Individuals of non-indigenous or exotic species, once in a new environment, may simply die, or may become reproduce leading to establishment of the species. Once established, such a species may result locally with little noticeable effect on its surroundings or may spread unimpeded, with devastating ecological or economic results. At least 4500 species of foreign origin, including plants, animals, and microorganisms, have established populations in the United States [46]. Approximately 2 percent of all insect and arachnid species are exotic to the United States [46]. In general, insects have high potential to become established due to their short generation time and high reproductive capacity.

Some exotic insect species are clearly beneficial. The European honey bee (Apis mellifera L.) forms the basis of commercial pollination of many US food crops. Non- indigenous organisms may also be beneficial as biological control agents, frequently controlling non-indigenous pests. For example, in Maine the parasitic wasp, Tetrastichus julis is a major mortality factor of the cereal leaf beetle, Oulema melanopus, an exotic pest of oats. Exotic insect species may have both beneficial and harmful effects. For example, the imported fire ants Solenopsis spp., which bite people and damage crops, also suppress populations of agricultural pests and contribute positively to nutrient cycling [54]. However, not all introduced exotic biocontrol agents have had positive results. For example, the seven-spotted lady beetle (Coccinella septempunctata), introduced to Maine from Europe, is a dominant aphid predator in the Maine potato ecosystem [44]. However, it has recently dispersed throughout much of the United States and appears to be out competing the native nine-spotted lady beetle (C novemnotata) in both alfalfa [28] and potato ecosystems [44].

Approximately 15 percent of exotic insects established in the United States have resulted in ecological damage [29,46]. High-impact exotic insect species which have become major pests include the gypsy moth (Lymantria dispar) (a major forest pest in New England, including Maine), the German cockroach (Blattella germanica), the imported fire ants (Solenopsis invicta, S. richteri), Russian wheat aphid (Diuraphis noxia) [29], and the African honey bee (Apis mellifera scutellata). Exotic insect species can pose serious threats to agriculture. The boll weevil (Anthonomus grandis) a pest of cotton, historically has had the highest documented impact, at least $50 billion (in 1991 dollars) of cumulative losses estimated for the years 1909-1949 [10]. Repeated outbreaks of the medfly in California necessitate costly control programs to avert projected annual losses of up to $897 million in damaged produce, control costs and reduced export revenues [30]. The European gypsy moth imposes the greatest measurable losses and requires the greatest expenditures for research, control, and eradication. The USDA estimated losses of $764 million from the European gypsy moth in 1981 alone, although the figure so far has been the all time high [46]. Loses due to the boll weevil, Mediterranean fruit fly, nun moth and spruce bark beetle, all exotic species, were estimated to be high as 73 billion dollars in $1991 [46]. It is not presently known how many exotic insects infest Maine or the cost they cause as agricultural, forest, or household pests; or by displacing indigenous species [16]. In Maine, a number of harmful exotic insect species were introduced from other states and countries. The Gypsy moth, Lymantria dispar, was brought into this country from Europe in 1869, and is now firmly established in southern and central Maine. The Japanese beetle, Popillia japonica  was accidentally introduced into New Jersey from Japan in 1916. Currently, it is present in all the states east of the Mississippi River except Florida and Wisconsin and is well established in the Atlantic Coast states including Maine. The European corn borer, Ostrinia nubilalis, originally occurred over most of Europe and western parts of Asia. It now occurs in all the US major corn-growing areas, as well as, Maine [41]. The Mexican bean beetle (Epilachna varivestis) was for a long time a serious pest only in the western US, from Colorado southward. This exotic insect species is now present throughout the United States except in the Pacific Coast states. In the last ten years this insect has become more severe in Maine.

The aforementioned exotic insects are well known as forest and agricultural pests. However, there are many insect invaders of the US Maine whose biologies are poorly documented or not brought to attention of the public. This review focuses on one such species populations of which have exploded in Maine, yet by unknown by most naturalists.

¹Maine Agricultural Experiment Station Journal Article no. 1953