Populations of the apple blotch leafminer,Phyllonorycter crataegella(Clemens), were sampled in two commercial apple orchards in Connecticut during 1978 and 1979. Each population developed three generations per year. Pupae of the third generation overwintered in fallen apple leaves, and adults emerged in late April and early May. In 1978, moth abundance increased sharply from the second to third flight period even though endosulfan, methomyl, or both were applied in the spring. In 1979, abundance of immatures and adults also increased markedly at the end of the growing season. Rise in population size was accompanied by an increase in the amount of larval food and a decrease in percent parasitism. Tissue feeders destroyed 38.9 and 52.3% of the photosynthetic area of attached and fallen leaves, respectively, collected in a severely infested orchard in October 1979. Impact of parasitism and other factors on populations is discussed.