Harnessing the rainbow: Effects of light quality on hemp and the cannabis aphid

LEDs (light emitting diodes) are used in horticulture to change the quality of light plants are grown under. Modifying the light quality of a growing environment with LEDs can help plants develop more desirable physical and chemical traits such as increased yield and nutrition. Pests can be affected by light quality as well. Light quality can decrease pest fitness directly by disrupting pest lifecycles and indirectly through chemical changes in the host plant. The strategy of increasing crop quality and suppressing pests through light quality manipulation may have utility in the cultivation of Cannabis sp. To test the effects of different light qualities on hemp growth, chemistry, and quality, we grew autoflowering CBD hemp, variety “Little Giant,” under four different ratios of blue, red, and broad spectrum (white) LEDs and measured the plants’ resulting yield, biomass, and concentration of the defensive hormone salicylic acid. As this variety of hemp is grown for the CBD (cannabidiol) in its flowers and for its comparatively low THC (tetrahydrocannabinol) content, the CBD and THC concentrations of the flowers were measured as a metric of crop quality. Low-green light quality resulted in reduced mass of vegetative tissues. Herbivory from aphids reduced concentration of CBD and THC in buds. Aphid herbivory reduced bud mass but increase leaf and stem mass.