The effects of late-season insect-simulated defoliation and premature harvest-aid application on cotton lint yield and physical fiber properties were evaluated in field experiments at two locations in Louisiana. Insect-simulated defoliation was accomplished by manually removing leaves with scissors, while all harvest-aids were applied with a CO2 backpack sprayer. Defoliation timings were NAWF5 + 450 heat units (HU), + 550 HU, + 650 HU, + 750 HU, and + 850 HU. Insect-simulated defoliation at NAWF5 + 450 HU reduced lint yield by 18% compared with the standard (chemical defoliation at 80% open, NAWF5 + 1050 HU); however, insect-simulated defoliation at NAWF5 + 550 HU and later developmental stages had no effect on lint yields. Chemical defoliation at NAWF5 + 450 HU, + 550 HU, and + 650 HU development stages reduced lint yield by 38, 37, and 15%, respectively, below the standard. Harvest-aid applications at NAWF5 + 750 HU and + 850 HU did not affect lint yields compared with the standard. Insect-simulated defoliation did not impact fiber properties, but chemical defoliation of plants at growth stages ≤NAWF5 + 550 HU significantly lowered fiber micronaire at one location in both years. Chemical defoliation did not influence fiber strength, length, elongation, or uniformity. These results indicate that management strategies for late-season bottom defoliating insects should be terminated at plant development ≥NAWF5 + 550 HU, while chemical defoliation should not be initiated until plant development ≥NAWF5 + 750 HU.