When plants are attacked by herbivores, they undergo profound phenotypic changes, including an activation of defenses. Although herbivory-induced plant responses are to a large extent tailored to the type of attacker, they can also have off-target effects, as they can: (a) operate beyond the site of attack; (b) persist over time, and; (c) modulate the induction by other attackers. It is through these kind of mechanisms that herbivores can alter the behavior and performance of other arthropod species on a shared host plant − even when they are separated from each other in time and/or space. Indirect, plant-mediated interactions between herbivores are important in nature and agriculture, as they are omnipresent and shape plant-associated arthropod communities. Unfortunately, despite their ecological relevance, we know very little about the mechanisms underlying plant-mediated interactions between herbivores.
In this project, we have investigated the indirect interaction between leaf-feeding Spodoptera frugiperda (fall armyworm) and root-feeding Diabrotica virgifera virgifera (western corn rootworm) on cultivated maize (Zea mays). Both insect species are major pests of maize and can co-occur throughout the growing season. The outcome of the plant-mediated interaction between the spatially separated larvae of these insect herbivores is mainly determined by the order of their arrival. That is, western corn rootworm larvae refuse to feed when a plant is already attacked aboveground by fall armyworms. This host avoidance behavior has been attributed to changes in root-emitted volatiles upon leaf herbivory. However, when western corn rootworms arrive first, they ‘canalize’ the plant’s metabolism, thereby making it largely unresponsive to subsequent attack by fall armyworms. Consequently, the rootworms are not repelled upon subsequent armyworm feeding, while the armyworms may now be negatively affected by plant responses to root-damage-associated water stress.
Previous research has suggested that the emission of a single volatile metabolite, a methoxy-nitrophenol, by maize roots − and the suppression thereof by first-arriving rootworms − is likely responsible for the sequence-specific interaction of leaf-feeding fall armyworm larvae and root-feeding western corn rootworm larvae. Note that this methoxy-nitrophenol (MNP) has not been described from any other plant species before, thus it is a novel plant metabolite with a putative function in plant resistance to an economically important insect pest.
The overall objectives of this project were, firstly, to determine when and how MNP is produced by maize and, secondly, to elucidate how MNP metabolism is suppressed by first-arriving western corn rootworms. Understanding these processes would greatly advance our understanding of plant-mediated interactions between herbivores and, moreover, it would deliver valuable new breeding targets for enhancing crop resistance.
