Light color and intensity affect biological balance
Light colour and intensity affect how well pests find plants, how they find their bearings in the greenhouse and how active they are. And the same applies to their natural enemies. The day length affects their life cycle, particularly when they enter diapause, or become dormant. Because of the many different effects, it is often difficult to predict how a change in lighting will affect the balance between what is beneficial and what is harmful. Despite the above, a number of principles are now clear.
Ultraviolet light
Most pests are attracted to ultraviolet (UV) light and are clearly more active in an environment with a lot of UV light. Unlike people, they are able to see this colour distinctly. Thrips, whitefly, leaf miner fly, aphids and red spider mite can find their way and spread better through the crop under UV light; in other words, they can find plants more easily and eat more. At lower UV intensities, they are less successful at doing so. This is therefore a starting point for the control of these pests. Scientific research in various countries has now shown that it is worthwhile keeping UV light out of the greenhouse. If glass or plastic film greenhouses are coated with UV-resistant materials, fewer pests fly in and those that do are far less active once they are in the greenhouse. An interesting point is that their natural enemies are less affected by the reduced UV levels. This is because they primarily find their prey by smell.
In a glass greenhouse the UV level is already low, but this does not apply to a plastic film greenhouse. In the latter it is certainly worthwhile to prevent the incidence of UV light (provided it is not necessary for the colouring of tree nursery crops, for example). ReduSol reflects UV radiation to a certain extent. Work is currently being carried out on special UV-reflecting coatings.
A disadvantage of UV protection is that pollinators, such as honeybees and bumblebees, can’t find their way as well in the absence of UV light. In the case of crops that have to be pollinated, the decision of whether to use UV protection must therefore not be taken lightly. Bees in particular need several days to get used to the situation if ReduSol is used. It helps if beehives are placed close to the greenhouse gables.
Other light colours
Other wavelengths of light, that is, different colours, also affect insects. There is a reason why sticky traps are yellow or blue: these are the colours by which insects are attracted. Red light can also be effective in reducing damage, but this works via a different mechanism. When exposed to a relatively high level of red light, a crop produces more antibodies, so that it is more resistant to pests. The ratio of red to far-red light is the determining factor in this mechanism and not the absolute level of red light. High-pressure sodium lamps (SON-T) produce relatively high levels of red light and therefore have a positive effect.
More light: more resistant crops
A high light intensity and a longer light period also yield a crop that is more resistant to insects. If a grower can take measures to increase the incident light, he or she will therefore have a healthier crop. Coatings such as ReduHeat, ReduFuse and ReduFuse IR are significant aids to this end.
Growers report that diffuse and/or heat-reflecting coatings also have a positive effect on biological pest control because breeding bags of these natural enemies heat up less when there is less light or the light is diffuse, as a result of which the beneficial insects remain more vigorous.
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