Hemlock sawfly infestations occur most commonly in the warm southern half of southeast Alaska. Biotic factors that limit sawfly populations are a fungus, Entomophthora sphaerosperma, which kills disproportionately large numbers of female larvae; food quality as affected by host site and defoliation; and three ichneumonid parasitoid species, Delomerista japonica diprionis, Oresbius tsugae tsugae, and Itoplectis quadricingulatus, which attack prepupal larvae in cocoons. E. sphaerosperma was the most effective natural control in recent years, especially during wetter than normal summers. Parasites which attack after the fungus has decimated larval populations were most abundant during two dry summers. Parasitization and sawfly emergence were directly related, and both were highly inversely correlated with percentage fungus killed sawflies. Sawflies reared on lightly defoliated trees produced significantly more eggs than those reared on heavily defoliated trees, and fecundity was directly related to cumulative heat units during the summer. Between 1952 and 1974, most negative sawfly population trends south of Frederick Sound occurred when two wet summers occurred consecutively. Apparent widespread collapse of sawfly populations in 1974 was associated with low temperatures in 1973 which delayed sawfly development and reduced opportunities for successful oviposition.