PlantLife Online

Answer

Because most corn damaging insects spend mot there time within the ear of corn most insecticides provide little control. The best method to protect your corn crop without the use of harmful chemicals is to protect the corn silk by using oil in combination with a naturally occurring soil bacterium Bacillus thuringiensis. The oil spreads down the silks and smothers the caterpillars, and the Bt toxin is carried by the oil into the ear, where it kills any caterpillars still feeding. Bt acts by producing proteins that reacts with the cells of the gut lining of susceptible insects. These Bt proteins paralyze the digestive system, and the infected insect stops feeding within hours. Bt-affected insects generally die from starvation, which can take several days (W.S. Cranshaw Colorado Sate University). 

http://www.ext.colostate.edu/pubs/insect/05556.html