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Artificial light at night has shown a dramatic increase over the last decades and continues to increase. Light at night can have strong effects on the behaviour and physiology of species, which includes changes in the daily timing of activity; a clear example is the advance in dawn song onset in songbirds by low levels of light at night. Although such effects are often referred to as changes in circadian timing, i.e. changes to the internal clock, two alternative mechanisms are possible. First, light at night can change the timing of clock controlled activity, without any change to the clock itself; e.g. by a change in the phase relation between the circadian clock and expression of activity. Second, changes in daily activity can be a direct response to light (‘masking’), without any involvement of the circadian system. Here, we studied whether the advance in onset of activity by dim light at night in great tits (Parus major) is indeed attributable to a phase shift of the internal clock. We entrained birds to a normal light/dark (LD) cycle with bright light during daytime and darkness at night, and to a comparable (LDim) schedule with dim light at night. The dim light at night strongly advanced the onset of activity of the birds. After at least six days in LD or LDim, we kept birds in constant darkness (DD) by leaving off all lights so birds would revert to their endogenous, circadian system controlled timing of activity. We found that the timing of onset in DD was not dependent on whether the birds were kept at LD or LDim before the measurement. Thus, the advance of activity under light at night is caused by a direct effect of light rather than a phase shift of the internal clock. This demonstrates that birds are capable of changing their daily activity to low levels of light at night directly, without the need to alter their internal clock.

The use of light emitting diodes (LEDs) in horticulture provides many new opportunities to improve crop production. LEDs reduce the energy costs of greenhouse production, allow for vertical and urban farming and provide the ability to alter the spectral composition of the light. Light quality is an important source of information for plants that strongly influences plant morphology, development and physiological processes ranging from photosynthesis to secondary metabolism. By altering light quality, growers can optimize crop yield and nutritional quality. However, light-induced changes in plant physiology can subsequently affect plant resistance to pests and the interactions with beneficial arthropods used for biological control and pollination. Most studies on the effects of LED lights in horticulture are focused on yield and product quality, and there is insufficient knowledge on the consequences for plant-arthropod interactions.The use of far-red LEDs (730 nm) is an excellent example. Far-red light is an important wavelength in natural canopies with pronounced effects on plant-herbivore interactions. Plants can perceive changes in the ratio between red and far-red light (R:FR). Red light is absorbed by plants and far-red light is reflected from vegetative plants tissue, which can be perceived by surrounding plants through a reduction in the R:FR and serves as a signal of neighbour proximity and competition for light. Plants respond to a reduction in R:FR with a range of morphological and physiological responses, collectively called the shade avoidance syndrome (SAS). These responses increase the plant’s light capturing ability and reproductive success, and for that reason far-red LEDs are employed in horticulture to induce shade avoidance responses and control crop morphology and development. However, to allow for the full and rapid expression of SAS, plant defensive signalling is inhibited and plants become more susceptible to pests and pathogens. Plants exposed to far-red light support higher herbivore performance and sustain increased damage by herbivory. This light-induced trade-off between growth and defence has been extensively studied on a mechanistic level, but the consequences of far-red LEDs in horticulture for plant-arthropod interactions have not.The aim of this thesis was to describe how supplemental far-red light influences plantarthropod interactions in greenhouse tomatoes. My study system included arthropod pests, biocontrol agents and pollinators. I studied how changes in the R:FR influences the performance of four arthropod herbivores with different feeding styles, including the tobacco hornworm (Manduca sexta), the green peach aphid (Myzus persicae), the two-spotted spider mite (Tetranychus urticae) and the greenhouse whitefly (Trialeurodes vaporariorum). I also studied how changes in R:FR influence the volatile-mediated attraction of two biocontrol agents, the predatory mite Phytoseiulus persimilis and the predatory bug Macrolophus pygmaeus, and how supplemental far-red light influences population dynamics and performance of these predatory arthropods. Lastly, I investigated how farred induced changes in tomato reproductive development influence the interaction with pollinating bumblebees (Bombus terrestris).Chapter 2provides a review on the effects of light quality on the balance between plant growth and defence. It describes how different wavelengths mediate plant morphology and photosynthesis between a “shade avoidance phenotype”, in which light harvesting is important, and a “photoprotected phenotype” in which protection from excess light is more important.

Artificial light at night has shown a dramatic increase over the last decades and continues to increase. Light at night can have strong effects on the behaviour and physiology of species, which includes changes in the daily timing of activity; a clear example is the advance in dawn song onset in songbirds by low levels of light at night. Although such effects are often referred to as changes in circadian timing, i.e. changes to the internal clock, two alternative mechanisms are possible. First, light at night can change the timing of clock controlled activity, without any change to the clock itself; e.g. by a change in the phase relation between the circadian clock and expression of activity. Second, changes in daily activity can be a direct response to light (‘masking’), without any involvement of the circadian system. Here, we studied whether the advance in onset of activity by dim light at night in great tits (Parus major) is indeed attributable to a phase shift of the internal clock.We entrained birds to a normal light/dark (LD) cycle with bright light during daytime and darkness at night, and to a comparable (LDim) schedule with dim light at night. The dim light at night strongly advanced the onset of activity of the birds. After at least six days in LD or LDim,we kept birds in constant darkness (DD) by leaving off all lights so birds would revert to their endogenous, circadian system controlled timing of activity.We found that the timing of onset in DD was not dependent on whether the birds were kept at LD or LDim before the measurement. Thus, the advance of activity under light at night is caused by a direct effect of light rather than a phase shift of the internal clock. This demonstrates that birds are capable of changing their daily activity to low levels of light at night directly, without the need to alter their internal clock.