Biological Control in Native
and Introduced Habitats:
Lessons Learned from the Sap-Feeding
Guilds on Hemlock and Pine
Mark S. McClure
Many of our most important and destructive pests are herbivorous insects that were introduced from abroad. Much time and many resources have been expended during this century to control our many introduced herbivorous pests by importing natural enemies from abroad ( DeBach 1974; Clausen 1978; Laing and Hamai 1976), yet examples of successful biological control have been relatively few ( Caltagirone 1981). Many biological control efforts have failed by using a "shotgun" approach of releasing any and all natural enemies with little knowledge of their ecological requirements and with no system in place for evaluating their performance following introduction ( Turnock et al. 1976; Pschorn-Walcher 1977).
Surprisingly few studies have examined the impact of natural enemies on the population dynamics of introduced pests in their homelands. Those that have been done have often focused on natural habitats where pests frequently persist at low, innocuous densities ( Caltagirone 1981). This situation can be quite different from that of introduced habitats where pests commonly persist at damaging outbreak population levels ( McClure 1983a). As we shall see, factors that affect the population dynamics of pests in native and introduced habitats differ greatly, and failing to understand and address these differences can impose significant constraints on our efforts to control introduced pest populations by introducing their native natural enemies.
There are many possible constraints on successful biological control of introduced pests. The common paucity of taxonomic and bionomic information on pests and their natural enemies in their homeland is among the most obvious and