The Dismissal

Nicotiana attenuata — coyote tobacco — grows across the western basins of North America, from British Columbia to northern Mexico. Its flowers open at dusk. The corolla tubes, two to three centimeters long, release benzyl acetone into the night air, and the hawkmoth Manduca sexta arrives to drink nectar and, in the process, pollinate. This is an ancient arrangement. The flower's shape fits the moth's proboscis. The timing fits the moth's activity cycle. The volatile fits the moth's olfactory receptors. Everything about the flower is designed to attract this specific visitor.

The moth's larvae eat the plant.

Manduca sexta — the tobacco hornworm — uses Nicotiana attenuata as a host. The female moth lays eggs on the leaves she visits to drink. The larvae hatch and consume the foliage, each caterpillar capable of defoliating a significant portion of the plant. The pollinator and the herbivore are not two unrelated problems. They are parent and child. The moth that carries pollen in is the moth that deposits the eggs that will eat the plant alive.

The plant's first line of defense is chemical. When hornworm larvae chew through the trichomes on the leaf surface, the plant produces trypsin protease inhibitors — proteins that interfere with the caterpillar's digestion, slowing its growth and reducing the damage per unit time. This is a direct defense: attack the attacker. But it doesn't sever the relationship. The moth still visits. The larvae still hatch.

The second line is recruitment. When larval saliva contacts the damaged leaf tissue, the plant emits green leaf volatiles — a specific blend of six-carbon aldehydes and alcohols released through the jasmonic acid signaling pathway. These volatiles undergo a conformational change on contact with the hornworm's saliva, producing a modified signal that attracts Geocoris bugs — big-eyed bugs that prey on hornworm eggs and early-instar larvae. The plant cannot kill the caterpillar. So it calls something that can. This is an indirect defense: recruit a third party. But it still doesn't sever the relationship. The moth still visits. Some larvae survive.

The third line is dismissal. Herbivory triggers a change not in the plant's defenses but in its reproductive strategy. The jasmonic acid cascade — the same signaling pathway that triggers protease inhibitors and volatile emissions — also reduces benzyl acetone production and shifts corolla opening from dusk to dawn. The flowers that were designed to attract the night-active hawkmoth now open in the morning, when the moth is inactive. Day-active hummingbirds arrive instead. The hummingbirds pollinate. They do not lay eggs. They do not eat the leaves.

In field trials with a native Utah population, the switch was measurable within eight days. Plants under hornworm attack shifted significantly toward morning-opening flowers. Remove the herbivory and the plant reverts to dusk opening. But as long as the larvae are present, the plant replaces one pollinator with another.

Dismissal is expensive. Hummingbird pollination is less efficient than hawkmoth pollination for this flower morphology — the tube shape evolved for the moth's proboscis, not the bird's bill. The plant pays in reduced pollination efficiency for the elimination of herbivory. The switch only makes sense when the cost of the larvae exceeds the cost of the inferior pollinator. The plant does not dismiss the moth preemptively. It dismisses the moth when the evidence — caterpillar saliva, jasmonic acid levels, tissue damage — exceeds a threshold.

What makes the case remarkable is not any individual defense but the fact that all three run through the same signaling pathway. Jasmonic acid triggers protease inhibitors, volatile emissions, and the flowering-time shift. The plant does not have three separate detection systems for three separate threats. It has one detection system — herbivore damage — connected to three responses calibrated to the severity of the signal. Low damage produces inhibitors. Higher damage recruits predators. Sustained damage fires the pollinator. The threshold structure is the strategy: the plant escalates through absorb, manage, and dismiss, and the escalation is encoded in the dose-response curve of a single hormone.

The moth has no counter-move. It cannot evolve to visit at dawn, because its entire sensory apparatus and flight physiology are adapted to nocturnal activity. It cannot evolve to suppress the jasmonic acid signal, because the signal is triggered by tissue damage, not by any moth-specific compound. The plant's dismissal exploits a constraint the moth cannot overcome: the moth is nocturnal, and the plant can choose when to open. The asymmetry is architectural. The moth is locked into its niche. The plant is not locked into its pollinator.