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Effects of herbivory amount and within‐plant variability by a specialist caterpillar on volatile emissions mediating inter‐plant signalling in wild cotton

Lucía Martín‐Cacheda,Yeyson Briones‐May,C. Bustos‐Segura,Ted C. J. Turlings,V. H. Ramírez‐Delgado,L. Abdala‐Roberts

Published 2025 in Ecological Entomology

ABSTRACT

Plant–plant signalling via volatile organic compounds (VOCs) has been widely documented, but its contingency on herbivory amount and its within‐plant distribution variability (i.e., how dispersed vs. aggregated it is throughout the canopy) remains poorly understood. Here we tease apart these factors by investigating whether signalling between wild cotton (Gossypium hirsutum) plants in response to herbivory by the specialist caterpillar Alabama argillacea was affected by different levels of herbivore density (proxy for herbivory intensity or amount) and patterns of within‐plant herbivore distribution (proxy of within‐plant damage variability). We conducted a greenhouse experiment for which we placed plant triplets in mesh cages, where in each case one plant acted as a VOC emitter and the other two as receivers. We subjected emitters to one of the following treatments: control (no larvae), low density (one larva on one leaf), high concentrated density (four larvae, two on each of two leaves) and high dispersed density (four larvae, one on each of four leaves). After 2 days of herbivory, we collected emitter VOCs to test for treatment effects on volatile emissions and placed A. argillacea larvae on receivers and measured the amount of damage to test for signalling effects on leaf resistance. Both high caterpillar density treatments similarly increased total VOC emissions, although the difference relative to controls was only significant under high concentrated density. Low density had virtually the same amount of emissions as controls. In addition, high dispersed density was the only treatment that caused a significant change in VOC composition relative to controls. In turn, our test of signalling effects indicated that only receivers exposed to highly dispersed density emitters showed a significantly lower amount of herbivory (i.e., greater resistance) than controls. These results suggest that the amount of herbivory primarily drives quantitative changes in VOC emissions, whereas within‐plant variability mainly causes qualitative changes. Further, the impact of these VOC induction patterns on inter‐plant signalling appears to be most pronounced when within‐plant herbivory variability is high.

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