Abstract
Plant induced defenses may benefit plants by increasing cannibalism among insect herbivores. However, the general efficacy of plant defenses that promote cannibalism remains unclear. Using a generalist Lepidopteran herbivore (Helicoverpa zea), we examined whether plant induced defenses in Solanum lycopersicum increased cannibalism among H. zea and whether defense-mediated cannibalism benefits both the plant and the cannibal. In a separate experiment, we also examined whether defense-mediated cannibalism has effects on H. zea herbivory that are comparable to the effects of pathogenic virus of H. zea (HzSNPV) and whether defense-mediated cannibalism modified pathogen efficacy. We found that both plant defenses and cannibalism decreased herbivory: H. zea consumed less plant material if plants were induced, if dead conspecifics were provided, or both. Cannibalism increased cannibal growth rate: cannibals effectively overcome the costs of plant defenses by eating conspecifics. Inoculating half of H. zea with virus strongly reduced caterpillar survival. Cannibalism occurred sooner among virus-inoculated groups of H. zea, and all caterpillars in virus-inoculated treatments died before the end of the 7-day experiment. Although the rise in mortality caused by HzSNPV occurred more rapidly than the rise in mortality due to defense-mediated cannibalism, overall H. zea mortality at the end of the experiment was equal among virus-inoculated and induced-defense groups. Defense-mediated cannibalism and viral inoculation equally reduced herbivory on S. lycopersicum. Our results provide evidence that defense-mediated increases in cannibalism can be as effective as other forms of classic herbivore population regulation, and that both viral pathogens and defense-induced cannibalism can have significant benefits for plants.
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The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Acknowledgements
We are indebted to Dina St. Clair and Amy Groh (UCD Plant Biology) and Raoul Adamchak (UCD Student Farm) for providing access to tomato fields and to Jake Goidell for helping with setup and data collection. Portions of this work were conducted while J.O. was hosted by the Department of Biology at Virginia Commonwealth University (VCU). VCU Biology, the Johnson, Vonesh and Damschen laboratories kindly shared space and equipment for conducting experiments.
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This research was supported by grants from UW Fall Competition, UW OVCRGE, and a UW Vilas Mid-Career Fellowship to JLO.
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JO conceived the study; JO and RK designed the study; VP, JO, and RK collected the data; JO and PG analyzed the data; JO and PG drafted the manuscript; all authors contributed to revising the final manuscript.
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Communicated by Laramy Enders.
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Orrock, J.L., Guiden, P.W., Pan, V.S. et al. Plant induced defenses that promote cannibalism reduce herbivory as effectively as highly pathogenic herbivore pathogens. Oecologia 199, 397–405 (2022). https://doi.org/10.1007/s00442-022-05187-8
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DOI: https://doi.org/10.1007/s00442-022-05187-8