Corn borer resistance

Damage from European Corn Borers in the U.S. Corn Belt has been minimized by use of the Bt genes. Although the damage from this pest has been most noticeable with stalk breakage and ear drop, early feeding also has been shown to cause yield loss.  First brood leaf feeding can interfere with movement of water and photosynthates, ultimately leading to poor ear development.  Non Bt corn and other grasses do produce defense compounds in reaction to chewing insects, such as corn borer larvae and sucking insects such as aphids.
 
One group of these chemicals are benzoxazinoids (acronym Bx). These are stored in the cell vacuoles within the cytoplasm.  These are at highest concentration in young plants in the roots and younger leaves. After the detection of cell injury by the insect, enzymes are activated to change a precursor called MBOA to the active form called DIMBOA.  This chemical negatively affects the insect’s ability to digest the leaf contents and thus reduces the damage of the first brood.  Varieties vary in DIMBOA content and all plants have a reduced concentration as they approach flowering.
 
Production of DIMBOA in plants does carry an energy cost and genetics involved is complex.  Genetics for DIMBOA production involves several genes with hybrids expression being intermediate between the parent inbreds.
 
There are varieties with less DIMBOA that still express first brood resistance. This appears to be related to cell wall components such as lignins and pectins, apparently making the tissue less easily digested.
 
There is energy expense associated with all forms of insect and microbe resistance. Some of this cost is reduced if the final product is produced as a response to the invader as opposed to being always present but I am not aware of a comparison of the energy cost of Bt versus more complex induced systems.  Certainly the Bt system involving only a single gene is easier from the corn breeding point of view.