Excerpts from the report: Patterns of gypsy moth behavior are described, especially those related to population density. Natural mortality-causing factors that operate against this insect are also described. Several agents kill subadult male and female gypsy moths at different rates. Major determinants of year-to-year changes in gypsy moth numbers are described. Gypsy moths, Porthetria dispar (L.), were imported to the United States from Europe in 1868 by a scientist who thought he could use them to make silk. They escaped, bred, thrived, and built up populations that soon began to damage trees in Massachusetts. The gypsy moth is a defoliator. It devours the foliage of hardwood trees— especially oaks— which can kill the trees. From the first infestations in Massachusetts, the gypsy moth has spread until it is now the major insect threat to the hardwood forests of the northeastern United States.