The Federal Executive Order 13112 of February 3, 1999 defines an invasive species
as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” The term alien means a species that is not native to here, and for our purposes, 'here' refers to Pennsylvania, but it can be an area as small as a remote pond or as big as a continent. Native species are those that have historically existed in an area such as Pa. Because these species exist here naturally, they have coevolved
predator and disease relationships that keep the native populations in check.

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
and their adults. The eggs hatch and the parasitoids burrow into the host, living off it and eventually killing the host prior to pupating, becoming adults and starting the parasitoid cycle all over again. This condition is a very simplified version of the balance of nature that we have heard so much about.

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
important bust part of the population cycle. The alien species population continues to grow until it runs into some other limiting factor such as food or space to grow. For invasive forest insects, such as the emerald ash borer, the invasive insect continues to attack and kill ash trees until there are no more ash trees upon which to feed.

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.

Gypsy moth females lay eggs on yellow birch tree trunk.

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
outbreaks which defoliate hundreds of thousands of acres. Countless millions of oak trees, a favorite of the gypsy moth, have been weakened and killed during the 141 years that the gypsy moth has been with us.

Adult emerald ash borer ready to emerge from its D-shaped exit hole.

The emerald ash borer (EAB), Agrilus planipennis,
was inadvertently introduced to Detroit, Michigan from China in the early 1990s. EAB attacks only ash trees. Since its introduction, the EAB has killed more that 40 million ash trees in the U.S. Each female lays approximately 250 eggs. The EAB eggs hatch into larvae that eventually girdle the trees, killing them within 3 to 5 years of infestation. Since the introduction of EAB, it has expanded into at least 12 states and a couple Canadian provinces. North American ash trees have no resistance to this insect. Scientists are working hard studying the biology of the insect and its parasitoids to look at possible ways to control the insect. Without some effective natural controls, virtually all native ash trees will be removed from America's woodlands as this invasive spreads.

Elongate hemlock scale insects on the underside of eastern hemlock needles.

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.

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