Why When And How Insects Are Introduced

Some introduced insects arrive in new locations on their own and are true immigrants. Of course, any simple range extension could bring a new introduced species in this sense, but as a rule this status is restricted to species arriving at a distant location by a discontinuous dispersal, rather than gradually diffusing from a neighboring site. Thus, the monarch butterfly (Danausplexippus) in Australia might not be considered to be introduced. It arrived around 1870 and established a population, having spread through the Pacific during the 19 th century largely unaided by humans. The monarch migrates long distances in its native North America, is an occasional straggler in Europe, and was recorded successively in Hawaii, the Caroline Islands, Tonga, and New Zealand before it reached Australia.

Some insects are introduced deliberately by humans. A few insects arrive as pets or pet food; recent pet price lists include mantids, walkingsticks, spider wasps, velvet ants, and dung and blister beetles. In Florida in 1989, giant Madagascan hissing cockroaches (Gromphadorhina portentosa) became highly popular as pets; at least some were released to the wild, where they survive. Similarly, caterpillars of a Chilean moth, Chilecomadia morrei, are sold as reptile food in the United States and Europe. The European honey bee (Apis mellifera) has been widely introduced for both the production of honey and crop pollination. The Asian silkworm (Bombyx mori) has been widely introduced along with its host plant, mulberry, for silk production. Even introductions that fail to establish a commercial industry can nonetheless establish a population. The Asian ailanthus moth (Samia walkeri) was brought to the United States in an attempt to found a silk industry on the ailanthus tree. Though the industry foundered, the moth remains. The gypsy moth (Lymantria dispar) was brought to North America to establish a silk industry; its predictable escape established one of North America's major pests.

Most deliberate insect introductions are for biological control. Although weeds and insect pests of agriculture are the usual targets, there are others. Thus, Paratrechina fulva, a Brazilian ant, was introduced to Colombia to control poisonous snakes, whereas over 45 dung beetle species were introduced to Australia to break down droppings of introduced livestock (~33% of established populations). Insects introduced to attack insects are either predators (primarily coccinellid beetles, but including other beetles, hemipterans, and neuropterans) or parasitoids (mostly hymen op terans, but including some dipterans). Insects introduced to control plant pests include primarily flies, beetles, and moths, although others such as bugs and thrips have been used. The number of insect species introduced for biological control purposes is substantial. For example, of approximately 2600 introduced insect species established in the Hawaiian islands, roughly 400 were introduced for biological control. About twice as many species introduced there for this purpose perished.

Far more insects are introduced inadvertently than deliberately by humans. Pathways are myriad. Soil ballast was an early predominant mode of entry to North America— many of the first introduced insects were soil beetles from southwestern England. This pathway is less common now, but insects are still carried in rootballs around cultivated plants and in soil on heavy equipment. Phytophages, particularly homopterans, dominated introductions to North America in the 19th century with the advent of fast steamships and a proliferation of imported nursery stock, and imported plants are still a major means of introducing insects worldwide. The grape phylloxera (Daktulosphaira vitifoliae) that devastated French vineyards in the 19th century arrived on saplings or cuttings of American vines. Insects can also be carried in water. The yellow fever mosquito (Aedes aegypti) probably arrived in colonial America in drinking water casks, while the Asian tiger mosquito (A. albopicta) reached North America in the 1980s in scrap tires from Japan. This is probably the route taken by A. japonicus, which arrived in the United States in 1998 and transmits West Nile virus. These latter two mosquitoes have recently been detected in used tires in New Zealand. Wooden packing material brought the Asian longhorned beetle (Anoplophora glabripennis) to the United States, whereas the European elm bark beetle (Scolytus multistriatus) that transmits Dutch elm disease arrived in North America on unpeeled veneer logs of European elm. The growth of international tourism can enhance the rate of insect introduction; in 1992 an Australian tourist returned from South America with a wound containing maggots of the New World screwworm Cochliomyia hominivorax.

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