The importance of root-produced volatiles as foraging cues for entomopathogenic nematodes

Abstract

Background Entomopathogenic nematodes (EPNs) are tiny parasitic worms that parasitize insects, in which they reproduce. Their foraging behavior has been subject to numerous studies, most of which have proposed that, at short distances, EPNs use chemicals that are emitted directly from the host as host location cues. Carbon dioxide (CO 2) in particular has been implicated as an important cue. Recent evidence shows that at longer distances several EPNs take advantage of volatiles that are specifically emitted by roots in response to insect attack. Studies that have revealed these plant-mediated interactions among three trophic levels have been met with some disbelief. Scope This review aims to take away this skepticism by summarizing the evidence for a role of root volatiles as foraging cues for EPNs. To reinforce our argument, we conducted olfactometer assays in which we directly compared the attraction of an EPN species to CO 2 and two typical inducible root volatiles. Conclusions The combination of the ubiquitous gas and a more specific root volatile was found to be considerably more attractive than one of the two alone. Hence, future studies on EPN foraging behavior should take into account that CO 2 and plant volatiles may work in synergy as attractants for EPNs. Recent research efforts also reveal prospects of exploiting plant-produced signals to improve the biological control of insect pests in the rhizosphere.

Host-seeking behavior of the Heterorhabditis amazonensis nematode in response to stimulant sources1

“…Carbon dioxide (CO 2 ) in the soil indicates a biological activity and has been widely implicated in locating a host (Rasmann et al 2012). Turlings et al (2012) reported that the combination of this gas and specific volatile root compounds is more attractive than when encountered individually, acting synergistically to attract entomopathogenic nematodes.…”

Behaviour and Population Dynamics of Entomopathogenic Nematodes Following Application

“…However, following a CO 2 gradient is likely to bring nematodes to plant roots (Dusenbery, 1987), where potential hosts might then be found by moving along the roots randomly and/or in response to further directional signals operating over a shorter scale. CO 2 may be seen as a response-activator that alerts EPN to the presence of living organisms and may enhance responsiveness to other more specific cues (Turlings, Hiltpold, & Rasmann, 2012) that are discussed in more detail below.…”

“…Use by EPN of plant-derived signals would be particularly important in finding hosts that are otherwise unattractive, such as vine weevil Otiorhynchus sulcatus Fabricius (Coleoptera: Curculionidae) larvae (Boff, Zoon, & Smits, 2001). However, the usefulness of the volatiles will vary depending on soil type and conditions; for example, being less effective in soil types with high levels of chemical activity (Turlings et al, 2012). Soil texture and moisture also affect diffusion of volatiles.…”

Rhizosphere 3: where plants meet soils down-under

“…The Marschner review by Turlings et al (2012) is especially noteworthy, first summarising the evidence for root volatiles as foraging cues for parasitic entomopathogenic nematodes and then presenting data to show that there may be a synergistic effect of root volatiles and CO 2 as attractants for the nematodes.…”