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In insects, the search for food is highly dependent on olfactory sensory input. Here, we investigated whether a single key odorant within an odor blend or the complexity of the odor blend influences the attraction of to a food source. A key odorant is defined as an odorant that elicits a difference in the behavioral response when two similar complex odor blends are offered. To validate that the observed behavioral responses were elicited by olfactory stimuli, we used olfactory co-receptor mutants. We show that within a food odor blend, ethanol functions as a key odorant. In addition to ethanol other odorants might serve as key odorants at specific concentrations. However, not all odorants are key odorants. The intensity of the odor background influences the attractiveness of the key odorants. Increased complexity is only more attractive in a concentration-dependent range for single compounds in a blend. Orco is necessary to discriminate between two similarly attractive odorants when offered as single odorants and in food odor blends, supporting the importance of single odorant recognition in odor blends. These data strongly indicate that flies use more than one strategy to navigate to a food odor source, depending on the availability of key odorants in the odor blend and the alternative odor offered.

Plant volatiles help herbivores to locate their hosts, and therefore, they could be used to help develop pesticide-free pest management strategies.To develop an attractant for tea leafhopper (Empoasca onukii), we screened nine tea plant volatile compounds for their attractiveness using Y-tube olfactometer assays. Results indicated that tea leafhoppers significantly preferred ocimene, limonene, (Z)-3-hexenol, and (Z)-3-hexenyl acetate over clean air. These compounds were combined in a blend which lost its attractiveness at concentrations below 10–2 g/ml in liquid paraffin. In field tests, the blend was attractive to leafhoppers only in autumn, but not in summer. Analyses of the tea field background odor showed that all four components of the blend were present at much higher concentrations in summer (0.05–0.001 ng/liter) than in autumn (∼10- to 25-fold lower). In fieldY-tube bioassays, compared with the tea field background odor, the blend was attractive at a concentration of 10–1 g/ml in liquid paraffin, but not at 10–2 g/ ml.These results suggest that field background odor can disrupt the attractiveness of an attractant based on plant volatiles to herbivores.

Animals use olfaction to search for distant objects. Unlike vision, where objects are spaced out, olfactory information mixes when it reaches olfactory organs. Therefore, efficient olfactory search requires segregating odors that are mixed with background odors. Animals can segregate known odors by detecting short differences in the arrival of mixed odorants (stimulus onset asynchrony). However, it is unclear whether animals can also use stimulus onset asynchrony to segregate odorants that they had no previous experience with and which have no innate or learned relevance (unknown odorants). Using behavioral experiments in honey bees, we here show that stimulus onset asynchrony also improves segregation of those unknown odorants. The stimulus onset asynchrony necessary to segregate unknown odorants is in the range of seconds, which is two orders of magnitude larger than the previously reported stimulus asynchrony sufficient for segregating known odorants. We propose that for unknown odorants, segregating odorant A from a mixture with B requires sensory adaptation to B.

Isoprene is the most abundant volatile compound emitted by vegetation. It influences air chemistry and is part of plant defense against abiotic stresses. However, whether isoprene influences biotic interactions between plants and other organisms has not been investigated to date. Here we show a new effect of isoprene, namely its influence on interactions between plants and insects. Herbivory induces the release of plant volatiles that attract the herbivore's enemies, such as parasitic wasps, as a kind of bodyguard. We used transgenic isoprene-emitting Arabidopsis plants in behavioral, chemical, and electrophysiological studies to investigate the effects of isoprene on ecological interactions in 2 tritrophic systems. We demonstrate that isoprene is perceived by the chemoreceptors of the parasitic wasp Diadegma semiclausum and interferes with the attraction of this parasitic wasp to volatiles from herbivore-infested plants. We verified this repellent effect on D. semiclausum female wasps by adding external isoprene to the volatile blend of wild-type plants. In contrast, the antennae of the parasitic wasp Cotesia rubecula do not perceive isoprene and the behavior of this wasp was not altered by isoprene emission. In addition, the performance of the 2 examined lepidopteran herbivores (Pieris rapae and Plutella xylostella) was not affected by isoprene emission. Therefore, attraction of parasitic wasps to host-infested herbaceous plants in the neighborhood of high isoprene emitters, such as poplar or willow, may be hampered by the isoprene emission that repels plant bodyguards.

The volatile plant compounds (VPC) alter pheromone perception by insects but mixture effects inside insect olfactory landscapes are poorly understood. We measured the activity of receptor neurons tuned to Z7-12Ac (Z7-ORN), a pheromone component, in the antenna and central neurons in male Agrotis ipsilon while exposed to simple or composite backgrounds of a panel of VPCs representative of the odorant variety encountered by a moth. Maps of activities were built using calcium imaging to visualize which areas in antennal lobes (AL) were affected by VPCs. We compared the VPC activity and their impact as backgrounds at antenna and AL levels, individually or in blends. At periphery, VPCs showed differences in their capacity to elicit Z7-ORN firing response that cannot be explained by differences in stimulus intensities because we adjusted concentrations according to vapor pressures. The AL neuronal network, which reformats the ORN input, did not improve pheromone salience. We postulate that the AL network evolved to increase sensitivity and to encode for fast changes of pheromone at some cost for signal extraction. Comparing blends to single compounds indicated that a blend shows the activity of its most active component. VPC salience seems to be more important than background complexity.

Herbivores emit plant-associated volatile organic compounds (VOCs) after feeding on plants. These plant-associated VOCs can be used by parasitoids to locate their hosts. It is hypothesized that certain compounds play key roles in the attractiveness of host-associated odor blends. The larval parasitoid, Microplitis croceipes (Hymenoptera: Braconidae) and its herbivore host, Heliothis virescens (Lepidoptera: Noctuidae), a major pest of cotton plant were used as model species to identify key compounds mediating attraction of parasitoids to hosts. Comparative GC/MS analyses of cotton-fed vs. artificial diet-fed hosts indicated that 12 of 17 compounds in the headspace of H. virescens larvae were exclusive to plant-fed hosts, and thus considered to be plant-associated. In order to identify key attractive compounds, a full blend of 15 commercially available synthetic compounds was modified by removing each of the 10 plant-associated compounds emitted by host larvae. In Y-tube olfactometer bioassays testing parasitoid responses to modified blends, 1-octen-3-ol, decanal, ( E )-β-caryophyllene, α-humulene, α-farnesene, and β-pinene were identified as key compounds contributing to attractiveness of the natural blend of VOCs emitted by cotton-fed hosts. The results showed that while various host-associated compounds act in concert to serve as useful host location cues, only a fraction of the natural blend mediates attraction in parasitoids. Furthermore, the role of a compound is better assessed in the context of other compounds, and odor blends are better perceived as a whole rather than as individual components.

Insects live in a highly complex odorant world. Within a variety of odor blends, they need to locate potential food sources, mates, and oviposition sites to gain reproductive success. In nature, volatile cues leading to a resource are always present with numerous other volatiles—here referred to as background odor—which may affect the parasitoid's response to resource-indicating cues. Three different types of background odor are discussed in this article: (a) irrelevant background odor, (b) background odor that may mask the resource-indicating signals, and (c) background odorants that may “sharpen the view” for resource-indicating odor and enhance the response to these. Odor orientation to resources especially in herbivorous and parasitic insects are addressed.

Habituation is a form of simple memory that suppresses neural activity in response to repeated, neutral stimuli. This process is critical in helping organisms guide attention toward the most salient and novel features in the environment. Here, we follow known circuit mechanisms in the fruit fly olfactory system to derive a simple algorithm for habituation. We show, both empirically and analytically, that this algorithm is able to filter out redundant information, enhance discrimination between odors that share a similar background, and improve detection of novel components in odor mixtures. Overall, we propose an algorithmic perspective on the biological mechanism of habituation and use this perspective to understand how sensory physiology can affect odor perception. Our framework may also help toward understanding the effects of habituation in other more sophisticated neural systems.

Summary Olfactory signals, often in synergy with visual signals, mediate the interactions between plants and animals. However, urbanization and agricultural practices are both sources of volatile organic compounds (VOCs) and reactive oxygen species (ROS) that have the potential to interfere with plant–animal communication and to disrupt mutualistic interactions. In this review, we explore how anthropogenic airborne pollutants may disrupt chemical information transfer between flowering plants and flower visitors. The emission of anthropogenic volatile pollutants (AVPs) including VOCs and formation of ROS, for example from traffic or industries, and non‐natural biogenic VOCs, for example from introduced crops, may have a number of effects: (i) changes in plant signalling as a consequence of plants experiencing physiological stress; (ii) chemical interference (chemical degradation/transformation of infochemicals); (iii) increased levels of background noise impeding signal detection; and (iv) changes in pollinator signal perception and behaviour. All of the above in turn could have consequences for the biological fitness of plants and animals that rely on olfactory information as pivotal functional signals. The study of anthropogenic airborne pollutants and their effects on plant signalling is just emerging and the impacts of this aspect of anthropogenic emissions are barely understood. Volatiles emitted from anthropogenically changed landscapes could, however, have far‐reaching consequences for ecosystem functioning in adjacent natural zones, particularly in fragmented landscapes. In response to the wide gap in our knowledge on the mechanisms that govern interference of anthropogenic VOCs with olfactory information, future research directions are proposed with the aim to inspire research to help elucidate the risks of anthropogenic VOCs for plant–pollinator communities and improve risk assessment strategies. A lay summary is available for this article. Lay Summary

Neophobic predator avoidance (NPA), the fear response exhibited by prey animals exposed to novel stimuli, is a plastic trait thought to be induced by exposure to elevated chronic risk of predation. Indeed, prey experiencing low levels of background risk fail to display NPA, while those experiencing high background risk do display NPA. Recent work has suggested that the trigger for inducing and maintaining NPA may not be background risk per se but rather uncertainty in the predation environment. Here, we designed two experiments on wild-caught neophobic Trinidadian guppies (Poecilia reticulata) to test the ‘uncertainty hypothesis’. In the first experiment, we tested if the diversity of novel stimuli encountered by guppies would affect the maintenance of their NPA. We found that exposure to a single novel odour once per day for 3 days was sufficient to extinguish the neophobic response to that odour (Experiment 1A) but not a new odour (Experiment 1B). However, when guppies were exposed to multiple novel odours so that each odour was encountered once a day for 3 days, they retained their NPA to both a previously encountered novel odour (Experiment 1A) and a new odour (Experiment 1B). In our second experiment, we repeatedly exposed guppies to a novel cue in a spatially predictable vs. unpredictable location and found that spatial unpredictability interfered with the reduction of NPA to that cue. Combined, our results suggest that uncertainty of risk experienced by prey in time and space shapes the retention of NPA.