Chemically-mediated host-plant location and selection by root-feeding insects

Recent studies have shown that root-feeding insects can be of considerable importance in terms of agricultural damage, their indirect impacts on above-ground herbivores and their efficacy as biocontrol agents of weeds. To date, isolated studies have made it difficult to identify the mechanisms by which soil-dwelling insects locate and select host-plant roots. This review synthesizes 78 studies describing root location and selection. Soil insect herbivores do not rely on encountering roots at random, but orientate towards them using semiochemicals that enable specialist insects to distinguish host-plants from unsuitable plants. Secondary plant metabolites released into the rhizosphere (alcohols, esters and aldehydes representing 37% of reported examples) underpin host-plant location and recognition, with 80% having 'attractant' properties. Insects feeding on a limited range of plants tend to exploit host-specific secondary metabolites, whereas nonspecialist feeders appear to use more general semiochemicals. When insects reach the roots, contact chemosensory cues act as either 'phagostimulants' (48% of the compounds being sugars) or feeding 'deterrents' (notably phenolic compounds). Twenty studies conclude that CO2 is the major primary plant metabolite that allows insects to locate to roots. However, several features of CO2 emissions from roots mitigate against it as a precise location cue. In addition to its lack of specificity, gradients of root emitted CO2 do not persist for long periods and vertical gradients of CO2 in the soil tend to be stronger than horizontal gradients. A conceptual model is presented, emphasizing the importance of soil properties (e.g. porosity, moisture) on chemical diffusion and insect motility.