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Unidirectional grass hairs usher insects away from meristems

Abstract

Grasses are major agricultural products worldwide and they are critical to ecosystem function in many terrestrial habitats. Despite their global importance, we know relatively little about their defenses against herbivory. Grasses tend to be tolerant of leaf loss because their valuable meristems are located underground, out of reach for above ground herbivores. Many grasses have unidirectional leaf hairs, prickles, and spines that make moving up the leaf blade easy, but make moving down, toward the meristem, difficult. We tested the hypothesis that unidirectional grass hairs direct small arthropod herbivores away from the meristems. In a field survey of the distribution of herbivore damage, we found that leaf tips received five times more damage than leaf bases for Avena barbata. Early-instar grasshoppers fed three times as often on leaf tops as on leaf bases of pubescent individuals in a common garden laboratory experiment. This effect was not observed for glabrous individuals where grasshoppers damaged leaf bases as often as leaf tops. A common generalist caterpillar, Heliothus virescens, was more than twice as likely to turn in the direction of the hairs, away from the meristems, when it encountered pubescent leaves of A. barbata. However, larger caterpillars of the generalist feeder Arctia virginalis showed no directional bias when they encountered pubescent leaves. In common garden experiments, selection on pubescence was weak and inconsistent over space and time. Under some circumstances, individuals of A. barbata with pubescent leaves were more likely to produce seeds than were individuals with fewer hairs. The surveys, behavioral experiments with small insects, and estimates of lifetime reproduction all support the hypothesis that unidirectional leaf hairs on A. barbata, and perhaps other grasses, serve as an unstudied defense that direct small herbivores away from the meristems.

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Acknowledgements

We thank Mikaela Huntzinger, Claire Karban, Jesse Karban, Katherine Toll, and Phil Ward for help with fieldwork. Ellen Dean identified the grasses. Mikaela Huntzinger and an anonymous reviewer improved the manuscript. The field studies were conducted at the UC McLaughlin Reserve, Hopland Research and Extension Center and Sierra Foothills Research and Extension Center, and we thank the staff for facilitating our work. We were supported by USDA multistate Grants NC-7 and NE-1501.

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Authors

Contributions

RK, EL, GV, and RL conceived the project. RK, EL, and RL designed the experiments. RK and RL performed the experiments. RK, EL, and RL analyzed the data. RK and RL wrote the manuscript, and EL and GV provided editorial advice.

Corresponding author

Correspondence to Richard Karban.

Additional information

Communicated by Colin Mark Orians.

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Karban, R., LoPresti, E., Vermeij, G.J. et al. Unidirectional grass hairs usher insects away from meristems. Oecologia 189, 711–718 (2019). https://doi.org/10.1007/s00442-019-04355-7

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Keywords

  • Defense
  • Herbivory
  • Leaf hairs
  • Movement
  • Poaceae
  • Pubescence
  • Tolerance