text Roelof Kleis photo Marco Hofsté
It is a bit of a festive moment, this Friday afternoon in the giant greenhouse at the chrysanthemum nursery G&C Flowers in Gameren. For entomologist Rob van Tol, at least. The still somewhat crude-looking cube he is going to test here is the prototype for a new generation of insect traps. Away with the flat coloured sticky boards with which many growers still monitor and/or control their pests. The new way of trapping insects entails using carefully composed light.
Insect traps in horticulture are essentially hopelessly old-fashioned and inefficient, explains Van Tol (Biointeractions & Plant Health). ‘Of the thrips that are attracted to a coloured board, only between five and 15 per cent actually land on it. The rest escape. Why is that? Apparently there is something visually wrong with the trap. The insects come towards it, but once they get close they lose their way and get disoriented. The few that do land actually just crash into it.’
LED trap as life’s work
The sticky boards are useful for monitoring insects but not for capturing them in large numbers. And yet that must be possible, thinks Van Tol. ‘Actually, so far all the research on insect traps has mainly been empirical. The whole spectrum of colours has been studied for attractiveness to insects. Comparisons have been made between different coloured boards, but no one has gone into it more deeply. What do insects actually see? How do they orientate themselves, and which aspects play a role in that? I want to use that basic knowledge to create better traps, on which insects do decide to land.’ With funding and support from the horticulture organization LTO-Glaskracht, Koppert Biological Systems and the Horticulture Top Sector, Van Tol has been working for over two years on what he has begun to see as his life’s work: the LED trap. And the prototype is the white cube made at the PSG workshop Tupola, which is about to be put through its paces in Gameren.
The cube consists of eight sections, each of which can be lit up separately. The sections are covered in foil on the outside. This apparently simple design has been thoroughly tested in advance. In a four-metre-long wind tunnel in Radix, Van Tol is testing the behaviour of thrips and tarnished plant bugs when exposed to LED light of different colours. Two infrared cameras make it possible to record the route of each insect in 3D. This setup has already produced some remarkable results. One example: thrips always approach a trap from downwind. Van Tol: ‘They always land against the wind. They can fly with the wind in their backs, but they can’t land. So they fly past the trap and turn to fly into the wind.’ This may seem a trivial point, but it is not, says Van Tol. ‘This is important for designing a good insect trap. The trap must not be too flat, but three-dimensional so that it is possible to land on it from all sides. That makes the trap more efficient.’
Another result: thrips prefer yellow light to blue. Which is interesting, given that blue boards are often used to lure the insects in greenhouses. So have the growers been doing it all wrong? No, showed further research. The reality, as so often, is more complex that we thought. Van Tol and his colleagues realized this when they heard about German research on thrips and their light preferences. ‘They found the exact opposite: their thrips had a preference for blue light over yellow.’ The German thrips still had that preference when they were tested in Wageningen. ‘The insects reacted in the same way as in Germany. So it was not question of a difference in the experiment. We are now researching whether learning behaviour is involved, perhaps with the host plant on which the thrips are bred playing a role. Another explanation might be genetic differences between the two families of thrips.’
UV light may play a role too. The Germans were working in a greenhouse with real sunlight instead of in a wind tunnel with sunlight simulated using LED lights. But the difference in UV light between the two locations did not explain the differences either. A new series of tests is planned to find the explanation by investigating all the possible combinations of yellow, blue, UV and green light. And to make it all even more complex: the hunger factor might play a role too, according to Van Tol. ‘Thrips fly towards yellow because they are hungry and they think it is a flower they are seeing. But a thrips that isn’t hungry might be orientated towards blue. We can figure all that out with the new LED trap.’
The trap must be three-dimensional so the insects can land from all sides
Besides colour and contrast, patterns play a role in the orientation. Van Tol shows us his latest toys: LED lights that produce a pattern of concentric circles or radial (starting from a central point) stripes or dots. The researchers are pulling out all the stops to seduce the insects into landing. They are also looking at how the insect’s eye works (see inset). The insect trap of the future will be a perfect seducer.
What does a thrips look like? PhD student Karla Lopez Reyes is trying to answer that question. She is doing research at Bristol University on how a thrips eye works, for the Wageningen thrips project. One of the things she is investigating is how each of the 60 facets of the eye helps the insect to see colours and patterns. She creates 3D images of the eye using CT scans. The idea is that this work will generate a model that shows us what thrips see. And that knowledge can then be applied to design a better trap.