Navigating on a chemical radar: Usage of root exudates by foraging Diabrotica virgifera virgifera larvae

Abstract

In the darkness of the soil matrix, the larva of the western corn rootworm (WCR) Diabrotica virgifera virgifera LeConte relies on chemical cues to locate and accept its host plant roots. For almost 40 years, entomologists and chemical ecologists have tackled the challenge of isolating and identifying active chemical compounds emitted by host plant roots (most of the research has been conducted on maize Zea mays L.) and used by the foraging insect pest larvae. A number of molecules of interest have been documented but have so far only been implemented with little success in the current arms race to manage WCR (and soil‐dwelling pests). An in‐depth understanding of the mechanisms underlying the insect foraging behaviour is certainly critical to the development of highly effective and sustainable pest control strategies. This contribution reviews the progress and highlights the gaps in our current knowledge on the chemical ecology of WCR larvae. An individual‐based model of the larval behaviour in response to root volatiles has been developed based on data from the literature. It also proposes avenues (tested or theoretical) to eventually implement this knowledge in integrated pest management of this major maize pest, a critical approach as WCR has evolved resistance to several control strategies.

The Role of Plant-Associated Microbes in Mediating Host-Plant Selection by Insect Herbivores

“…Many insect species use plant-produced volatile compounds to locate and evaluate potential host plants [133] as these cues effectively transmit useful information over both short [134] and long [135] distances in a variety of environments. Olfactory cues can be general indicators of plant presence, for example the respiratory biproduct CO 2 [19], or complex blends of volatile organic compounds (VOCs) that convey detailed information about plant identity [4], nutrient content [136], defense status, or risk of predation by natural enemies [137,138]. Plants emit characteristic blends of VOCs that vary by plant species, genotype, developmental stage, and tissue [139].…”

“…In contrast, gustatory cues require plant contact and provide information about suitable tissues for feeding or oviposition [17]. The use of different cues varies among insect herbivore species [18] and particular cues may be more useful in certain habitats, like soil environments [19], or during certain times of the day, such as diurnal, nocturnal, or crepuscular activity [20]. Although visual, olfactory, and gustatory cues vary in relative importance during host-plant selection among different herbivore species and environmental conditions, these cues are often used in combination by foraging or ovipositing insects [15].…”

Complex plant metabolomes guide fitness-relevant foraging decisions of a specialist herbivore

“…Here, we developed a manipulative approach to evaluate the importance of maize primary and secondary metabolites for the foraging and foraging-dependent performance of the western corn rootworm (Diabrotica virgifera virgifera). The western corn rootworm is an economically damaging maize pest (58). Its larvae are highly specialized on maize roots (59).…”

“…Its larvae are highly specialized on maize roots (59). Over the last decades, the chemical ecology of the western corn rootworm has been studied in detail (13,58,59).…”

Plant-derived CO2mediates long-distance host location and quality assessment by a root herbivore

“…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 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, 2015b;Hu et al, 2018;Huang et al, 2017;Robert et al, 2012c).…”