Animals that feed on plant tissues or plant products are often called herbivores. This term applies not only to insects that injure a plant by chewing leaves or sucking sap but also to more benign species who only collect pollen, nectar, or plant resins. Entomologists frequently use the noun "phytophagy" and the adjective "phytophagous" when referring to any of these nutritional strategies. Both words are derived from Greek roots: "phyton" meaning plant and "phagein" the verb to eat or devour.
Phytophagous insects generally use visual or olfactory (odor) cues to locate a host plant. Visual cues may be as simple as the vertical silhouette of a tree or the contrast of white flowers against a dark background of foliage. Some insects are strongly attracted to certain shapes or colors which they evidently associate with "food". Red spheres, for example, attract adult apple maggots, white pans of water attract aphids, and bright yellow sticky traps attract leafhoppers. Odor cues are plant volatiles such as the saponins in alfalfa, the mustard oils in crucifers, or the terpenes in conifers. Sometimes these attractants are primary plant compounds such as sugars (e.g. glucose), nucleotides (e.g. adenine), or amino acids (e.g. alanine) that a plant needs for its own survival and growth. But in other cases, the attractants are secondary plant compounds that have no nutritional value to either the plant or the insect. These substances may be manufactured by the plant as a chemical defense against herbivores but they unwittingly serve as token feeding stimulants to a select group of specially adapted species. Milkweed plants, for example, produce cardenolides that deter feeding by most phytophagous insects. These chemicals, however, attract monarch butterflies, oleander aphids, milkweed beetles, and a few other species that have the ability to digest or detoxify the compounds.
Insect herbivores often have a cyclical pattern of feeding behavior. After an initial phase of attraction to the host plant, appropriate tactile (touch) and olfactory (odor) cues trigger the impulse to take a first bite. Additional gustatory (taste) stimuli must be present in order for continued feeding to occur. After a bout of feeding is complete, the insect may leave the host plant to engage in other activites.
Since many plants conduct chemical warfare against insect herbivores by manufacturing repellents or deterrents, it is common for insects to be rather narrow and specialized in their choice of host plant. A monophagous insect restricts itself to a single host species -- it is a consummate specialist, adapting its behavior and physiology to a single nutritional resource. Some of these insects must rely on intestinal symbionts to supply essential dietary components that are not supplied by their host (see Insect Nutrition). Oligophagous insects have a slightly broader host range -- often adopting any plant within a close circle of related genera or the members of a single taxonomic family. These insects are less likely to starve if a preferred host plant is unavailable. A few insects are polyphagous. These species are equipped with "broad-spectrum" detoxification enzymes that can overcome a wide range of plant defenses. It can be metabolically "expensive" to produce these enzymes, but on the other hand, there is no shortage of available food!
Some of the more polyphagous insects (like grasshoppers and armyworms) will consume every part of their host plant. But most insect herbivores are more selective: they specialize as leaf chewers, sap suckers, stem borers, root pruners, gall makers, leaf miners, collectors of pollen or nectar, etc. Each of these feeding strategies represents a separate ecological niche and all of the species that feed on the same plant in the same way are known as members of a feeding guild. Within a feeding guild, all species compete directly with each other for exactly the same resource. Between members of different guilds, competition is usually less direct and less severe. As a result, there is strong selective pressure limiting the number of species within each guild. Direct competitors usually are not closely related to each other (phylogenetically) and their association tends to be relatively recent in origin and short-lived in duration compared to more symbiotic (mutualistic) interactions. Natural selection tends to favor adaptations that minimize competition between species within a feeding guild.
Herbivory has had both positive and negative impacts on plants over evolutionary time. Flowering plants (the Angiosperms) have certainly benefited by attracting insect herbivores and exploiting them as pollinators. These plants often provide nectar (or other nutritional "rewards") to their insect accomplices. Bright colors, distinctive odors, geometrical patterns, and in some cases even subterfuge are tactics used by plants to attract specific pollinators and maintain their interest from blossom to blossom (pollinator fidelity). On the other hand, insects are also vectors of plant diseases. Aphids and leafhoppers (Hemiptera: Homoptera) are notorious for spreading plant viruses and mycoplasmas as they feed. Bark beetles (Coleoptera) invade the woody tissues of living trees, inoculating them with fungal pathogens that weaken and eventually kill the tree. Bacteria, protozoa, and nematode pathogens are also carried from plant to plant by insect herbivores. Pathogens may be carried externally on an insect's feet, mouthparts, or ovipositors, or internally in the salivary glands, digestive tract, or reproductive system. Some plant diseases like fire blight (a bacterium) and mummy berry (a fungus) are collected and spread by insect pollinators that are attracted to sticky-sweet exudates produced by the infected plants.