The push-pull intercrop Desmodium does not repel, but intercepts and kills pests.

Over two decades ago, an intercropping strategy was developed that received critical acclaim for synergizing food security with ecosystem resilience in smallholder farming. The push-pull strategy reportedly suppresses lepidopteran pests in maize through a combination of a repellent intercrop (push), commonly Desmodium spp., and an attractive, border crop (pull). Key in the system is the intercrop's constitutive release of volatile terpenoids that repel herbivores. However, the earlier described volatile terpenoids were not detectable in the headspace of Desmodium, and only minimally upon herbivory. This was independent of soil type, microbiome composition, and whether collections were made in the laboratory or in the field. Furthermore, in oviposition choice tests in a wind tunnel, maize with or without an odor background of Desmodium was equally attractive for the invasive pest Spodoptera frugiperda. In search of an alternative mechanism, we found that neonate larvae strongly preferred Desmodium over maize. However, their development stagnated and no larva survived. In addition, older larvae were frequently seen impaled and immobilized by the dense network of silica-fortified, non-glandular trichomes. Thus, our data suggest that Desmodium may act through intercepting and decimating dispersing larval offspring rather than adult deterrence. As a hallmark of sustainable pest control, maize-Desmodium push-pull intercropping has inspired countless efforts to emulate stimulo-deterrent diversion in other cropping systems. However, detailed knowledge of the actual mechanisms is required to rationally improve the strategy, and translate the concept to other cropping systems.

Developmentally regulated volatiles geosmin and 2-methylisoborneol attract a soil arthropod to Streptomyces bacteria promoting spore dispersal.

Volatile compounds emitted by bacteria are often sensed by other organisms as odours, but their ecological roles are poorly understood1,2. Well-known examples are the soil-smelling terpenoids geosmin and 2-methylisoborneol (2-MIB)3,4, which humans and various animals sense at extremely low concentrations5,6. The conservation of geosmin biosynthesis genes among virtually all species of Streptomyces bacteria (and genes for the biosynthesis of 2-MIB in about 50%)7,8, suggests that the volatiles provide a selective advantage for these soil microbes. We show, in the present study, that these volatiles mediate interactions of apparent mutual benefit between streptomycetes and springtails (Collembola). In field experiments, springtails were attracted to odours emitted by Streptomyces colonies. Geosmin and 2-MIB in these odours induce electrophysiological responses in the antennae of the model springtail Folsomia candida, which is also attracted to both compounds. Moreover, the genes for geosmin and 2-MIB synthases are under the direct control of sporulation-specific transcription factors, constraining emission of the odorants to sporulating colonies. F. candida feeds on the Streptomyces colonies and disseminates spores both via faecal pellets and through adherence to its hydrophobic cuticle. The results indicate that geosmin and 2-MIB production is an integral part of the sporulation process, completing the Streptomyces life cycle by facilitating dispersal of spores by soil arthropods.

Ecological and Phylogenetic Relationships Shape the Peripheral Olfactory Systems of Highly Specialized Gall Midges (Cecidomiiydae).

Insects use sensitive olfactory systems to detect relevant host volatiles and avoid unsuitable hosts in a complex environmental odor landscape. Insects with short lifespans, such as gall midges (Diptera: Cecidomyiidae), are under strong selection pressure to detect and locate suitable hosts for their offspring in a short period of time. Ephemeral gall midges constitute excellent models for investigating the role of olfaction in host choice, host shift, and speciation. Midges mate near their site of emergence and females migrate in order to locate hosts for oviposition, thus females are expected to be more responsive to olfactory cues emitted by the host compared to males. In this study, we explored the correlation between host choice and the function of the peripheral olfactory system in 12 species of gall midges, including species with close phylogenetic relationships that use widely different host plants and more distantly related gall midge species that use similar hosts. We tested the antennal responses of males and females of the 12 species to a blend of 45 known insect attractants using coupled gas chromatographic-electroantennographic detection. When the species-specific response profiles of the gall midges were compared to a newly generated molecular-based phylogeny, we found they responded to the compounds in a sex- and species-specific manner. We found the physiological response profiles of species that use annual host plants, and thus have to locate their host every season, are similar for species with similar hosts despite large phylogenetic distances. In addition, we found closely related species with perennial hosts demonstrated odor response profiles that were consistent with their phylogenetic history. The ecology of the gall midges affects the tuning of the peripheral olfactory system, which in turn demonstrates a correlation between olfaction and speciation in the context of host use.