No abstract

Plants produce complex mixtures of primary and secondary metabolites. Herbivores use these metabolites as behavioral cues to increase their fitness. However, how herbivores combine and integrate different metabolite classes into fitness-relevant foraging decisions in planta is poorly understood. We developed a molecular manipulative approach to modulate the availability of sugars and benzoxazinoid secondary metabolites as foraging cues for a specialist maize herbivore, the western corn rootworm. By disrupting sugar perception in the western corn rootworm and benzoxazinoid production in maize, we show that sugars and benzoxazinoids act as distinct and dynamically combined mediators of short-distance host finding and acceptance. While sugars improve the capacity of rootworm larvae to find a host plant and to distinguish postembryonic from less nutritious embryonic roots, benzoxazinoids are specifically required for the latter. Host acceptance in the form of root damage is increased by benzoxazinoids and sugars in an additive manner. This pattern is driven by increasing damage to postembryonic roots in the presence of benzoxazinoids and sugars. Benzoxazinoid- and sugar-mediated foraging directly improves western corn rootworm growth and survival. Interestingly, western corn rootworm larvae retain a substantial fraction of their capacity to feed and survive on maize plants even when both classes of chemical cues are almost completely absent. This study unravels fine-grained differentiation and combination of primary and secondary metabolites into herbivore foraging and documents how the capacity to compensate for the lack of important chemical cues enables a specialist herbivore to survive within unpredictable metabolic landscapes.

Plants produce complex mixtures of primary and secondary metabolites. Herbivores use these metabolites as behavioral cues to increase their fitness. However, how herbivores integrate primary and secondary metabolites into fitness-relevant foraging decisions in planta is poorly understood. We developed a molecular manipulative approach to modulate the availability of sugars and benzoxazinoid secondary metabolites as foraging cues for a specialist maize herbivore, the western corn rootworm. By disrupting benzoxazinoid biosynthesis in maize and sugar perception in the western corn rootworm, we show that sugars and benzoxazinoids act as distinct and dynamically integrated mediators of short-distance host finding and acceptance. While sugars improve the capacity of rootworm larvae to find a host plant and to distinguish post-embryonic from less nutritious embryonic roots, benzoxazinoids are specifically required for the latter. Host acceptance in the form of root damage is increased by benzoxazinoids and sugars in an additive manner. This pattern is driven by increasing damage to post-embryonic roots in the presence of benzoxazinoids and sugars. Benzoxazinoid- and sugar-mediated foraging directly improves western corn rootworm growth and survival. Interestingly, western corn rootworm larvae retain a substantial fraction of their capacity to feed and survive on maize plants in the absence of both cues. This study unravels fine-grained differentiation and integration of primary and secondary metabolites into herbivore foraging and documents how the capacity to compensate for the absence of important chemical cues enables a specialist herbivore to survive within unpredictable metabolic landscapes.

Insect herbivores can use volatile and visual cues to locate and select suitable host plants from a distance. The importance of CO 2 , arguable the most conserved volatile marker of metabolic activity, is not well understood in this context, even though many herbivores are known to respond to minute differences in CO 2 concentrations. To address this gap of knowledge, we manipulated CO2 perception of the larvae of the western corn rootworm (Diabrotica virgifera virgifera; WCR) through RNA interference and studied how CO 2 perception impacts their interaction with their host plant, maize (Zea mays). We show that the expression of a putative Group 2 carbon dioxide receptor, DvvGr2, is specifically required for larval responses to CO2. Silencing DvvGr2 has no major effect on the ability of WCR larvae to locate host plants at short distance (<10 cm), but increasingly impairs host location at greater distances, suggesting that WCR larvae integrate CO 2 with other volatile cues for host finding in a distance-specific manner. We further show that the larvae use CO 2 as a fitnessrelevant long-distance indicator of plant nutritional status: Maize plants that are well-fertilized emit more CO2 from their roots and are better hosts for WCR than plants that are nutrient-deficient, and the capacity of WCR larvae to distinguish between these plants depends exclusively on their capacity to perceive CO 2 through DvvGr2. This study unravels how CO 2 can mediate plant-herbivore interactions by serving as a distance-dependent host location and quality assessment cue.Keywords: Plant-herbivore interactions, foraging, volatile perception, behaviour, host location. 2012).So far, this model has not been experimentally validated, and the precise role of plant-derived CO2 as a host location cue by herbivores in general, and root herbivores in particular, remains unclear (Eilers et al., 2016). To the best of our knowledge, no studies so far have investigated the role of plant-derived CO 2 in plant-herbivore interactions in vivo using molecular manipulative approaches.The larvae of Diabrotica virgifera virgifera (western corn rootworm, WCR) feed exclusively on maize roots and cause major yield losses in the US and Eastern Europe (Ciosi et al., 2008; Gray et al., 2009; Meinke et al., 2009; Toepfer et al., 2015). The larvae rely on a number of volatile and non-volatile chemicals to identify and locate host plants, distinguish between suitable and less-suitable maize plants and forage within the maize root system (Hiltpold et al., 2013; Johnson and Gregory, 2006; Johnson and Nielsen, 2012; Robert et al., 2012c; Schumann et al., 2018). Non-volatile primary metabolites such as sugars and fatty acids as well as secondary metabolites such as benzoxazinoids and phenolic acid conjugates modulate larval behaviour (Bernklau et al., 2011; Bernklau et al., 2015; Bernklau et al., 2016a; Bernklau et al., 2018b; Erb et al., 2015a; Hu et al., 2018; Huang et al., 2017; Robert et al., 2012c). Volatiles including (E)-βcaryophyllene, ethylene and CO2 attra...