It is well-established now that phytochrome detection of high plant density signals reduces plant resistance against herbivores and pathogens. In a new Functional Structural Plant (FSP) modeling paper, Jorad de Vries (in the Jochem Evers group @ Wageningen University) et al show how actual competition for light impacts on herbivore damage.  They built an FSP model for Brassica nigra and simulated growth in presence/absence of herbivores with different feeding preferences and under different planting densities. Check out their paper here: https://academic.oup.com/aob/advance-article/doi/10.1093/aob/mcx212/4823055