Hyperparasitoids use herbivore-induced plant volatiles to locate their parasitoid host

Abstract

Plants respond to herbivory with the emission of induced plant volatiles. These volatiles may attract parasitic wasps (parasitoids) that attack the herbivores. Although in this sense the emission of volatiles has been hypothesized to be beneficial to the plant, it is still debated whether this is also the case under natural conditions because other organisms such as herbivores also respond to the emitted volatiles. One important group of organisms, the enemies of parasitoids, hyperparasitoids, has not been included in this debate because little is known about their foraging behaviour. Here, we address whether hyperparasitoids use herbivore-induced plant volatiles to locate their host. We show that hyperparasitoids find their victims through herbivore-induced plant volatiles emitted in response to attack by caterpillars that in turn had been parasitized by primary parasitoids. Moreover, only one of two species of parasitoids affected herbivore-induced plant volatiles resulting in the attraction of more hyperparasitoids than volatiles from plants damaged by healthy caterpillars. This resulted in higher levels of hyperparasitism of the parasitoid that indirectly gave away its presence through its effect on plant odours induced by its caterpillar host. Here, we provide evidence for a role of compounds in the oral secretion of parasitized caterpillars that induce these changes in plant volatile emission. Our results demonstrate that the effects of herbivore-induced plant volatiles should be placed in a community-wide perspective that includes species in the fourth trophic level to improve our understanding of the ecological functions of volatile release by plants. Furthermore, these findings suggest that the impact of species in the fourth trophic level should also be considered when developing Integrated Pest Management strategies aimed at optimizing the control of insect pests using parasitoids.

Figure 1. Experimental study system of the four-trophic-level community on Brassica oleracea plants.

The gregarious primary parasitoid Cotesia glomerata (CG) and the solitary C. rubecula (CR) attack caterpillars of Pieris (PR) butterflies, which are in turn attacked by several hyperparasitoids: Acrolyta nens (1), Lysibia nana (2), Pteromalus semotus (3), Mesochorus gemellus (4), and Baryscapus galactopus (5). Hyperparasitoids at the fourth trophic level find their primary parasitoid host at the third trophic level via information derived from the plant at the first trophic level. Larvae of primary parasitoids that develop in their herbivorous host at the second trophic level inflict changes in their herbivore host, and the combination of herbivore and parasitoid (parasitized herbivores) inflict changes in plant volatile emission (I). These changes in plant volatile emission are used by hyperparasitoids as a cue of host presence (II). Photograph credit: Tibor Bukovinszky.

Figure 2. Performance of Lysibia nana on pupae of two parasitoid species.

Lysibia nana dry mass plotted against the mass of the Cotesia cocoon before L. nana had parasitized the cocoon. Orange symbols represent wasps emerging from C. glomerata cocoons, and black symbols those emerging from C. rubecula cocoons. Females are represented by dots, and males by triangles. Photograph credit: Tibor Bukovinszky.

Figure 3. Lysibia nana responses in choice tests with primary parasitoid cocoons.

Lysibia nana preference (top bar) for gregarious broods of Cotesia glomerata (grey) or solitary cocoons of C. rubecula (white) in a Petri dish bioassay. Lysibia nana preference (lower bar) for gregarious broods of Cotesia glomerata (grey) or the same number of cocoons of C. rubecula (white). Numbers between brackets indicate the fraction of wasps that responded to cocoons within 10 min from the start of the experiment. * p<0.05, ** p<0.001. Photograph credit: Tibor Bukovinszky.

Figure 4. Preference of hyperparasitoids for herbivore-induced plant volatiles.

Preference of the hyperparasitoid Lysibia nana for herbivore-induced plant volatiles was tested by using a full factorial design of two-choice olfactometer tests including pair-wise comparisons of the treatments: undamaged plants (white bars), Pieris rapae damaged plants (light grey), plants damaged by Pieris rapae caterpillars parasitized by Cotesia glomerata (dark grey bars), or plants damaged by Pieris rapae caterpillars parasitized by C. rubecula (black bars). The two lowest pairs of bars show the preference of L. nana for plants treated with caterpillar regurgitant. The first pair shows hyperparasitoid preference when plants are artificially damaged and regurgitant of unparasitized (light grey) or parasitized (dark grey) caterpillars was applied. The second and lowest pair shows that hyperparasitoids do not respond to the application of regurgitant without artificially damaging the plant. Numbers between brackets indicate the number of wasps that made a choice within 10 min from the start of the experiment versus the total number of wasps tested. * p<0.05, ** p<0.001. Photograph credit: Tibor Bukovinszky.

Figure 5. PLS-DA plot based on comparisons among volatile blends of Brassica oleracea plants under herbivory by parasitized or unparasitized caterpillars.

Plants were either undamaged (red, UD), damaged with two unparasitized Pieris rapae caterpillars (blue, PR), or P. rapae caterpillars parasitized by Cotesia rubecula (orange, PR-CR) or C. glomerata (green, PR-CG). Photograph credit: Tibor Bukovinszky.

Figure 6. Herbivore-induced plant volatiles mediate hyperparasitism in the field.

Percentage of Cotesia glomerata (CG, left) and C. rubecula (CR, right) cocoon clutches hyperparasitized on plants that had been induced with herbivory by unparasitized or parasitized caterpillars of P. rapae. Pieris rapae (PR), P. rapae parasitized by C. glomerata (PR-CG), P. rapae parasitized by C. rubecula (PR-CR), and undamaged (UD). Letters indicate significant differences between treatment groups (GLM, p<0.05). Photograph credit: Tibor Bukovinszky.

Figure 7. Relative abundance of primary parasitoids and hyperparasitoids from primary parasitoid cocoons in the field.

Primary and hyperparasitoid wasps were reared from cocoons of the gregarious Cotesia glomerata (left) and solitary C. rubecula (right) that had been collected from Brassica oleracea during three field seasons. Colors indicate the different parasitoid species; the white segment of the bars depicts the primary parasitoids (a) C. glomerata and (b) C. rubecula; colored bar segments represent the most abundant hyperparasitoids: Acrolyta nens (1, blue bar), Lysibia nana (2, orange bar), Pteromalus semotus (3, yellow bar), Mesochorus gemellus (4, red bar), and Baryscapus galactopus (5, green bar). Photograph credit: Tibor Bukovinszky.