Effects of low and high red to far-red light ratio on tomato plant morphology and performance of four arthropod herbivores

Changing the spectral composition of light has numerous advantages for the production and quality of horticultural crops. However, changing the light spectrum to stimulate growth can have negative consequences for plant responses to biotic stress. Especially changes in the ratio between red and far-red light (R:FR) have an important effect on plant-herbivore interactions. A low R:FR is indicative of shading and competition for light and induces strong growth responses which are known as shade avoidance responses, including stem elongation and leaf hyponasty. Exposure to low R:FR also inhibits plant defensive responses against biotic agents and increases the performance of pests and pathogens. In this study we tested whether an increase in R:FR, through the supplementation of red light LEDs can be used to reduce the performance of arthropod herbivores. Tomato plants (Solanum lycopersocum) were exposed to three different R:FR ratios; 0.5 (shading), 1.2 (sunlight) and 5.2 (artificial) and infested with either caterpillars (Manduca sexta), spider mites (Tetranychus urticae), aphids (Myzus persicae) or whiteflies (Trialeurodes vaporariorum). Plants exposed to a low R:FR showed clear stem elongation and leaf hyponasty consistent with the shade-avoidance responses, while plants exposed to high R:FR showed reduced stem length and reduced hyponasty. The performance of all four herbivore species was significantly increased after exposure to low R:FR, although the strength of this effect varied between phloem feeding and tissue feeding herbivores. Increasing the R:FR only reduced the performance of the caterpillars and did not significantly affect the other herbivore species. These results indicate that herbivore species with different feeding strategies respond differently to changes in light quality. These findings have implications for the use of LEDs in greenhouse horticulture.