When we eat our vegetables, we pick or cut off the part of the vegetable we eat and consume the entire piece shortly thereafter. Vegetable preparation often includes removing parts of the vegetable where the plant has mounted a “wound response” at the site of a cut. Plant wound responses can add bitter or anti-biotic substances to the local area. The wound site often has a different texture and is tougher than the fresh tissues. Humans will process vegetables to be frozen or stored by heating that inactivates the enzymes responsible for the wound response.
Caterpillars will induce wound responses in plants when they bite and chew the plant. Unlike humans who eat the whole vegetable, caterpillars may feed on a single plant for weeks. Caterpillars must adapt to the wound responses of the plant. Caterpillars can move from a damaged site and initiate a new feeding site at an undamaged area. This avoids the longer term wound response. Short term (within minutes) wound responses require other adaptations.
It has long been observed that the mechanical damage by cutting plants does not precisely imitate insect feeding damage. Caterpillars secrete saliva and other oral secretions that are not present in mechanical damage. The interactions between caterpillar oral secretions and plant wound response are complex. Some caterpillar oral secretions will induce wound and anti-biotic responses in plants. Other caterpillar oral secretions will suppress the expression of plant genes.
A recent study of caterpillars feeding on plants has found that addition of some caterpillar oral secretions to feeding sites will enhance the performance of the caterpillar.
The oral secretions act (through a mechanism that is not known) to suppress some of the genes that are otherwise induced during wounding. The identity of the active components of the oral secretions are under investigation.
The interactions that enable plants to defend against feeding by insect herbivores and enable insects to successfully use a plant as food to grow and develop are quite complex. Advances in our knowledge could lead to improvements in our ability to breed crop plants that are more resistant to insect feeding and less dependent on synthetic insecticides.