The Federal Executive Order 13112 of February 3, 1999 defines an invasive species
Some of these predator species are parasitoids. A
parasitoid - unlike a parasite that lives off a host plant or
animal, not doing the host any good but not killing it -
actually kills its host. Parasitoid populations prey upon
the host species, laying eggs on their eggs, their larvae
When a native population of host species has a very successful reproduction year, due to temperature, moisture, food, lack of predators or otherwise, the population very quickly increases. The co-evolved parasitoid species now have greater access to egg laying sites. Depending upon how many generations the parasitoid species has per year, the parasitoid population can grow very quickly, overwhelming the native host population. The native host population crashes and the parasitoid population crashes immediately afterwards because of a lack of egg laying sites. The populations of both the native host and the parasitoid are again in an endemic or naturally-occurring, balanced population. These are called population boom and bust cycles and they are normal.
With invasive alien species, there are no co-evolved
parasitoids, predators or diseases to hold the alien
species in check. When an alien is introduced and has a
successful reproduction year there are no parasitoid species to attack the alien species and cause the
Invasive species are by their nature highly competitive and are capable of reproducing very rapidly. Here is a sample of some invasive insects and diseases in Pennsylvania.
The gypsy moth (Lymantria dispar) is one of our best
known invasive insect species. The gypsy moth was
introduced into Massachusetts in 1869 by an amateur
entomologist who was attempting to cross gypsy moths
with silk moths from Asia. Some of the gypsy moths
escaped and have been expanding their range ever
since; it is now as far west as Minnesota. A single egg
mass laid by a female gypsy moth can have 500 to
1,000 eggs in it, making it possible for gypsy moth
populations to explode very rapidly. It continues to have
The emerald ash borer (EAB), Agrilus planipennis,
Hemlock wooly adelgid (HWA) and elongate hemlock scale (EHS) are tiny alien insects partnering to destroy our state tree, the eastern hemlock. HWA was introduced to the west coast in the 1920s and was moved to the east coast in the early 1950s on landscape plants from the west. More that one half of Pennsylvania is already infested with HWA, while EHS is still mostly confined to the eastern half of the state. Both HWA and EHS live almost their entire lives attached to new hemlock twigs at the base of needles. They attach to the tree by a feeding tube through which they drink the sap, causing the trees to weaken and eventually die. PA DCNR is cooperating in a study to develop a spray using a naturally occurring fungus to control HWA.