It has been demonstrated that bollworm and boll weevil damage to developing cotton (Gossypium hirsutum L.) bolls declines dramatically at approximately 350 heat units after flower fertilization. Use is made of this phenomenon for timing the cessation of insecticide application to the cotton crop. This study was designed to explain this phenomenon by investigating physical, anatomical and biochemical changes in the capsule wall in relation to boll age and insect feeding. The effect of plant growth regulators, PIX and PGR-IV, on the development of the boll wall was also investigated. The largest change in resistance to penetration occurred at 350 heat units after anthesis. Light and electron microscopy showed massive lignification of the endocarp cells of the boll wall at about 350 heat units which may be related to resistance to insect feeding. This study should explain the decline in attractiveness of the cotton boll with age to bollworm and boll weevils, and provide additional confidence utilizing this phenomenon to terminate insecticides.