Plants have many defenses against animals that want to eat them, but no plant defense remains entirely impregnable. Eventually, some animal consumer finds a way to deal with the defense and consume the plant tissues.
Vertebrates are relatively large, compared to insects, so they have some options for dealing with difficult plant tissues that insects do not have. Vertebrates can eat a larger variety of plants in a day, eat larger meals, and retain those meals longer than insects can. However, because they are small, insects have alternative tactics that vertebrates do not have.
Some insects have become very specialized, utilizing just a few food plants, thus avoiding the need for dealing with a wide array of toxins. There are some that are ultra-specialists on just one food plant. And some insects are so specialized that every population of a single species uses a different food plant in different parts of the species' geographic range.
Many herbivorous insects use gut-associated communities of micro-organisms to help digest plant material or deal with plant toxins. Termites, for example, use protozoa (or sometimes bacteria) to help them digest cellulose. Very small herbivores do not have internal space for huge communities of helpful micro-organisms. So they find or even cultivate helpful micro-organisms outside their bodies.
Some termites cultivate fungus gardens in their nests and the fungi break down cellulose into usable bits. Certain wood-boring beetles rely on wood-digesting fungi to reduce the wood to a usable condition. Leaf-cutter ants in the tropics cultivate (and feed on) underground fungal gardens that grow on bits of leaf and flower brought in by the ants.
How do young insect herbivores acquire their necessary digestive helpers? The young of some species eat their mother's feces. Other female insects put a colony of micro-organisms into their eggs or smear them on the outside of the eggs when they are laid.
Many insect herbivores avoid the problem of dealing with tough fibers by piercing directly into plant cells and sucking out the juices. Some have special behavioral adaptations for avoiding sticky sap and toxins: Milkweed beetles cut the sap-containing veins in the leaf, which stops the flow of milky sap to the outer part of the leaf, and then they feed on the sapless portion of the leaf.
Perhaps most interesting of all are the insects that use defensive plant chemicals for their own purposes. Some have evolved the necessary means of detoxifying the compounds in a particular plant species and actually home in on the smell of those chemical compounds to find their special food plant. Some insects even use defensive plant compounds for their own defense. For example, caterpillars of the monarch butterfly, which feed on milkweeds, store milkweed toxins in their bodies. Any bird that tries to eat a monarch (caterpillar or adult) soon vomits and learns that monarchs are nasty and distasteful. Such predators learn to avoid insects that look like monarchs, so some harmless species have evolved to look like monarchs, and thus gain indirect protection.
Some sawfly larvae, feeding on conifers, separate out and sequester conifer resin in special pockets behind the mouth, and they regurgitate this resin onto would-be attackers. Birds, ants and spiders are all repelled by this resinous vomit. A moth larva that eats pine needles makes an "igloo" of pine resin, to which it is tolerant, but the igloo protects the enclosed moth larva from other insects.
Lubber grasshoppers vomit brown juice when pestered, and the repulsiveness of this juice depends on what they have been eating; their natural foods add noxious chemicals that the grasshoppers recycle, but the vomit of grasshoppers fed on lettuce lacks this useful repulsiveness.
Plant toxins may confer other benefits on the leaf-eaters that have the means of dealing with them. For example, monarch caterpillars that feed on milkweeds with high concentrations of toxins are more resistant to parasites than caterpillars that ingest low concentrations of toxins. (Since we humans eat many plants that have low levels of toxins, I wonder if we can obtain similar benefits from some of our food plants!)
Still more entrancing are the stories of insects that recycle plant compounds for mating purposes.
For example, a gaudy pink and white moth, whose larval diet consists entirely of tissues of Crotalaria plants, not only tolerates the nasty defensive compounds but also uses a plant-derived chemical in courtship. Males that have taken in large quantities of this chemical are more successful in courting females. They apparently smell good to a female, but the smell is also an indication of further benefits to be obtained by the female. When a male copulates with a female, he transfers a package containing not only sperm, but also some nutrients and this special protective chemical. The female adds this chemical to her own storehouse and uses the combined quantity to protect both herself and her eggs.
Bigger males usually contain larger amounts of the magical compound, and females prefer bigger males as mates. Thus they ensure that they will receive a large mating gift of protective chemical (and nutrients) and their offspring will be well-protected, in turn.
There is another benefit too: Because body size is genetically controlled, large fathers tend to produce large sons, which will tend to be successful in mating when their time comes. And large daughters of big males will be able to produce more eggs than smaller females.
This is a tiny sample of findings from hundreds of researchers who have spent entire careers ferreting out some of the myriad ways that insect herbivores deal with their food plants. There are thousands of fascinating stories out there in the literature, and thousands more yet to be discovered.
Mary F. Willson is a retired professor of ecology and a Trail Mix board member.