When cells within a tobacco leaf are touched by an external force, stretch receptors cause them to release calcium ions. The signal spreads into adjacent cells in a propagating wave triggering the release of more calcium ions and the subsequent production of a chemical called GABA. Tobacco leaf parasites that ingest GABA suffer tremors, excitablilty, paralysis, and death.
"Bristles and crotchets on tarsi of insect larvae crawling on a leaf may inflict minor scratches on a plant that lead to rapid synthesis of γ-aminobutyric acid (GABA)...Very high levels of dietary GABA may have detrimental effects on growth and survival rates of insect larvae." (Hilker 2010:268)
"Since touch can be sensed by a plant, then walking of herbivorous arthropods on a plant when searching for mates or an oviposition site may inform the plant of a future danger of feeding damage. It is possible that touch could prime the plant to get 'ready for the battle.' (Hilker 2010:273)
"...disruption of GABA signaling in insect larvae causes physiological and developmental damage. Blockage of the peripheral neuronal GABA-gated Cl- channels in invertebrates leads to tremors, hyperexcitability, paralysis and death." (Bown 2006:424)
"Within 5–10 min of tobacco budworm (Heliothis virescens) larvae crawling on plant-attached tobacco (Nicotiana tabacum) leaves the GABA levels increase four- to fivefold. Similarly, crawling of oblique-banded leafroller larvae on soybean leaves raises GABA levels 10- to 12-fold within 10 min. Larval footsteps on the leaf surface can be visualized through superoxide production or transient increases in chlorophyll synthesis" (Bown 2006:425)
"Touch stimulation elicited a relatively prolonged (2-5 min) transient increase in [Ca2+ ] at the cellular level...The touch stimulus led to elevated [Ca2+] that moved away from the site of the stimulus in what may be a propagating wave of Ca2+." (Legue 1997:795)