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Seeds and seedlings are particularly vulnerable to herbivory. Unlike mature plants, which can wait until herbivory is experienced to induce defense, seeds and seedlings face mortality if they wait. Slug mucus functions as a kairomone, a non-attack-related substance emitted by consumers that is detected by a prey species (in this case, plants). While snail mucus has been shown to induce defense in seedlings, it is not widely confirmed whether slugs have the same effect and whether seeds can also detect and react to such herbivore cues. We investigated how exposure to Arion subfuscus mucus affected growth and defense in Brassica nigra seeds and seedlings. Seeds exposed to slug mucus germinated 5% faster than control (water only) seeds, but the resulting seedlings weighed 16% less than control seedlings. To test whether this difference results from herbivore-exposed plants allocating energy from growth to defense, we conducted choice bioassays assessing slug preference for control seedlings versus seedlings that were either (A) exposed to mucus only as a seed; or (B) exposed to mucus as a seed and seedling. While slugs did not differentiate between control seedlings and ones exposed to herbivore cues only as a seed, they ate 88% less biomass of seedlings exposed to mucus as both seeds and seedlings. These results suggest that slug mucus induces changes in plant traits related to defense and growth/competitive ability. Future research should determine the chemical mechanisms of this induced defense.

Avoiding detection is perhaps the ultimate weapon for both predators and prey. Chemosensory detection of predators via waterborne or airborne cues (predator-released kairomones) is a key prey adaptation in aquatic ecosystems. Pirate perch, Aphredoderus sayanus, a largely insectivorous mesopredatory fish, are considered to be chemically camouflaged because they are unavoided by all colonizing organisms tested, including treefrogs and aquatic insects, despite stronger predatory effects on target taxa than several avoided fish. To address the mechanism behind camouflage we used aquatic insect colonization as a bioassay to test (1) whether increasing pirate perch density/biomass leads to increased avoidance, and (2) whether pirate perch mask heterospecific fish kairomones. Insect abundances, species richness, and community structure showed no response to pirate perch density. Last, pirate perch did not mask the kairomones of heterospecific predatory fish. Results support the idea that fish kairomones are species-specific, and chemical camouflage is driven by a unique chemical signature that is either undetectable or has no negative associations for colonists.

Predators can modify population and community dynamics not only through direct predation, but also through nonconsumptive effects. Predator-induced changes in the traits of prey species are important components of these nonconsumptive effects. While these are well studied in simplified one-predator one-prey settings, relatively little is known about how kairomones act on prey across heterogeneous aquatic ecosystems with diverse identities of predator and prey. Kairomones are, to some extent, predator specific, and can be classified as diet dependent or diet independent. This classification depends on whether the identity of the prey plays a role in eliciting a defense response. In an effort to elucidate how prey and predator identity determines the defense responses in water fleas, we used inducible morphology and life history traits of Daphnia mitsukuri as a model to systematically explore to what extent predator and prey identity, and species composition of the prey community determined the expression of inducible defenses, and to what extent predator-induced responses differed among genotypes. Our results showed that the defense responses of D. mitsukuri mainly relied on diet-dependent kairomones, which were not influenced by fish species identity but were highly dependent on the phylogenetic distances between D. mitsukuri and the prey ingested by fish. This phylogenetic signal is strong, with D. mitsukuri responding only to the kairomones of fish that feed on cladocerans. We also found that the amplitude of the responses to fish kairomones increased with the amount of cladoceran in the dietary community. We observed significant differences in defensive traits among three D. mitsukuri genotypes, but the differences were minor compared with the effects of kairomones themselves. The results of our systematic analyses point to the role of prey phylogeny in eliciting inducible defenses of D. mitsukuri, thereby enriching our understanding of nonconsumptive effects in aquatic ecosystems.

Prey are under selection to minimize predation losses. In aquatic environments, many prey use chemical cues released by predators, which initiate predator avoidance. A prominent example of behavioral predator-avoidance constitutes diel vertical migration (DVM) in the freshwater microcrustacean Daphnia spp., which is induced by chemical cues (kairomones) released by planktivorous fish. In a bioassay-guided approach using liquid chromatography and mass spectrometry, we identified the kairomone from fish incubation water as 5α-cyprinol sulfate inducing DVM in Daphnia at picomolar concentrations. The role of 5α-cyprinol sulfate in lipid digestion in fish explains why from an evolutionary perspective fish has not stopped releasing 5α-cyprinol sulfate despite the disadvantages for the releaser. The identification of the DVM-inducing kairomone enables investigating its spatial and temporal distribution and the underlying molecular mechanism of its perception. Furthermore, it allows to test if fish-mediated inducible defenses in other aquatic invertebrates are triggered by the same compound.

The southern pine beetle, Dendroctonus frontalis Zimmermann is an important mortality agent of Pinus in the eastern United States of America where it commonly shares hosts with the black turpentine beetle, Dendroctonus terebrans (Olivier), which infrequently kills trees. Unlike D. frontalis, which must kill its hosts to become established in the bark and reproduce, D. terebrans can occupy living hosts as a parasite. Olfactory mechanisms whereby D. frontalis initially locates hosts have not been demonstrated, whereas D. terebrans responds strongly to host odors. Because D. terebrans produces frontalin, the primary aggregation pheromone component for D. frontalis, and commonly arrives on hosts prior to D. frontalis, it has been hypothesized that D. terebrans pheromone components can mediate D. frontalis location of suitable, living trees. We assessed this possibility with studies of the semiochemical interactions between D. frontalis and D. terebrans. Coupled gas chromatography-electroantennographic detection analyses indicated that D. terebrans produces nine different olfactory stimulants for D. frontalis, nearly all of them known semiochemicals for D. frontalis. A trapping experiment designed to address the potentially confounding influence of lure contamination confirmed that the D. terebrans pheromone component exo-brevicomin enhances attraction of D. frontalis and thus could be an attractive kairomone. In ambulatory bioassays, male D. frontalis were strongly attracted to odors of frass of solitary female and paired D. terebrans, indicating their attraction to the naturally occurring semiochemicals of D. terebrans. Cues from D. terebrans may influence host and mate-finding success of D. frontalis and, thereby, the latter's virulence.

In 2009, we determined the effects of the enantiomeric composition of the kairomone, α-pinene, on trap catches of arboreal beetles (Coleoptera) in stands of eastern pine trees with resin dominated by (+)-α-pinene. We hypothesized that the responses of beetles would correlate with the predominant enantiomer of α-pinene found in host pines. Lures of (+)-, racemic (±), and (–)-α-pinene were added separately to ethanol-baited multiple-funnel traps. Species such as Monarthrum mali (Fitch), Dendroctonus terebrans (Olivier), Ips grandicollis (Eichhoff), and Pachylobius picivorus (Germar) (Coleoptera: Curculionidae) showed a preference for traps co-baited with (–)-α-pinene. α-Pinene enhanced attraction of Hylastes salebrosus Eichhoff, Hylastes porculus Erickson and Hylastes tenuis Eichhoff (Coleoptera: Curculionidae) to ethanol-baited traps with no effects from enantiomeric composition of α-pinene. The attraction of the ambrosia beetles, Xyleborinus saxesenii (Ratzeburg) and Dryoxylon onoharaense (Murayama) (Coleoptera: Curculionidae) to ethanol-baited traps was interrupted by the addition of α-pinene, regardless of enantiomeric composition. Species such as Xylosandrus germanus (Blandford), Cnestus mutilatus (Blandford) and Stenoscelis brevis (Boheman) (Coleoptera: Curculionidae) were unaffected by the presence of α-pinene. Trap catches of some species of longhorn beetles and bark beetle predators (Coleoptera: Cerambycidae, Cleridae, Elateridae, Histeridae, and Trogossitidae) were increased by the addition of α-pinene, although results varied by location. Platysoma spp. (Coleoptera: Histeridae) showed a marked preference for traps co-baited with (+)-α-pinene in Florida and Georgia. In summary, we found that the enantiomeric composition of α-pinene in hosts was not a good predictor of enantiomeric preferences by beetles.

To test the hypothesis that non-consumptive predation pressures favor genotypes that have higher equilibrium densities or mictic ratios, we expose replicate populations of the rotifer Brachionus angularis to low (LK) and high (HK) concentrations of predator Asplanchna kairomones. Over 75 days (25 asexual generations) of selection, LK-treated populations have higher equilibrium densities and lower mictic ratios than the control populations. The opposite occurs for HK-treated populations. These results suggest that compared with the control, LK and HK select for genotypes with higher equilibrium densities and mictic ratios, respectively. Common garden bioassays reveal that in the absence of Asplanchna kairomones, populations with an LK selection history have a higher average intrinsic rate of population increase, and those with an HK selection history have a higher average mictic ratio than those with the control selection history, which suggest that adaptations to LK and HK increase average intrinsic rate of population increase and mictic ratio of prey populations, respectively. Increased intrinsic rate of population increase and mictic ratio can serve as strategies of offsetting mortality from predation and escaping the predator, and thus enable prey to survive low and high predation pressures, respectively.

The shy-bold behavioral continuum is an element of animal behavior which is often studied for its ecological relevance, particularly in the context of predation risk. How individuals respond to various predation cues is well studied at the individual level, but relatively little is known about how these responses can differ among closely related species. We exposed individual wild-caught juvenile bluegill (Lepomis macrochirus) and pumpkinseed (L. gibbosus) to kairomones of a common predator (Northern pike, Esox lucius), conspecific alarm cues, or a lake water control in a Z-maze trial commonly used to assess relative levels of bold and exploratory behaviors. Neither species exhibited any significant behavioral responses to either predation cue, but bluegill consistently displayed more bold-type behaviors than pumpkinseed. Although the lack of a behavioral response to predation cues in this study is equivocal, we identify clear differences in boldness between these two congeners and discuss the possibility of ecological niche differentiation driving these behavioral differences.Significance StatementClosely related species exposed to similar predator guilds may differ in their behavioral responses to predation risk, particularly during different life-history stages. We exposed juveniles of two co-occurring sunfish congeners (Lepomis spp.) to chemical predation cues (predator kairomones or conspecific alarm cues) in a Z-maze assay to assess relative levels of bold and exploratory behaviors. Bluegill (L. macrochirus) were consistently bolder and more proactive than pumpkinseed (L. gibbosus). Ontogenetic ecological niche differentiation between these species may be the cause of these observed differences and life-history trajectories should be considered in the design of future research questions.

Natural enemies locate their herbivorous host and prey through kairomones emitted by host plants and herbivores. These kairomones could be exploited to attract and retain natural enemies in crop fields for insect pest control. The parasitoid Encarsia formosa preferentially parasitises its whitefly host, Trialeurodes vaporariorum, a major pest of tomato Solanum lycopersicum, thus offering an effective way to improve whitefly control. However, little is known about the chemical interactions that occur in E. formosa-T. vaporariorum-S. lycopersicum tritrophic system. Using behavioural assays and chemical analyses, we investigated the kairomones mediating attraction of the parasitoid to host-infested tomato plants. In Y-tube olfactometer bioassays, unlike volatiles of healthy tomato plants, those of T. vaporariorum-infested tomato plants attracted E. formosa, and this response varied with host infestation density. Coupled gas chromatography/mass spectrometric analyses revealed that host infestation densities induced varying qualitative and quantitative differences in volatile compositions between healthy and T. vaporariorum adult-infested tomato plants. Bioassays using synthetic chemicals revealed the attractiveness of 3-carene, β-ocimene, β-myrcene and α-phellandrene to the parasitoid, and the blend of the four compounds elicited the greatest attraction. Our results suggest that these terpenes could be used as an attractant lure to recruit the parasitoid E. formosa for the control of whiteflies in tomato crop fields.

Insect-derived volatiles seem to provide reliable chemical cues that plants could employ to defend themselves. Here we investigated the effect of pheromone emission from a closely associated (Anthonomus grandis; boll weevil) and an unassociated (Tibraca limbativentris) herbivore on cotton volatile emission. Exposure to A. grandis aggregation pheromone induced cotton defence response by enhancing the emission of volatiles attractive to the natural enemy of A. grandis, the parasitic wasp Bracon vulgaris, but only when the pheromonal blend was complete (all four components). Individual components of A. grandis aggregation pheromone were not able to induce cotton plants to increase the release of volatiles. On the other hand, T. limbativentris sex pheromone did not induce any change in the cotton constitutive volatile profile. Our results support the hypothesis that plants are able to detect pheromones of tightly co-evolved herbivores. Moreover, A. grandis pheromone exposure induced similar volatile compounds to herbivore-induced cotton, such as linalool, (E)-ocimene, (E)-4,8-dimethylnona-1,3,7-triene (DMNT), and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT). We also showed that the larval ectoparasitoid B. vulgaris relies on boll weevil’s aggregation pheromone and pheromone-induced plant volatiles as kairomones to locate suitable hosts.