Abstract
Microbes play crucial roles in the biology of herbivorous insects, and the last decade has provided exciting new evidence for a prominent role of microbial symbiosis in detoxification of plant toxins, manipulation of plant defences and defence against natural enemies. We provide an order by order update of symbioses across herbivorous insects, particularly focusing on recent published evidence, and on how symbionts interact with the defensive system of the plant. While the hemimetabolous Hemiptera order largely relies on obligatory microbial symbioses, we did not find such a close relationship between symbionts and hosts in the other three orders Orthoptera, Phasmatodea and Thysanoptera. These three orders mostly harbour transient gut symbionts and/or rely on laterally transferred genes from microbes. Despite the radical changes and harsh conditions during metamorphosis, numerous holometabolous species transmit symbionts vertically and show close associations with both intra- and extracellular symbionts. The last section of this book chapter discusses the role that symbionts will play in future scenarios of global warming, but also their implications for the transmission of plant viruses and modern agriculture.