Some of these chemicals discourage feeding.
Others mimic insect juvenile hormones. These hormones are responsible for insect larval development and govern the transition to adulthood. The mimics disrupt the normal hormone balance causing premature or incomplete development. In some cases they completely interrupt development. In any event, because the larva is normally the``destructive'' stage, the damage to the plant is reduced.
Some of these compounds, like nicotine (used as far back as 1690 as an insecticide), are toxic to insects. The Pierid butterflies, one species of which is the Cabbage Butterfly, have managed to overcome the toxic mustard oils in their food plant and even use the scent to find oviposition sites. The Monarch incorporates the toxic cardenolides of the milkweeds as a larva and transmits these defenses to the adults in their blood (hemolymph). While some insects have been able to detoxify secondary metabolites, others store them for later use.
The production of these compounds represents a considerable investment of energy by the plant. Because different conditions occur in different regions of the plant, the compounds are not necessarily distributed uniformly. A study on the leaves of oak (Quercus robur) showed that the mature leaves had a higher concentration of tannins than the new leaves. Tannins reduce digestability. The new leaves had a higher nutrient content but also contained more toxic substances. The plants became a mosaic of nutrient availability. This effect may have some impact on the distribution herbivorous insects, perhaps forcing them to bunch together. This bunching could produce a corresponding effect on their insect predators.
These compounds have been important in shaping our environment. For instance, flower colors are produced by secondary metabolites. In some areas, they affect the spacing of plants and brush fire occurrence. In addition, as we all know, many of these compounds affect animals and people too.
Ron Lyons (volunteer 1990-1999)