The predatory mite Phytoseiulus persimilis does not perceive odor mixtures as strictly elemental objects

Abstract

Phytoseiulus persimilis is a predatory mite that in absence of vision relies on the detection of herbivore-induced plant odors to locate its prey, the two-spotted spider-mite Tetranychus urticae. This herbivorous prey is feeding on leaves of a wide variety of plant species in different families. The predatory mites respond to numerous structurally different compounds. However, typical spider-mite induced plant compounds do not attract more predatory mites than plant compounds not associated with prey. Because the mites are sensitive to many compounds, components of odor mixtures may affect each other's perception. Although the response to pure compounds has been well documented, little is known how interactions among compounds affect the response to odor mixtures. We assessed the relation between the mites' responses elicited by simple mixtures of two compounds and by the single components of these mixtures. The preference for the mixture was compared to predictions under three conceptual models, each based on one of the following assumptions: (1) the responses elicited by each of the individual components can be added to each other; (2) they can be averaged; or (3) one response overshadows the other. The observed response differed significantly from the response predicted under the additive response, average response, and overshadowing response model in 52, 36, and 32% of the experimental tests, respectively. Moreover, the behavioral responses elicited by individual compounds and their binary mixtures were determined as a function of the odor concentration. The relative contribution of each component to the behavioral response elicited by the mixture varied with the odor concentration, even though the ratio of both compounds in the mixture was kept constant. Our experiments revealed that compounds that elicited no response had an effect on the response elicited by binary mixtures that they were part of. The results are not consistent with the hypothesis that P. persimilis perceives odor mixtures as a collection of strictly elemental objects. They suggest that odor mixtures rather are perceived as one synthetic whole.

Fig. 1

Behavioral responses by Phytoseiulus persimilis to various odors. Below each group of three bars is the reference number of the experiment (also present in Table 2). Depicted is, for each experiment, the difference between the observed attraction (N = 225 mites divided over 9 replicate choice tests) and predicted attraction under the averaging, the overshadowing and the additive response model respectively. A * indicates a significant difference (Χ ² _d.f. = 1 P < 0.05) between the observed response and the response under the model. The results are divided in three blocks representing experiments with both components of the mixture not eliciting a response, one component eliciting a response and both components eliciting a response, respectively

Fig. 2

The dose response relation of binary mixtures and their components. The experiments were conducted to assess if a function which describes the response of Phytoseiulus persimilis to a binary odor mixture as a function of the response to its components at one concentration also describes the response to the mixture at a different concentration. The y-axis represents the attraction of P. persimilis to the odor (N = 120 mites divided over 6 replicates = 100%), the x-axis represents the four odor concentrations in decreasing sequence (pure, 10×, 100× and 1000× –diluted in hexane). The lowest concentration was omitted from the analysis as the mites hardly responded to the mixture. Above each odor concentration is an abbreviation which qualitatively describes the response to the odor mixture as a function of the response to its components. Abbreviations: ovA/ovB: overshadowing by compound A or B, eq: equal to the mixture’s components, me: mean, sy: synergism, op: opposite of its components, at: a mixture of neutral compounds which is attractive, TMTT: (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene