Eastern deciduous forests are subject to a number of serious insect and disease outbreaks. Among the most conspicuous is that of the introduced gypsy moth (Lymantria dispar), which infests primarily oaks, causing severe defoliation and tree mortality. But it also has the benefit of eliminating weak individuals, and thus improving the genetic stock, as well as creating rich habitat of a type through accumulation of dead wood. Because hardwoods sprout so readily, this moth is not as harmful as the hemlock woolly adelgid. Perhaps more serious is the introduced beech bark disease complex, which includes both a scale insect (Cryptococcus fagisuga) and fungal components.
What does the hemlock woolly adelgid help improve?
{'text': 'genetic stock', 'answer_start': 336}