Selection by parasitoid females among closely related hosts based on volatiles: Identifying relevant chemical cues

Abstract

Parasitoid fitness is influenced by the ability to overcome host defense strategies and by the ability of parasitoid females to select high-quality host individuals. When females are unable to differentiate among hosts, their fitness will decrease with an increasing abundance of resistant hosts. To understand the effect of mixed host populations on female fitness, it is therefore necessary to investigate the ability of female parasitoids to select among hosts. Here, we used behavioral assays, headspace volatile collection, and electrophysiology to study the ability of Asecodes parviclava to use olfactory cues to select between a susceptible host (Galerucella calmariensis) and a resistant host (Galerucella pusilla) from a distance. Our studies show that parasitoid females have the capacity to distinguish the two hosts and that the selection behavior is acquired through experiences during earlier life stages. Further, we identified two volatiles (α-terpinolene and [E]-β-ocimene) which amounts differ between the two plant-herbivore systems and that caused behavioral and electrophysiological responses. The consequence of this selection behavior is that females have the capacity to avoid laying eggs in G. pusilla, where the egg mortality is higher due to much stronger immune responses toward A. parviclava than in larvae of G. calmariensis.

Keywords: Asecodes parviclava; electrophysiology; headspace volatile collection; host–parasitoid system; olfactometer.

Figure 1

(a) Mummified larva of Galerucella calmariensis (Coleoptera: Chrysomelidae), showing pupae of the parasitic wasp Asecodes parviclava (Hymenoptera: Eulophidae) inside. (b) Adult female of A. parviclava. Scale bars: 1 mm

Figure 2

General setup of the Y‐tube glass olfactometers used to test the behavioral responses in Asecodes parviclava to different volatiles

Figure 3

(a) Behavioral responses in Asecodes parviclava females originating from Galerucella calmariensis and Galerucella tenella respectively to G. calmariensis vs Galerucella pusilla larvae feeding on Lythrum salicaria. (b) Behavioral responses in A. parviclava originating from G. calmariensis to host plant (L. salicaria) odors alone vs a blend of host plant odors and synthetic chemical compounds

Figure 4

Selected ion chromatogram showing compounds released from Lythrum salicaria with feeding larvae of either Galerucella calmariensis or Galerucella pusilla. Compounds: (1) (Z)‐β‐ocimene, (2) γ‐terpinene, (3) (E)‐β‐ocimene, (4) p‐cymene, (5) α‐terpinolene, (6) (3E)‐4,8‐dimethyl‐1,3,7‐nonatriene (DMNT), (7) (Z)‐3‐hexenyl acetate, (8) (Z)‐3‐hexenol, (9) hexyl 2‐methyl‐butanoate, (10) (E)‐2‐hexenyl butanoate, (11) (Z)‐3‐hexenyl 3‐methyl‐butanoate, (12) (E)‐2‐hexenyl 3‐methyl‐butanoate, (13) β‐elemene, (14) caryophyllene, (15) (E)‐β‐farnesene, (16) unidentified sesquiterpene, (17) (Z,E)‐α‐farnesene, (18) (E,E)‐α‐farnesene, (19) methyl salicylate, (20) (3E,7E)‐4,8,12‐trimethyl‐1,3,7,11‐tridecatetraene (TMTT), (21) unidentified sesquiterpene, (22) benzeneethanol, (23) caryophyllene oxide, (24) (E)‐nerolidol, (25) hexyl benzoate, (26) (Z)‐3‐hexenyl benzoate, (27) eugenol, (IS) internal standard, (*) compound from a blank sample

Figure 5

EAG response of Asecodes parviclava (mean ± SE, n = 14) to volatile compounds that were (a) differentially released by the two larval species when feeding on the host plant, (b) released at equal rates by the two larval species, and (c) not detected in the headspace collections (serving as controls). The rightmost bar (and the dotted line) corresponds to the response of control antennae not exposed to any volatile. Black bars indicate responses that significantly differed from the control at p < .01