European corn borer (ECB) larvae are better adapted to the boring behavior than are cotton bollworm (CBW) larvae in 7-, 14-, 21-, and 28-day old cotton bolls. None of these four boll age classes were safe from ECR entry into the boll for any larval instar tested. CBW larvae were less able to penetrate larger bolls (21 & 28 days old) and did so appreciably only in the 4th and 5th instars. Younger CBW larvae did not appreciably penetrate younger bolls (i.e., 1st instars in 7 & 14 day old bolls, and 2nd instars in 14 day old bolls). Given adult and larval behaviors, bolls ò 21 days old are probably protected from CBW entry; however, ECB entry into cotton bolls of any age is almost assured. Management considerations and pitfalls are discussed.