which improve the effectiveness of biological pest control. “Various natural predators eat both the pest as well as the plant. If you supply the plant with an endophyte – a useful microorganism that lives in the plant – the plant becomes less attractive to the natural predators and so these focus more on the harmful organisms,” says Zijlstra.
One of the most important parts of the project is the development of precision spraying. Cameras on the spray robot “see” the chlorophyll and only spray the plant and not the ground or into space. This saves 50% of the crop protection product. The next step is to combine it with the CF-camera. This detects firstly the suspicious areas so the robot only needs to spray these areas. This can save up to 70% of the product use.
The project Gezonde Kas has come to an end but a follow-up project for pot and container cultivation (tree nurseries) is in the pipeline. Researcher Zijlstra hopes that growers and horticultural advisors will quickly pick up on the techniques that have now been adapted for use in practise.
The project Gezonde Kas (Healthy Greenhouse) has developed an innovative four-step system in the field of crop protection. Key words are prevention, early detection, better control of climate and disease development and a reduction of chemicals. In all cases innovative techniques have been made adapted for use in practise and are described in this article.
Text: Tijs Kierkels. Photos: Wilma Slegers
Bees and bumblebees are, thanks to their build and behaviour, good pollinators. But they have additional potential. They are also useful for disease control by transferring antagonistic microorganisms and for retrieving information about diseases in the fields. An important aspect for all these tasks is: How do we keep them working under the ‘new’ growing conditions in the greenhouse?
Sjef van der Steen, researcher in the department Bio Interactions and Plant Health at Plant Research International, Wageningen, the Netherlands, researched the additional possibilities for these pollinators such as in disease control and air quality detection.
His research considered two questions: Is crop protection via bees and bumblebees effective and is it safe for these creatures? He provided the bees with the antagonist Trichoderma that works against Botrytis. The bees introduced sufficient Trichoderma onto the plant to prevent it becoming diseased. The substance had no effect on bees and bumble bee families.
Exit and entry opening
Then he carried out exploratory research into possible ways to establish a system in which the incoming and outgoing flow of bees could be separated. He placed a small type of aquarium pump at the exit route that continually released fresh spores.
The principle was picked up and further developed by Biobest. Meyers Softfruit, of Riemst, Belgium, has been using this method for two years. The bumblebee hives are equipped with an entry and exit opening. A powder dispenser is integrated into the exit opening. When a bee flies away it automatically takes with it some of the fine powder to the leaves and flowers. It was first tried out with the biofungicide VerderaB4. This is the first substance that has been permitted for this application. This year Van der Steen will join on-going Dutch research in Bleiswijk once new antagonists have being selected.
During their flight back from the flower the bees carry with them not only pollen but also other ‘information’ such as bacteria and fungi which have carried by air into or onto the flower. By analysing which disease or pest is present it’s possible to take timely action before the symptoms become visible. In order to get a good picture of what exists, the researcher had to make a number of choices. Where to place the colony of bees? How many colonies do you need? How do you ensure that the bee colony flies to a certain crop?
The next step was to take samples. He used two methods for obtaining the bacteria or fungi for analysis. “One method involves pulling the bees off the flight path, killing them and then immersing them. By using the other method the bees remain alive. We get them to walk through a tube containing a sticky material. Part of what they carried with them remains in the tube.” For both methods the researcher created a solution containing the pathogens for analysis later.
Diseases on flowers
Van der Steen decided to use two different methods of analysis: One uses a reagent; the other DNA-analysis. For the first method he uses a lateral flow device (lfd): a double plastic strip with a piece of reagent paper sandwiched between that discolours when a certain fungus or bacteria is present. Prime Diagnostics of Plant Research International supplies such papers containing a reagent that detects various common diseases.
For DNA-analysis he uses a portable PCR-analyser (poly chain reaction). “We used this method two years ago in cooperation with Agis, an Austrian institute for quality in agri- and horticulture to detect fire blight in fruit. We have used them in the Netherlands for detecting bacteria in strawberry and brown rot in cabbage. The results show that bees and bumblebees only carry with them diseases that are present on flowers. This method doesn’t work for leaf diseases, such as brown rot in cabbage.”
Air quality detector
Thinking further ahead Van der Steen sees opportunities for bees to take samples ‘passively’. A honeybee colony spreads itself out over the landscape while searching for food. As well as pollen and nectar they carry ‘other material’ with them to the hive. By taking samples of this and analysing it you get a picture of the diseases that are present in the environment. “The limitation of this method is that it only works with flowers. On the other hand, the possibilities are great. Everything that is spread through the air can be found on the flowers, not only plant diseases, but also bacteria that cause Q-fever, for example,” says the researcher. Van der Steen does see possibilities for using bee colonies as ‘air quality detectors’. “Outside West-Europe, where you don’t find these poles that detect air quality we can use bee colonies as cheap indicators of the environment.”
One of the conditions for using bees and honeybees, whether it is for pollination or disease control, is that the (flying) conditions must be good. Among other things this depends on the greenhouse roof material and the lighting.
Effect of lighting
With the arrival of assimilation lighting the greenhouse conditions, which are already different to outside, are changed even further. Ten years ago, colleague Tjeerd Blacquière who was doing research into the direct effects of high light levels on pollinating insects saw that assimilation lighting affected both these pollinators and the crop as, among other things, the amount of light, duration of light, light spectrum (more heat radiation), the day length and the direction of the light changed. Artificial lighting also altered the vigour of the crop and the day-night rhythm of flowering.
A few years ago Blacquière offered a few suggestions: Set the timing of the lighting so that the opening of the flowers occurs at the most optimal hour for pollination. Close the exit opening of the hives (automatically) well before sunset to prevent (a lot of) workers trying to fly away from the hive in poor light. A few hours of light during the peak in flower opening - during the middle of the day - is sufficient. Research showed that in February around 10 am all sweet pepper flowers had already been stripped of pollen by honeybees.
Use lamps with a better spectrum. Pure red LEDs are probably the most ideal for bees and honeybees. They don’t see red light at all.
Different roofing materials
Blacquière also carried out exploratory research into the impact of using various roofing materials such as glass, polymethylmethacrylate and polycarbonate on the foraging behaviour of bees. In the glass greenhouse the sun was visible as a bright spot in the sky. In the polycarbonate greenhouse the light was scattered into a light arch along the entire firmament. An arch shaped distribution of light was visible to a lesser effect in the polymethylmethacrylate greenhouse.
All the greenhouse roofs allowed PAR-light to enter well. Ultraviolet is hardly transmitted by polycarbonate, partially by glass and completely by polymethylmethacrylate. After introduction to the greenhouse, bees and bumblebees showed normal orientation behaviour under a greenhouse roof of glass and polymethylmethacrylate. Under polycarbonate the bees and bumblebees failed to return to the hive.
In addition to being pollinators, bees and bumblebees are also useful for crop protection. They can carry antagonists to pests and thereby prevent diseases from developing. In addition they can act as air quality detectors because apart from pollen they also carry back bacteria and fungal spores, providing the flying conditions are good.
Text/photos: Marleen Arkesteijn