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How do plant secondary compounds discourage herbivores from feeding?

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A pine grosbeak with pink and gray plumage perches on a branch, eating red berries among autumn foliage. — Chemoreception Animals, such as this pine grosbeak, use chemoreception to respond to chemicals that can be tasted and smelled in their environments. Many of these chemicals affect behaviors such as food preference and defense.

Plant secondary compounds discourage herbivores from feeding primarily by acting as chemical deterrents that are detected through chemoreception. When an animal bites into a plant, these compounds—such as alkaloids, phenols, terpenoids, and tannins—are detected by sensory cells, often triggering aversive behavior.12

These compounds function in several ways:

Taste Deterrence: Many secondary compounds are bitter or astringent, signaling potential toxicity to herbivores. Animals, particularly herbivores, have evolved to detect these substances and reject the plant material to avoid poisoning.23
Sensory Irritation: Some chemicals stimulate receptors that convey sensations of burning or irritation to the nervous system, effectively discouraging further consumption.3
Post-ingestive Effects: If a plant's defensive chemicals are not immediately detected, animals may learn to associate the flavor of the plant with subsequent illness, leading them to avoid it in the future.23
Specialized Deterrence: Certain insects possess specific "deterrent cells" that respond directly to these compounds, correlating with an immediate refusal to feed.1