The obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris), is one of the most destructive pests of tree fruits in Washington. The development of insecticide resistance in OBLR has led us to explore new management strategies. The use of very low doses of insecticides that have strong sublethal effects represents an environmentally-friendly option to improve the existing integrated pest management strategies. We tested the insect growth regulator pyriproxyfen (Esteem® 0.86EC, Valent U.S.A. Corporation) to determine its lethal and sublethal effects on OBLR. A leaf-disk bioassay was used to test seven doses of pyriproxyfen ranging from 0.0 to 3.0 µg per final instar OBLR. Male and female larvae were assessed separately for mortality or other subsequent parameters of growth and development. We found that response to pyriproxyfen was dose-dependent: only 5-6% of the larvae treated with the highest dose emerged as morphologically normal adults, compared to 86% emergence in the controls. Time to pupation and adult emergence was significantly increased at doses higher than 0.1 µg. The weights of OBLR pupae and adults were also significantly increased at a dose of 0.03 µg per larva. The fecundity and fertility of OBLR were highly reduced even at LD₁₀, with males being more affected than females. A proposed discriminating dose of pyriproxyfen for testing field-collected larvae is between 0.1 and 0.3 µg. We conclude that a combination of lethal and sublethal effects contribute to observed reductions of OBLR densities in tree fruit orchards treated with pyriproxyfen.