Root herbivores: More than meets the eye
Root herbivores have a bad reputation. They are destructive, often monstrous looking pests (think alien-like scarab larvae; Figure 1) and sworn enemies of many gardeners. Although such a negative view of the activity of these soil-dwelling invertebrates might be deserved in many cases, there is more to them than meets the eye.
Root herbivores are key components of below-ground biodiversity for reasons other than their economic importance. They can drive the community dynamics of microorganisms and plants, even those found above-ground which superficially seem to have no connection to what’s going on in the below-ground ecosystem. Their role in terms of ecosystem functioning is increasingly being recognised, as research in this area continues to uncover their indirect competitive and facilitating effects on other organisms.
As for other groups of soil biodiversity, it is difficult to study the biology and ecology of root-feeders in situ because of the inability to see what is happening in real-time. The impracticality of working with such organisms in their natural environment has undoubtedly had an impact on the type of studies that can be carried out and therefore hypotheses that can be tested. However, the application of new and emerging technologies, including those used to study above-ground insect herbivores, means that it is becoming progressively easier to focus on root herbivores in the context of functional diversity and ecosystem processes.
One area that has great potential to improve our understanding of root herbivore activity and behaviour is molecular genetics, recently reviewed (Benefer and Blackshaw, 2013) in the new volume of Advances in Insect Physiology on the Behaviour and Physiology of Root Herbivores (edited by Scott Johnson, Ivan Hiltpold and Ted Turlings; Figure 1).
It highlights the numerous DNA (and protein) based techniques that are available and have been used to study species interactions (predator-prey, plant-herbivore, endosymbiont-host), genetic diversity (population dynamics and structure, gene flow), phylogenetics (species identity and distributions) and quantification of populations of insect root herbivores; relevant pre-requisite information required to determine their beneficial (and damaging) impacts in the wider ecosystem. Despite this, only 10 of the 249 articles published in Molecular Ecology and Molecular Ecology Resources on insect herbivores are specifically on root-feeding taxa, reminiscent of a general research bias towards above-ground insect herbivores.
So far, these and other studies which have used molecular genetics techniques have, unsurprisingly, been of an applied nature, concentrating on significant worldwide pests such as wireworms (mainly Agriotes spp.), corn rootworm (Diabrotica spp.) and white grubs (Scarabidae larvae, also known as cane grubs and chafers). This trend is likely to continue in the future, but with increasing access to next generation sequencing (NGS) technologies and decreasing costs associated with their use more genome- or community-wide analyses can take place.
For other groups of the soil biota, such as microorganisms and nematodes (also including some root herbivores), the use of sequence-based methods is already relatively commonplace. However, further opportunities remain to carry out large scale studies on, for example, root herbivore interactions in whole soil food webs (as extensions of current feeding ecology studies employing PCR-based methods), the composition and diversity of soil communities (metagenetics) in relation to land-use, management and climate change, and gene expression profiling to allow assessment of root herbivore responses to biotic and abiotic components in the soil environment.
The other reviews in this volume of Advances in Insect Physiology are testament to the good progress that has been made in understanding the behaviour and physiology of root herbivores. This is only likely to continue as awareness and development of new methods to study them expands.
Benefer, C.M. and Blackshaw, R.P. (2013) Chapter Five - Molecular Approaches for Studying Root Herbivores, in: Johnson, S.N., Hiltpold, I., Turlings, T.C.J. (Eds.), Advances in Insect Physiology. Academic Press, vol. 45, pp. 219-255.