Nutrition – how animals acquire, process and allocate dietary resources – underpins many ecological processes.  This statement applies particularly to herbivorous insects because their diet of plant material is of low, unbalanced and variable nutritional quality, with major consequences for host plant choice, insect abundance, and community composition.   Time and again, plant utilization by herbivorous insects is mediated by resident microorganisms that variously promote digestion, provide supplementary nutrients and detoxify plant allelochemicals.  The prevalence of these microbial symbioses in herbivorous insects raises three broad questions.  (1) What is the impact of the microbiota on ecologically-important traits and fitness of herbivorous insects?  (2) How does the composition of the microbiota affect the traits of insects, including host plant range? (3) Can manipulation of the microbiota contribute to novel strategies for management of beneficial and pest insect species?  

Results/Conclusions

Two broad patterns are emerging from recent research.  First, the impact of many microorganisms on the traits of herbivorous insects is strongly context-dependent.  Some microbes can be beneficial, harmless or deleterious to their insect host, depending on the nutritional composition of the diet or the rearing plant.  Second, the composition of the microbial community in an insect is not fixed, but can vary with host age, condition and genotype; and the impact of the microbiota on host traits can vary with microbial identity and diversity.  A key issue for the future is to develop strategies that use our increasing understanding of dietary factors and microbial determinants of the fitness and traits of herbivorous insects to manage beneficial and pest species.