Advances in understanding plant root responses to root-feeding insects

In terms of the number of animal and plant species that occupy our planet, around 26% are herbivorous insects (Strong et al., 1984). They represent enormous diversity, particularly in terms of how they consume and exploit plant resources. A comparatively small proportion of herbivorous insects, limited to six orders, feed belowground on plant roots (Brown and Gange, 1990). While some root herbivores spend their entire lifecycle belowground (e.g. grape phylloxera), it is likely that the majority of species have aboveground life stages (usually adult) and it is the juvenile (larval) life-stages that attack roots. Rootfeeding insects differ markedly from shoot-feeding insects in terms of their ecology and life-history traits. While there are many exceptions to the rule, these differences can be broadly generalized (see Table 1) (Johnson et al., 2016b). In particular, root herbivores are probably represented in just 17% of families: they are predominantly chewers, have relatively long lifespans, live in highly aggregated populations and are in constant contact with the immense microbial communities found in the soil (Johnson et al., 2016b). As a group, there are still major gaps in our knowledge about how they interact with roots compared with our understanding of aboveground insect herbivore interactions (Hunter, 2001).