In a new study conducted at the Satakunta forest diversity experiment we have shown that tree species diversity effects on pineapple gall adelgid on Norway spruce were mediated by changes in canopy cover around the host spruce trees. Our study therefore demonstrates that changes in forest structure may be critical to understanding the responses of herbivores to plant diversity and may underpin associational effects in forest ecosystems. Read the paper at http://dx.doi.org/10.1111/oik.03307