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Population Dynamics of a Pernicious Parasite: Density‐Dependent Vitality of Red Pine Scale

McClure, Mark S.
Ecology 1983 v.64 no.4 pp. 710-718
Cecidomyiidae, Matsucoccus matsumurae, Pinus resinosa, branches, climatic factors, correlation, fecundity, mortality, natural enemies, nymphs, overwintering, parasites, population density, population growth, temperature, trees, Connecticut
The relationships between biotic and climatic factors and three parameters of fitness (survival, developmental rate, fecundity) and population density were examined for the exotic red pine scale, Matsucoccus resinosae (Homoptera: Margarodidae), in a plantation of red pine, Pinus resinosa, in Connecticut, USA, over a period of population increase and decline from 1975 to 1980. When scale density was relatively low and injury to trees was minor, measures of fitness were not correlated with density. However, when density increased markedly, each measure of fitness was significantly negatively correlated with scale density. Also, as scale density increased so too did injury to trees indicated by an increasing percentage of branches exuding resin. A greater percentage of nymphs dispersed and died as branches became resinous and unsuitable for colonization. Even though scale density decreased sharply after 1977, survival, developmental rate, and fecundity continued to decline, indicating that deterioration of red pine as a host was progressive and irreversible. The scale killed all trees in the plantation within 7 yr of its initial colonization. The single most important factor leading to the collapse of the red pine scale population was change in the magnitude of overwintering mortality incurred by nymphs. Drastic reductions in scale numbers during winter more than offset population growth during the rest of the year and resulted in overall annual population decline. Steadily increasing overwintering mortality from year to year was due to density—dependent reduction in developmental rate which resulted in nymphs overwintering in a developmental substage incapable of surviving even relatively mild winter temperatures. Natural enemies were not important to the population dynamics of M. resinosae. The only such species encountered, the cecidomyiid fly Lestodiplosis nr. grassator, consumed only 5—7% of the scale ovisacs in each year, showing no response to the changes in scale population density.