Two adelgids,Pineus boerneriAnnand andP. coloradensis(Gillette), are serious pests of red pine,Pinus resinosaAiton, in parts of New England. Both species are parthenogenetic, trivoltine, and complete six stages of development. The life stages were readily distinguished from one another within and between species by body size and by several morphological features including antennae, setae, and wax-producing glands. Generation overlap was so pronounced that all life stages of these adelgids were present nearly all times during the year.Pineus boerneri, an introduced species, spread at an average rate of 15 km per year during a 5-yr period between surveys. It invaded 21 of the 48 red pine stands sampled in Connecticut and Massachusetts, which is south of the natural range ofP. resinosa, and injured and killed trees at all of the inhabited sites.P. coloradensis, a native and heretofore innocuous species, occurred in all 10 stands sampled in New Hampshire and Vermont, which is within the natural range ofP. resinosa, and in 35 of the 48 plantations located in Connecticut and Massachusetts. Surprisingly, it injured and killed trees in more than 40% of these stands in both areas.Population trends of these adelgids during a 5-yr period indicated thatP. boernerihad displacedP. coloradensisin all 3 previously cohabited pine stands and in 7 of 12 others whereP. coloradensisinitially had occurred alone and whereP. boerneriinvaded subsequently. In four of the five remaining stands in the latter category,P. coloradensishad been reduced to very low density. These results reaffirm the superior competitive ability ofP. boerneridemonstrated in laboratory experiments.