Since herbivory causes a loss of available energy, plants have developed different strategies to control or limit it. In some cases, plants divert energy into the production of chemicals, like tannins, which generally affect insects (and other herbivores) unfavorably (e.g. taste bad, reduce digestibility, disrupt hormone system). Some herbivores have adapted, thriving on the chemicals or neutralizing them, perhaps even using them to aid their own survival. Monarch butterflies, for instance, obtain some protection from cardiac glycosides (heart poisons, emetics at low doses) ingested while feeding on milkweed plants as caterpillars. In the distant past, whatever strategy was adopted may have discouraged activity from herbivores far more voracious than the ones we find now.
Some plants respond to herbivory by modifying their architecture. Bushier plants can result when lateral bud development is stimulated by the destruction of growing tips. Root-feeding insects may stimulate similar changes in root systems. Increased bushiness is usually good for the herbivore; microclimate changes that result from increased architectural complexity may also improve the environment for the plant.
Some plant species have developed mutually beneficial relationships with ants. Most ant species do not feed on plants, but by wandering all over the plant they discourage the presence of other insects, some of which are herbivores at some stage in their lives. Bull-horn acacias provide homes for ants in their thorns. Some plants provide extrafloral nectaries to attract ants. Some species of ants "farm" honeydew-producing insects like aphids and scales. Hosting colonies of these sap-sucking insects might be regarded as a tradeoff that some plants make to enlist the protection of ants. Because their colonies are so large, leaf cutter ants, which do feed on plants, can influence the local distribution of plants. Harvester ants assist in the distribution of plant seeds.
All insects, whether or not they feed on plants, are important to plants because they transport, concentrate and recycle precious nutrients such as carbon, nitrogen, and trace minerals. Insects excrete waste products such as gases, water, urine and feces (insect fecal matter is called frass). One Malaysian epiphyte gets about 40% of its carbon from carbon dioxide exhaled by ants living in its sacklike leaves. In addition 30% of the its nitrogen comes from ant products and activities. Carnivorous plants, like sundews and pitcher plants, supplement their diets with trapped insects. Insects attract other creatures, compounding this nutrient enrichment.
Recently, scientists found that even some grasshopper species give something back. One of the protein components in their saliva increases the efficiency of auxin, the plant growth hormone, allowing grazed plants to grow faster.
Pollination services, provided by many insects and generally considered beneficial, do not come without cost - plants must divert energy from growth and reproduction to attract pollinators. Sometimes, they must even tolerate the more destructive stages of their pollinators' lives.
Long ago, plants and insects came to terms with each other, selecting with various strategies the interactions that now occur. This mutual interaction continues to evolve as plants and insects shape each others' world.
Ron Lyons (volunteer 1990-1999)