Prey and non-prey arthropods sharing a host plant: effects on induced volatile emission and predator attraction

Abstract

It is well established that plants infested with a single herbivore species can attract specific natural enemies through the emission of herbivore-induced volatiles. However, it is less clear what happens when plants are simultaneously attacked by more than one species. We analyzed volatile emissions of lima bean and cucumber plants upon multi-species herbivory by spider mites (Tetranychus urticae) and caterpillars (Spodoptera exigua) in comparison to single-species herbivory. Upon herbivory by single or multiple species, lima bean and cucumber plants emitted volatile blends that comprised mostly the same compounds. To detect additive, synergistic, or antagonistic effects, we compared the multi-species herbivory volatile blend with the sum of the volatile blends induced by each of the herbivore species feeding alone. In lima bean, the majority of compounds were more strongly induced by multi-species herbivory than expected based on the sum of volatile emissions by each of the herbivores separately, potentially caused by synergistic effects. In contrast, in cucumber, two compounds were suppressed by multi-species herbivory, suggesting the potential for antagonistic effects. We also studied the behavioral responses of the predatory mite Phytoseiulus persimilis, a specialized natural enemy of spider mites. Olfactometer experiments showed that P. persimilis preferred volatiles induced by multi-species herbivory to volatiles induced by S. exigua alone or by prey mites alone. We conclude that both lima bean and cucumber plants effectively attract predatory mites upon multi-species herbivory, but the underlying mechanisms appear different between these species.

Fig. 1

Effect of multi-species herbivory on volatile emission by lima bean plants. (a) Volatile emission (mean + SE peak area units) upon single herbivory by Tetranychus urticae (20 mites per leaf) (open bars), single herbivory by Spodoptera exigua (two larvae per leaf) (filled bars), or multi-species herbivory (hatched bars). Asterisks indicate compounds that were significantly affected by herbivore treatment (P < 0.05, Kruskal–Wallis tests). (b) Ratio of emission rates upon multi-species herbivory to the sum per volatile emitted by the two single herbivore treatments (see “Methods and Materials” for detailed explanation). Symbols indicate mean ratio, and error bars indicate upper and lower 95% confidence limits. Asterisks indicate compounds that were significantly more strongly induced by multi-species herbivory than by the sum of T. urticae and S. exigua (lower 95% confidence limit larger than 0). N = 4 for all treatments. Compound numbers: (1) (Z)-3-hexen-1-ol, (2) methyl salicylate, (3) (E)-β-ocimene, (4) (E)-4,8-dimethyl-1,3,7-nonatriene, (5) (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene, (6) p-mentha-1,3,8-triene, (7) (Z)-3-hexen-1-ol acetate, (8) 2-methylbutanal-O-methyl oxime, (9) 3-methylbutanal-O-methyl oxime, (10) linalool, (11) 1-octen-3-ol, (12) hexyl acetate, (13) limonene, (14) β-caryophyllene, (15) nonanal, (16) indole, (17) (E)-2-hexen-1-ol acetate, (18) 3-pentanone, (19) 3-octanone, (20) 2-methylbutanal nitrile, (21) (Z)-β-ocimene, (22) rose furan, (23) unknown 95B, 150, (24) 3-methylbutanal nitrile, (25) unknown 91B, 148, (26) unknown 41, 69B, 164, (27) 2-methylpropanal-O-methyl oxime

Fig. 2

Effect of multi-species herbivory on volatile emission by cucumber plants. (a) Volatile emission (mean ± SE peak area units) upon single herbivory by Tetranychus urticae (100 mites per leaf, N = 5, open bars), single herbivory by Spodoptera exigua (2 larvae per leaf, N = 4, filled bars), or multi-species herbivory (N = 4, hatched bars). Asterisks indicate compounds that are significantly affected by herbivore treatment (P < 0.05, Kruskal–Wallis tests). (b) Ratio of emission rates upon multi-species herbivory to the sum per volatile emitted by the two single herbivore treatments (see “Methods and Materials” and for detailed explanation). Symbols indicate mean ratio, and error bars indicate upper and lower 95% confidence limits. Asterisks indicate compounds that were significantly less induced by multi-species herbivory than by the sum of T. urticae and S. exigua (upper 95% confidence limit smaller than 0) (N = 4) Compound numbers: (1) (E)-2-hexenal + (Z)-3-hexanal, (2) (Z)-3-hexen-1-ol acetate, (3) indole, (4) (E,E)-α-farnesene, (5) butyl aldoxime, (6) 3-methylbutanal-O-methyl oxime, (7) (E)-4,8-dimethyl-1,3,7-nonatriene, (8) (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene, (9) (E)-β-ocimene. Note that peak area units in Fig. 2a cannot be compared with Fig. 1a because measurements were done on a different GC-MS system

Fig. 3

Responses of P. persimilis to volatile blends in the Y-tube olfactometer. Odor sources consisted of four leaves from lima bean (a and b) or cucumber plant (c and d) simultaneously infested by Tetranychus urticae and Spodoptera exigua or infested by one of the herbivore species alone (S. exigua: a and c, T. urticae: b and d). Bars present the overall percentages of predators choosing for each odor source. Numbers in bars are the total numbers of predators responding to each odor source. Choices between odor sources were analyzed with a two-sided binomial test (*P < 0.05; ***P < 0.001)