Abstract
BackgroundInsect predators and parasitoids exploit attractive chemical signals from lower trophic levels as kairomones to locate their herbivore prey and hosts. We hypothesized that specific chemical cues from prey non-hosts and non-habitats, which are not part of the trophic chain, are also recognized by predators and would inhibit attraction to the host/prey kairomone signals. To test our hypothesis, we studied the olfactory physiology and behavior of a predaceous beetle, Thanasimus formicarius (L.) (Coleoptera: Cleridae), in relation to specific angiosperm plant volatiles, which are non-host volatiles (NHV) for its conifer-feeding bark beetle prey.Methodology/Principal FindingsOlfactory detection in the clerid was confirmed by gas chromatography coupled to electroantennographic detection (GC-EAD) for a subset of NHV components. Among NHV, we identified two strongly antennally active molecules, 3-octanol and 1-octen-3-ol. We tested the potential inhibition of the combination of these two NHV on the walking and flight responses of the clerid to known kairomonal attractants such as synthetic mixtures of bark beetle (Ips spp.) aggregation pheromone components (cis-verbenol, ipsdienol, and E-myrcenol) combined with conifer (Picea and Pinus spp.) monoterpenes (α-pinene, terpinolene, and Δ3-carene). There was a strong inhibitory effect, both in the laboratory (effect size d = −3.2, walking bioassay) and in the field (d = −1.0, flight trapping). This is the first report of combining antennal detection (GC-EAD) and behavioral responses to identify semiochemical molecules that bypass the trophic system, signaling habitat information rather than food related information.Conclusions/SignificanceOur results, along with recent reports on hymenopteran parasitoids and coleopteran predators, suggest that some NHV chemicals for herbivores are part of specific behavioral signals for the higher trophic level and not part of a background noise. Such bypass-trophic signals could be of general importance for third trophic level players in avoiding unsuitable habitats with non-host plants of their prey.
Highlights
Insect predators and parasitoids exploit a variety of chemical signals from different trophic levels as kairomones and synomones to locate their herbivorous prey and hosts in tri-trophic systems [1,2,3,4]
Antennal responses The antennae of T. formicarius gave consistent and strong responses, to the common bark beetle pheromone components, cis-verbenol, trans-verbenol, and verbenone (Figure 2A), and to two volatiles from trees not exploited by the prey of the clerids (i.e. non-host volatiles (NHV) for the prey)
Our electrophysiological and behavioral data show that the checkered beetle can detect olfactory signals directly from the trophic chain, e.g., kairomonal compounds such as prey pheromone components or host plant volatiles of its prey, and doi:10.1371/journal.pone.0011063.g003
Summary
Insect predators and parasitoids exploit a variety of chemical signals from different trophic levels as kairomones and synomones to locate their herbivorous prey and hosts in tri-trophic systems [1,2,3,4]. The Eurasian spruce engraver, Ips typographus (L.) (Coleoptera: Scolytidae), can recognize and avoid three specific alcohols from green leaves (1-hexanol, (Z)-3-hexen-1ol, and (E)-2-hexen-1-ol = GLV); two C8-alcohols (3-octanol and 1-octen-3-ol); and a spiroacetal trans-conophthorin from angiosperm bark [8,9,10]. Such specific olfactory recognition and inhibitory behavioral effects on attraction of angiosperm non-host volatiles (NHV) have been reported for several other conifer bark beetle genera in both Eurasia and in North America [7,11]. We studied the olfactory physiology and behavior of a predaceous beetle, Thanasimus formicarius (L.) (Coleoptera: Cleridae), in relation to specific angiosperm plant volatiles, which are non-host volatiles (NHV) for its conifer-feeding bark beetle prey
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