Foraging in the dark – chemically mediated host plant location by belowground insect herbivores

Root-feeding insects are key components in many terrestrial ecosystems. Like shoot-feeding insect herbivores, they exploit a range of chemical cues to locate host plants. Respiratory emissions of carbon dioxide (CO2) from the roots is widely reported as the main attractant, however, there is conflicting evidence about its exact role. CO2 may act as a ‘search trigger’ causing insects to search more intensively for more host specific signals, or the plant may ‘mask’ CO2 emissions with other root volatiles thus avoiding detection. At least 74 other compounds elicit behavioral responses in root-feeding insects, with the majority (>80 %) causing attraction. Low molecular weight compounds (e.g., alcohols, esters, and aldehydes) underpin attraction, whereas hydrocarbons tend to have repellent properties. A range of compounds act as phagostimulants (e.g., sugars) once insects feed on roots, whereas secondary metabolites often deter feeding. In contrast, some secondary metabolites usually regarded as plant defenses (e.g., dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA)), can be exploited by some root-feeding insects for host location. Insects share several host location cues with plant parasitic nematodes (CO2, DIMBOA, glutamic acid), but some compounds (e.g., cucurbitacin A) repel nematodes while acting as phagostimulants to insects. Moreover, insect and nematode herbivory can induce exudation of compounds that may be mutually beneficial, suggesting potentially significant interactions between the two groups of herbivores. While a range of plant-derived chemicals can affect the behavior of root-feeding insects, little attempt has been made to exploit these in pest management, though this may become a more viable option with diminishing control options.