COS 160-6 - Auditory predator cues reduce herbivore survival and plant damage

Background/Question/Methods

The high fitness costs of even unsuccessful predation attempts select for prey capable of detecting, avoiding, and deterring attack. The resulting changes in behavior and/or physiology can yield risk-induced trait responses (RITRs) in prey, and research has highlighted the importance of auditory cues in terrestrial predator-prey interactions. Work on this sensory modality has focused primarily on vertebrate taxa; with the notable exception of bat-insect interactions, we know little about how auditory predator cues affect invertebrates.
We conducted experiments assessing the effect of auditory predator cues on caterpillar survival in the lab and herbivory in the field. Larvae of the moth Spodoptera exigua are preyed upon by social wasps; we exposed S. exigua to recordings of predatory wasp buzzing, a ‘harmless’ sound of similar volume, and a no-sound control to remove confounding visual and/or chemical cues and separate predator-specific responses from that of sound per se. Over a three-week period, we assessed how these three treatments affected 1) larval survival and weight at/time to pupation in the lab; and 2) S. exigua herbivory on tomato plants in the field. At the end of the latter experiment, the plants were harvested to assess herbivore damage and plant biomass.

Results/Conclusions

When S. exigua larvae were reared in the lab, only 67% of larvae in the wasp buzzing treatment pupated, versus 84% and 91% in the mosquito buzzing and control treatment respectively. Among larvae that survived to pupation, there were no significant differences in either time to pupation or weight at pupation.
Auditory risk cues also decreased S. exigua herbivory and increased plant biomass in the field. Plants in the control treatment experienced 17% more herbivory (measured as % leaves with S. exigua damage) than plants in the mosquito buzzing treatment and 24% more than plants in the wasp buzzing treatment. The difference in herbivory rates was also reflected in aboveground biomass: plants in the control treatment were significantly (21%) lighter than wasp plants but did not differ from plants in the mosquito treatment. Although there were no treatment-level differences in belowground biomass, the differences in aboveground biomass were reflected in total plant biomass. Plants in the control treatment were 19% lighter than plants in the wasp treatment, a significant difference, but did not differ from plants in the mosquito treatment. Our results suggest that RITRs stemming from auditory predator cues may have cryptic but important impacts on terrestrial plant-herbivore interactions.