Fungal Infection Reduces Herbivore-Induced Plant Volatiles of Maize but does not Affect Naïve Parasitoids

Abstract

Plants attacked by insects release volatile compounds that attract the herbivores' natural enemies. This so-called indirect defense is plastic and may be affected by an array of biotic and abiotic factors. We investigated the effect of fungal infection as a biotic stress agent on the emission of herbivore-induced volatiles and the possible consequences for the attraction of two parasitoid species. Maize seedlings that were simultaneously attacked by the fungus Setosphaeria turcica and larvae of Spodoptera littoralis emitted a blend of volatiles that was qualitatively similar to the blend emitted by maize that was damaged by only the herbivore, but there was a clear quantitative difference. When simultaneously challenged by fungus and herbivore, the maize plants emitted in total 47% less of the volatiles. Emissions of green leaf volatiles were unaffected. In a six-arm olfactometer, the parasitoids Cotesia marginiventris and Microplitis rufiventris responded equally well to odors of herbivore-damaged and fungus- and herbivore-damaged maize plants. Healthy and fungus-infected plants were not attractive. An additional experiment showed that the performance of S. littoralis caterpillars was not affected by the presence of the pathogen, nor was there an effect on larvae of M. rufiventris developing inside the caterpillars. Our results confirm previous indications that naïve wasps may respond primarily to the green leaf volatiles.

Benzoxazinoid Metabolites Regulate Innate Immunity against Aphids and Fungi in Maize

“…For HPLC analysis of BX content, leaf material was collected from four to six infested leaves from different plants. S. turcica cultivation, spore collection, and inoculation of 8-d-old seedlings (5 3 10 4 spores mL 21 ) were performed as described by Rostás et al (2006). Sixteen randomly collected segments (2-3 cm) from inoculated leaves of four different plants per line were collected at indicated time points and divided for HPLC analysis and microscopic analysis following trypan blue lactophenol blue staining (Koch and Slusarenko, 1990).…”

Plants under dual attack : consequences for plant chemistry and parasitoid behavior

“…Some studies have also examined effects on the third trophic level when both the JA and SA signal-transduction pathways are assumed to be induced, by combinations of herbivores and/or pathogens. While in one case attack by two herbivore species respectively affecting JA and SA signaling led to a decrease in carnivore attraction compared to single-species herbivory (Zhang et al, 2009), in all other cases carnivores were either equally attracted to, or preferred dually damaged plants over plants damaged by the host herbivore alone, for both dual attack by two herbivores (Moayeri et al, 2007;Erb et al, 2010) and a combination of herbivore and pathogen (Rostás et al, 2006). Interactions between the two primary signal-transduction pathways may play a role in the changes in VOC emissions in response to dual attacker events.…”

“…Concomitant attack by a hemibiotrophic fungus, Setosphaeria turcica, and a leaf chewing herbivore, S. exigua, resulted in strongly suppressed volatile emissions in comparison to herbivory alone (Rostás et al, 2006). Classically categorized as a necrotroph, recent work suggests that S. turica is in fact a hemibiotroph (Chung et al, 2010), and the plant defense response to the fungus in the early infection stages appears to be SA and ET pathway regulated (Erb et al, 2009).…”

“…The reduction in (E)-β-ocimene emission in response to whitefly infestation correlated with reduced JA titre and reduced transcription of the JA-regulated gene encoding for the enzyme ocimene synthase that is crucial for in (E)-β-ocimene production (Zhang et al 2009). Likewise, a sharp decrease in overall volatile emissions may not have any consequences for parasitoid attraction if the compounds involved in attraction remain relatively unaffected in their emission rates (Rostás et al, 2006). Thus, in order to understand the consequences of dual infestation on the behavioral responses of carnivorous insects, knowledge of the underlying mechanisms is crucial.…”

“…It appears that as with multiple insect herbivory, concomitant challenge by a pathogen and herbivore can affect HIPV composition and emission rates . In maize, for instance, plants simultaneously challenged by the fungus Setosphaeria turcica and Spodoptera littoralis caterpillars had total volatile emission rates nearly 50% lower than with herbivory alone, yet the host searching behavior of two parasitic wasp species was not affected (Rostás et al, 2006). Meanwhile, in peanut plants, dual infestation with a pathogen and a herbivore led to enhanced wasp attraction (Cardoza et al, 2003b).…”